News:



  • March 29, 2024, 09:16:46 AM

Login with username, password and session length

Author Topic: Fox 35 Hop Up?  (Read 9159 times)

Offline Shawn Kuntz

  • Trade Count: (0)
  • Lieutenant
  • ***
  • Posts: 121
  • Ringmasters Forever!
Fox 35 Hop Up?
« on: June 26, 2007, 11:10:48 AM »
I have a late 70's Fox 35 new in the box engine.  I would like to put in a Nobler or Stunt King type of classic stunt model.  Is the stock Fox 35 plenty good or should I have it hopped up?  What hop up tips do you guys have?  Who could I send it too?  Any recommendations?
Shawn Kuntz
Bismarck, North Dakota USA

Offline Ralph Wenzel (d)

  • Trade Count: (0)
  • Captain
  • *****
  • Posts: 848
Re: Fox 35 Hop Up?
« Reply #1 on: June 26, 2007, 06:04:46 PM »
The only two "hop-up" techniques I know for the Fox .35 are: 1) a hemi-head, and 2) tip the Nitro can a bit farther. Oh, and if you feel particularly lucky, you might try to locate an ABC Piston/Liner setup.

You can get a better-running, smoother performing Fox in several ways, #1 being the addition of a Smith-Nelson "Hi-Zoot" crankshaft which is hardened and much better-balanced. Also, you will want to add a "stuffer" backplate, which prevents the connecting rod from wandering off the end of the crankpin.

I have my Foxes built by Larry Foster, but Randy Smith and Lew Woolard also do excellent work on them.

Randy sells a Super-duper Fox with all the above for $200. See    aeroproduct.net   - - and click "Engines"
(Too many irons; not enough fire)

Ralph Wenzel
AMA 495785 League City, TX

steven yampolsky

  • Guest
  • Trade Count: (0)
Re: Fox 35 Hop Up?
« Reply #2 on: June 26, 2007, 07:07:49 PM »
You can get a better-running, smoother performing Fox in several ways, #1 being the addition of a Smith-Nelson "Hi-Zoot" crankshaft which is hardened and much better-balanced. Also, you will want to add a "stuffer" backplate, which prevents the connecting rod from wandering off the end of the crankpin.

Stuffer backplate has a much more important function than keeping the connecting rod from slipping. It dicreases the volume of the crank case thus creating more vacuum suction which improves fuel draw considerably.
Another superb mod is PA NVA. Randy designed them specifically for Fox35's. Unlike ST NVA's, it has small diameter neck. This is important in Fox35's since they don't have a venturi that can be opened up.


Offline phil c

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2480
Re: Fox 35 Hop Up?
« Reply #3 on: June 28, 2007, 07:24:38 PM »
Steve, how did you measure the increased fuel suction from the stuffer backplate?  Bigiron told me he made the first stuffer backplates strictly to keep the rod in place so the engine would last longer.  It had no effect on how the engine ran.
phil Cartier

steven yampolsky

  • Guest
  • Trade Count: (0)
Re: Fox 35 Hop Up?
« Reply #4 on: June 30, 2007, 09:54:14 PM »
Steve, how did you measure the increased fuel suction from the stuffer backplate?  Bigiron told me he made the first stuffer backplates strictly to keep the rod in place so the engine would last longer.  It had no effect on how the engine ran.

Actually, this was done by a fella working for Fox back a few decades. I don't remember the guy's name but he came up with an adjustable backplate. He was able to adjust the amount of crank case volume while the motor was running. There was an article describing the experiments. Based on the results of testing, Fox Manufacturing came out with the stuffer back plate that they sell up until this day.

Mind you, I was not around when all this happened, so this recollection is NOT from my own absolute knoweldge but from others that were around back then and relayed the story on SSWF(or was it in Stunt News?).

Offline L0U CRANE

  • AMA Member
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1076
Re: Fox 35 Hop Up?
« Reply #5 on: July 01, 2007, 12:52:41 AM »
Steve, Phil and others...

The original OEM Fox 35 backplate was more like a pie tin than a good cylindrical plug for volume in the lower case. Working from different ideas, in different parts of the country, without coordination, about the same time as Marvin Denny to improve that situation, I had my approach. That was to "lower" the rear face of the casting (i.e., towards the prop end) to where the piston pin touched the backplate, then use a 0.010" gasket to provide clearance. I'd heard that the 'spec' fore/aft float for the shaft was about that 0.010"...

It may not have done much of itself, but I made several other 'touches' to Fox 35s of the mid/late 70's era, and since, that somewhat improved 'balance', 4-/2-cycling, economy and usable torque - or so it seemed to me, anyway. At least, working from as-assembled-and-sold Fox 35s - rarely called for replacement pieces - I did 'make-overs' rather than hop-ups or rebuilds. ...and got them through the critical initial break-in phase, for several other fliers. And I worked cheap, but good value for cost.

Marvin Denny developed the first GOOD cylindrical, teflon or other low friction material faced after-market "stuffer" backplate, and I believe he developed the first button hemi-head and clamp set-up for the Fox Stunt 35. Fox Mfg came out with a similar design, "idiot-proofed" with a locating pin to keep the button slot aligned with the piston baffle - which BigIron considers not as good as his design. I'll go with MD on that, but feel that for practical success, with a bunch of factory "blessed" button/clamp sets going to inexperienced hands, Fox Mfg was probably better off to keep the bits aligned...

However, for good success with a "stock" (iron piston/leaded steel sleeve) Fox Stunt 35, the most important thing is a GOOD break-in! The other touches add, of course, but at a level that a poor break-in will never let the engine benefit from. It takes time, and plenty of 28%-29% Castor oil fuel, and associated mess, to 'bring-in' a stock Fox 35.

The ABC piston/sleeve, and the counterweighted shaft, now available after-market (piston/cylinder occasionally from Fox Mfg), change what was the first "modern"  stunt 35 - in 1949-50, anyway - into a much nicer engine, which is still late-1940's in design concept. And do so at considerable cost. The thing is, that the 4/2 run the Fox 35 "created" has become a very desirable thing, often demanded from engines designed for an entirely different power profile.

The 4/2 run IS a glorious thing to hear! It isn't necessarily a standard that MUST happen. Engines designed for the RC market, which calls for less fussy break-in, less oil, and more power in a broader RPM band, can serve beautifully for stunt - if we accept that we have to go with what the engines were designed to do. We also gain - usually - from less vibration. More recent engines have relatively massive crank-disks - flywheels - and schneurle porting... and light alloy pistons in expansion-matched brass or aluminum-alloy sleeves... that damp a lot of vibes. The Fox 35 is a lightly-built engine - someone called it the max engine in the minimum frame - so there isn't that much crank-disk mass, except with the Super-Zoot shaft and the lighter reciprocating-mass ABC upper end.

The Fox 35 can still do what it has always done - serve modest size/weight stunt models with CONSISTENT performance for a long time. Not that the RC-based high-production stunt engines are Kleenex items, but you do have to spend a lot more to get one that will last for a few seasons of serious practice and competition. They ARE the solution for models heavier and larger than a Fox 35 can handle well, but for such models a Fox 35 was never a proper choice anyway.

We have more choice available today, so we're not limited to what a Fox can give us. Still, for what it can do, that Fox Stunt 35 is still there. Stock, or goodied-up to do significantly more than Duke intended, it is still available.

May that remain true for a long time!
\BEST\LOU

Offline RandySmith

  • Administrator
  • Trade Count: (0)
  • Admiral
  • *****
  • Posts: 13747
  • Welcome to the Stunt Hanger.
    • Aero Products
Re: Fox 35 Hop Up?
« Reply #6 on: July 01, 2007, 12:26:34 PM »
Actually, this was done by a fella working for Fox back a few decades.

Yep, his name is George Aldrich. He talked about this on one of his columns.   If you decrease the crankcase area, the result is to increase crankcase pressure. This forces the fuel into the port with more velocity and helps to mix the air fuel mixture. The increased velocity causes a greater pressure differential. Or suction as many refer to it erroneously.  This might help eliminate the side mounted "burp" or not. But the end result is a slightly smoother burn and running engine.  As to the ABC set up, my info, heard on here. is that the result is the displacement goes to .32, not .35. Not that it matters. H^^

Hello Ty

Well not exactly right, the displacement only goes  to 32 on the afertmarket piston/sleeve/head combos that people bought from Australia.  The FOX products that I have seen all use the standard  full size head  and  are still  35  size.
Regards
Randy

Offline L0U CRANE

  • AMA Member
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1076
Re: Fox 35 Hop Up?
« Reply #7 on: July 01, 2007, 04:05:09 PM »
Randy,

Couple of thoughts:

The Aussie sleeve/piston set: is that theone I've heard called 'plasma coated'? Seem to remember a displacement reduction was mentioned for that one.

Gordon Jennings masterwork, the Two-Stroke Tuners' Handbook, mentioned "base compression". In the then-modern 2-stroke motorcycle engines the book basically dealt with, Jennings said that earlier motorcycle engines had been built with very excessive case volume, and those benefitted from 'stuffing' the case more than newer engines. Newer engines were designed with lower crankcase volumes that gave good 'draw' and 'pumping' velocities through the transfer passages. Stuffing them further had been tried, and found to be no advantage, and occasionally a disadvantage by increasing pumping losses. More power was absorbed in pumping fuel/air into the case, then up the transfers, than was gained.

It seems our newer engines have a similar condition. There isn't much spare volume under the piston. Older designs, like the Fox stunt, did have more than ample volume in the crankcase. I think Duke Fox also did some testing, and found that extreme stuffing did cost power. However, Fox 35 stuffer backplates, like the ones Marvin Denny pioneered, certainly work better than the older stock tapered backplate.

So, as Ty mentioned, moderate stuffing helps a Fox 35's fuel draw. It also helps the piston pump more charge up the bypass to the combustion chamber. Both are 'good things' and - for stunt anyway - don't degrade usable power.
\BEST\LOU

Offline phil c

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2480
Re: Fox 35 Hop Up?
« Reply #8 on: July 01, 2007, 04:54:01 PM »
Just asking, how did you measure the increased fuel suction resulting from using a stuffer backplate?
phil Cartier

Offline RandySmith

  • Administrator
  • Trade Count: (0)
  • Admiral
  • *****
  • Posts: 13747
  • Welcome to the Stunt Hanger.
    • Aero Products
Re: Fox 35 Hop Up?
« Reply #9 on: July 01, 2007, 10:23:19 PM »
Randy,

Couple of thoughts:

The Aussie sleeve/piston set: is that theone I've heard called 'plasma coated'? Seem to remember a displacement reduction was mentioned for that one.

Gordon Jennings masterwork, the Two-Stroke Tuners' Handbook, mentioned "base compression". In the then-modern 2-stroke motorcycle engines the book basically dealt with, Jennings said that earlier motorcycle engines had been built with very excessive case volume, and those benefitted from 'stuffing' the case more than newer engines. Newer engines were designed with lower crankcase volumes that gave good 'draw' and 'pumping' velocities through the transfer passages. Stuffing them further had been tried, and found to be no advantage, and occasionally a disadvantage by increasing pumping losses. More power was absorbed in pumping fuel/air into the case, then up the transfers, than was gained.

It seems our newer engines have a similar condition. There isn't much spare volume under the piston. Older designs, like the Fox stunt, did have more than ample volume in the crankcase. I think Duke Fox also did some testing, and found that extreme stuffing did cost power. However, Fox 35 stuffer backplates, like the ones Marvin Denny pioneered, certainly work better than the older stock tapered backplate.

So, as Ty mentioned, moderate stuffing helps a Fox 35's fuel draw. It also helps the piston pump more charge up the bypass to the combustion chamber. Both are 'good things' and - for stunt anyway - don't degrade usable power.


Hi Lou

My comments to TY's post were about the  Aussie  FOX plasma P/S, I have set up several of them  and they are a smaller  bore than the  FOX ABC  P/S. I think this was done mainly because it is hard to make  sleeves  that thin, The FOX 35 brass or Alum sleeve is a very thin sleeve, they are a little hard to keep round when they get  thin. Would be interesting to ask the guys that made them if this was the main reason or not.

I had years  ago (almost 30 years now)ran FOX 35s with several differant backplate settings  yielding much differant crankcase volumes and  really didn't find much of any difference in fuel draw  or  power difference  either  way.  You also have to remember if you did do something  that dramatically increased the FOX  fuel draw, it may NOT  have that typical FOX 4-2  break, An increase in the suction may do away with or shorten the break where it may not even be a useful stunt engine.......  I do know that many times  running  pressure gives the same  results, you get a shorter and  delayed  "break",  sometimes this makes the engines dumb as far as coming on and off 4-2 in the correct  places.

Jennings  book  and  findings  are a  real  jewel, however  not everything Gordan  found  applies to our stunt motors. adding a  carb  makes things  way differant.

Regards
Randy

Offline L0U CRANE

  • AMA Member
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1076
Re: Fox 35 Hop Up?
« Reply #10 on: July 06, 2007, 03:33:26 PM »
 >"You also have to remember if you did do something  that dramatically increased the FOX  fuel draw, it may NOT  have that typical FOX 4-2  break, An increase in the suction may do away with or shorten the break where it may not even be a useful stunt engine.......

Agreed, if all that's involved is case volume and fuel feed. Personal opinion: 2/4 break run depends more on sleeve timing than any other single factor - at least on an iron-in-steel Fox 35 and similar layouts. The usual suspects: nitro, prop load, oil%, plug, adjustment to the actual flight conditions all help a lot, but, if the timing is way off, can't do the entire job.

Fox engines, as sold, are usually a set of pieces that do fit each other quite well, but which may or may not be within several thousandths of design spec dimensions. I begin "fitting" a Fox by taking the actual dimensions in that particular bunch of pieces, and determining if, and how much, I have to cut or shim (usually a little cutting) to get the sleeve timing I want. No secret: 130° exhaust duration, 120° bypass.

That's later exhaust open/earlier close than any stock Fox 35 I've played with, and longer bypass duration. Port open/close "lag" is only 5°. Runaway RPM is unlikely - hot exhaust will short-circuit to the lower case if there isn't enough time to vent it out before bypass opens. There are very few hundred RPM - on a usable prop/fuel/plug/dialing setting - above the 2-cycle break. The setting is a solid 4 with a clean break to 2 but without a sharp power jump. Airspeed is needle limited in level and unloaded flight, but the power is mostly there when loads call on it. When all works well, I've had people come over and ask what engine I'm using. They didn't recognize the 'angrier' sound as Fox 35. (Wish I could fly more often lately, to keep things working well. Crumbs, dust, spiderwebs, etc., in the model, engine and me take several flights to blow clean.)
\BEST\LOU

Offline Marvin Denny

  • Trade Count: (0)
  • Captain
  • *****
  • Posts: 889
  • Welcome to the Stunt Hanger.
Re: Fox 35 Hop Up?
« Reply #11 on: July 06, 2007, 07:35:29 PM »
  I think I shall chime in here as there is a little misconception here.
  Both Randy and Lou are correct when they say that Duke Fox had a "variable Volume" backplate that he used to determine the "correct" crankcase volume for the Stunt 35.  Whether he (Duke) or George Aldrich designed it is a matter of "discussion"  I have heard both say that it was their idea.  Both are deceased now so who knows?? Not me.  I DID design the Stuffer back plate solely to keep the rod fully engaged on the crank pin.  In doing so, some (a considerable amount) of vibration was eliminated.  That was a fringe benefit that we did not expect, but gladly accepted.  Duke checked the "ideal crankcase volume" using Fox Superfuel (5% nitro/ 29% castor oil and 2% ignitors) a 10 X 6 Top Flite prop, and a Fox standard long plug.  He checked it flat on a bench and got the 4-2-4 break by raising and lowering the tank.  Not by loading the engine as it would in a plane.
  When ANYTHING is done to raise or lower the RPM, (more nitro, less oil, less pitch or diameter on the prop, then the "ideal" Case volume  will change.  That is why most speed engines really have the cases packed to the max.
  I was wearing out three (3) shafts and two (2) piston/sleeve assemblies on my foxberg race engines per year until I started using the Stuffers.  I could only use them during practice as they are not legal for actual races.   Of all the racing and stunt flying I have done using the Fox Stunt 35 both with and without the Stuffer backplate, I have NEVER found any increase in RPM nor power.  I will condition that by saying that ANYTIME you eliminate vibration, you will realize an increase in power as vibration robs power. After installing the Stuffer backplate, I have yet to wear out a shaft (over 15 years) and my piston/cylinder assemblies started lasting over two years.
  I personally think that the Stuffer is the best single thing you can do to help  your Fox Stunt 35.  I think that the ABC  Piston/cylinder assembly is still available from Fox Mfg, but it doesn't 4-2-4 break as well as the old steel/iron ones (my opinion).
  The Hemi-Head (Fox brand) is more or less my design.  I was developing them and was NOT happy with them when Fox Mfg put them into production.  The "locator roll pin that they used to index the button with the piston baffle caused abour a 50% failure rate due to cracking out of the squish band into the combustion chamber and thus a loss of compression.  It has since been eliminated and a set of installation instructions included to assure correct installation per my directions.  The squish band and combustion dome is STILL not as the final development wound up. If anyone buys one from Fox mfg and is not happy with it, I will make the proper changes to it at no cost to the customer other than shipping.

  I hope all this makes sense.

  Bigiron
marvin Denny  AMA  499

steven yampolsky

  • Guest
  • Trade Count: (0)
Re: Fox 35 Hop Up?
« Reply #12 on: July 06, 2007, 09:03:29 PM »
  I hope all this makes sense.

Man this thread is great! Amazing info!

Offline Jim Oliver

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1407
Re: Fox 35 Hop Up?
« Reply #13 on: July 06, 2007, 09:32:18 PM »
Marvin ain't called Bigiron for nothin'.

Jim
Jim Oliver
AMA 18475

Offline L0U CRANE

  • AMA Member
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1076
Re: Fox 35 Hop Up?
« Reply #14 on: July 07, 2007, 02:21:40 AM »
BigIron,

Thanks! That's an excellent summary. It's great to get the accurate word, when the fog of time and hearsay have allowed things to blur.

I play with the sleeve timing because I'd noticed the dimensional scatter back in the 1970's - not an era of Fox Mfg's best QC for the Stunt 35... Took an engine that ran as I wanted, figured the timing from the dimensions, and over the next few, settled in on the numbers.

The 130°/120° durations are the same, or very nearly, as Lew W does in his Silver Foxx schneurles, aren't they? Different case, different porting, different RPM band, and it works for him. The light-frame 40s may be more alike in dimensions than the 35 Stunts still seem to be, and it's great that they have that thick flange to allow sinking the sleeve into the case, too.

Thanks, again!
\BEST\LOU

Offline Bill Little

  • 2017
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12676
  • Second in COMMAND
Re: Fox 35 Hop Up?
« Reply #15 on: July 07, 2007, 04:13:34 AM »
Just as an aside;  I *met* Lou many years ago.  He kindly offered to rework a Fox 35 Stunt for me.  He was not, and AFAIK still not in the "reworking business".  At first run, the engine was noticeably smoother running, and the break was easier to set.  It quickly became my favorite Fox 35 for Classic use.  I still have it and use it. 

I also had Larry Foster rework one of my 40th Anniv. Fox 35s.  Same thing, a noticeably smoother engine!  So far, I have not had a Fox 35 set up by Randy.  Face it, I don't use the Fox 35 much anymore since the advent of the Aero Tiger 36.  But it is a completely different animal!

One thing I DID do was have Larry send me his instructions on doing the mods to the Fox 35.  Simple work and a very smooth running engine also.
Big Bear <><

Aberdeen, NC

James Hylton Motorsports/NASCAR/ARCA

AMA 95351 (got one of my old numbers back! ;D )

Trying to get by

Offline L0U CRANE

  • AMA Member
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1076
Re: Fox 35 Hop Up?
« Reply #16 on: July 07, 2007, 12:48:10 PM »
Bill,

Thanks for the kind words. Right - I'm not in the 'business' of reworking Foxes now. I still do mine and an ocasional engine for a friend or so.

A while back, Larry Foster made a sizeable investment in time, effort, and never-assembled Fox parts to develop and offer his L&J Fox 35s in his spare time. They were excellent from the start. He's continues to refine them and to be interested in people who are using them. I never considered my efforts on the Fox Stunt to be a source of income, so chose to sit back enjoy Larry's successes with his engines. 

Amateurs often care more about what they do, than some professionals - in many fields. Professionals must have a sharp sense of cost-effectiveness regarding their time. The eventual professional product is the most that can be done within cost restraints, which could be restated as the least that the client is willing to pay for. Not suggesting that a pro doesn't wish to make it better, just that he has to factor what the client is willing to pay for.

I get a little uncomfortable when someone says that a -whatever- amateur activity should be done more professionally. A profession is a life's method of earning a living. An amateur activity, say a hobby or recreational sport, is much more optional than that. 'Professional' doesn't always mean as well as someone will do where costs and earnings are not the controlling consideration. ..where the wish to do well doesn't have a price tag.

L&J Foxes aren't Larry Foster's day job, but he does them lovingly - that 'amateurish' zeal that isn't measured by a dollar sign.
\BEST\LOU

Offline Marvin Denny

  • Trade Count: (0)
  • Captain
  • *****
  • Posts: 889
  • Welcome to the Stunt Hanger.
Re: Fox 35 Hop Up?
« Reply #17 on: July 08, 2007, 06:26:39 PM »
  I,like you, Lou, do not do rework for others.  I do my own engine "tinkering" but leave the semi professional rework to others such as Larry Foster or Lew Woolard.  I want my hobby to just remain that--- a hobby--- not work.    When I get something that seems to work, I present it to the engine manufacturer and leave it up to him to decide if it can be beneficial and profitable to produce.
   Seems to work fairly well for me.

  Bigiron
marvin Denny  AMA  499

Offline L0U CRANE

  • AMA Member
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1076
Re: Fox 35 Hop Up?
« Reply #18 on: July 10, 2007, 12:20:17 AM »
Bigiron,

I would offer what I do to manufacturers, and have on occasion, except that the steps I take are more foreplay than series production. (I did offer a modified image of the muffler-bolt guides to J Lowry while he was there, which is now the full-length guides on the most recent series Fox Stunt, but I hardly expect I was the only one offering the same idea...)

Each engine is a unique set of compromises around the design specs. Each engine needs to be tickled, caressed, polished and obliged to perform, based on what it is in the package shipped from the manufacturer. (Larry F's great advantage - with never-assembled parts, he can mix/match optimum sets for the eventual individual engine!)

The time and sensitivity needed to coax a random manufactured engine into the most beautiful girl at the dance is a personal devotion. It ISN"T applicable to mass production. ...and I DON'T have 4" whitewalls on my tires...

ALL Fox 35s run alike, unless they've been destroyed by a bad break-in, or the wrong fuel, later on. True, but like the pigs in Orwell's 1984, some pigs is more equal than others. I just try to get the built-in problems settled, and to let the engine be what it can be -with what I prefer as sleeve timing, anyway.

...Aay, works for me...
\BEST\LOU

Offline Peter in Fairfax, VA

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1101
Re: Fox 35 Hop Up?
« Reply #19 on: January 19, 2019, 10:01:34 AM »
Is it a good idea to "fit" my Fox .35 40th Anniversary engine by lapping the piston/sleeve? 

It is still really, really tight after half a dozen flights.  So tight that it does not even "flip over" with authority.

Any other thoughts about hop-up parts for that model?  Do I need to change the head and/or backplate?

Online Dan McEntee

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6823
Re: Fox 35 Hop Up?
« Reply #20 on: January 19, 2019, 10:21:49 AM »
Is it a good idea to "fit" my Fox .35 40th Anniversary engine by lapping the piston/sleeve? 

It is still really, really tight after half a dozen flights.  So tight that it does not even "flip over" with authority.

Any other thoughts about hop-up parts for that model?  Do I need to change the head and/or backplate?

     A have dozen flights is not nearly enough to break in an engine. It's best to put it on a test stand and bench run it. Make sure you have fuel with at least 25 to 29% total oil. Take a 10-4 or 10-5 prop and cut it down to 9 inches and rebalance it. Use that for your break in/bench run prop. It may take an hour or more of running on the bench. Run it in what we call a four stroke, or pretty rich for the first several tanks. Then a couple clicks leaner. Every now and then, pinch the fuel line so it breaks into a higher RPM run and see how it goes back to the needle setting. Let it cool in between runs. All this time, look at the exhaust residue and what color it is. It make be dark, because that is the metal coming out of the engine from rubbing together. If after 15 or 20tank fulls see how it flips over cold. If it seems better, and/or the exhaust is coming out clear now, you are in the ball park for flying the engine. For flying, I would stay with 10-5 props for a while.
    Now, having said all of that, you need to also make sure you are not putting things into a bind when you mount the engine. The Fox .35 and some others are really pretty flimsy and if the mounting surface is not flat, solid and even, you can actually distort the case and cylinder when bolting it down. Check this out on your model also. Sometimes the mounting lugs on the engine are not flat. They can have a shallow draft angle to allow the case to pop out of the casting mold. These can be machined or carefully filed flat. It pays to carefully check any engine that is new to you whether it is new or used.
   What airplane, fuel and prop are you using? You can search out Fox.35 on thelist here and find enough stuff to read for days!
    Type at you later,
      Dan McEntee
AMA 28784
EAA  1038824
AMA 480405 (American Motorcyclist Association)

Offline Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13717
Re: Fox 35 Hop Up?
« Reply #21 on: January 19, 2019, 11:56:05 AM »
Is it a good idea to "fit" my Fox .35 40th Anniversary engine by lapping the piston/sleeve? 

It is still really, really tight after half a dozen flights.  So tight that it does not even "flip over" with authority.

Any other thoughts about hop-up parts for that model?  Do I need to change the head and/or backplate?

 As Dan notes, half-a-dozen flights is nowhere near enough run time to break in a Fox 35, at least a stock engine. It may be tight for other reasons, too, like uneven head bolt tension. Think several hours of short (few minutes) runs, with cooling cycles between, on a bench, working up to full diameter props. Foxes are known for getting better and better with time.

   Since you have already run it, lapping is probably not a good idea. If it is not loose through bottom dead center, the issue is probably in the bearing section, not the cylinder/piston. Since you have already flown it, probably too late to be really careful, so I think you are talking about bench time. If you can peak it and hold a peaked setting for a minute or so on the bench (without it starting to sag off, requiring the needle to be opened to keep it peak), and it goes into a two-stroke with a pinch of the line and then *immediately* returns to a 4 when released, then you are done. If it's not ready, when you release the pinched line, it will lag a bit returning to a 4. Broken in, it will snap back very quickly.

     Of course, finding an expert to assist you with these subjective evaluations would help. I would suspect that volunteers to spend a day at the field running your unmuffled Fox for 4 minutes and then waiting 15 until you get to 2 hours or run time will be few and far between, but a simple inspection might identify where the problem might be.

   An alternative (presuming having flown it has done about as much damage as you can do)-  this is not recommended, but was done a lot in the good old days - hook it up to  quart fuel can, set it at a lean 4-stroke, and then walk away. Monitor from a distance, tweak it back as necessary to hold the setting. I have a quart-size Fox Superfuel can with a uniflow vent squirreled away somewhere for just such a operation. I have heard stories of the same thing except with a GALLON can, and I beoeve them. A gallon is about 2 hours and people have claimed to do it 5 times.

   Also, since you didn't mention it, you need fuel with at least 25% oil, at least half of it castor, and more (up to 30%) would not hurt anything

    This is why a half-dozen flights sounds so inadequate, you are off by a factor of 10 or more from the general consensus. And also why you don't see too many Fox 35s winning big contests any more, too.

     I *would not* fly it any more until it feels right, because it only takes one missed needle to cause damage, and they aren't making Fox cylinder/piston assemblies any more.

     Brett

p.s. I know this is paleo-thread (from 2007!) but the only real hop-up tip for a Fox, as noted many times above, is more nitro. 10 or 15% (instead of 5) will make a tremendous difference in the power - and the noise. More than 15% and you might get more noise for a while, followed by stunned silence, when the crankshaft breaks and flies off into the weeds. You don't need or want to adjust the compression with more nitro, it's plenty low enough already to safely run on any amount of nitro you have, the shaft is the limiting factor.

Offline Peter in Fairfax, VA

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1101
Re: Fox 35 Hop Up?
« Reply #22 on: January 19, 2019, 02:04:50 PM »
I am running high castor oil 5% nitro fuel, like Fox Supefuel or similar from Brodak.  When the weather turns better here if Northern Virginia, I'll make an effort to set up a test stand and break it in.

Kind of surprised that lapping by hand is not popular for stunt, as it was very popular when I flew Combat for both Super Tigre and Fox. Lapping after running was tolerated, too.  An evening to fit the engine's piston/sleeve, followed by one or two test runs, was adequate.

The particular plane this is on, a Sterling Navion, has some good virtues - it is light, straight and well balanced (about 2 oz lead in the tail.)  However, this Fox .35 did not hold a needle well on a 10x6; better on a 9x6.  I've changed over the tank to a uniflow with a chicken hopper, as well, which should help some near the end of the run.  The last time I flew it, the motor suddenly quit at a difficult point of a vertical eight on 52' lines.

As far as the Fox .35 being popular, I had one on a Nobler about 1974.  Broke several cranks and never did have enough power for a decent cloverleaf.  My main engine this season will likely be an O.S. LA .46, or, if that doesn'.t work out, a Super Tigre .46

thanks for the break-in suggestions,

Peter

Online Dan McEntee

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6823
Re: Fox 35 Hop Up?
« Reply #23 on: January 19, 2019, 02:31:28 PM »
   Well it sounds like you do have some engine experience. This here computer thing makes it difficult to feel out what a persons capabilities are. If in talking with some one about lapping a particularly stubborn engine, I usually suggest lapping the piston backwards to the cylinder, and do so lightly. If there is any taper to the cylinder, it makes the opposite taper on the piston and releaves the skirt just a bit. I have done this on several Fox .35s with some good success. It's oneof those little things that doesn't do much by itself, but when combined with a bunch of other little things, it adds up and helps a lot. While the engine is apart, take a 4-40 tap to all the bolt holes just to clean them up. You will some times be surprised at how much metal can come out of them, especially on engines of certain vintages. Makes me wonder just how far Duke made his employees push the limits of a tap, or the employees couldn't tell if one was getting dull! This helps with everything tightening up nice and even. Check the crank also and see how it fits in the main bushing. And while you got it out drill some oil holes in the rod ends so that they can get a bit more juice when running. This is probably all stuff you already know but helps a Fox out a lot. There is information out there on doing a balance procedure on the crank also that helps a Fox run a bit smoother, and also never forget the stuffing of the bypass port to help with the infamous Fox burp on profile models. Like I mentioned before, there is tons of stuff on the list that would take you days to go through but is all useful and interesting. The late Gary Frost used to refer to the Fox .35 and merely an engine "kit" that comes assembled! But I think they are a big part of the history of the hobby and I like to run them. It's just not right not having a Ringmaster ( or your Navion) with a Fox .35 in it hanging on the wall ready to fly. Good luck and have some fun with it.
   Type at you later,
    Dan McEntee
AMA 28784
EAA  1038824
AMA 480405 (American Motorcyclist Association)

Offline Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13717
Re: Fox 35 Hop Up?
« Reply #24 on: January 19, 2019, 02:58:45 PM »
I am running high castor oil 5% nitro fuel, like Fox Supefuel or similar from Brodak.  When the weather turns better here if Northern Virginia, I'll make an effort to set up a test stand and break it in.

Kind of surprised that lapping by hand is not popular for stunt, as it was very popular when I flew Combat for both Super Tigre and Fox. Lapping after running was tolerated, too.  An evening to fit the engine's piston/sleeve, followed by one or two test runs, was adequate.

   The underlying issue is that most people have *no idea* what they are doing when it comes to engines, and the more they think they know, the more likely they are to screw it up. We have thread after thread of "grind this, lap that, polish something else" etc. almost always ending in failure.

   On the Fox 35 in particular, as Motorman notes, everything is crooked to begin with, and it's also very flimsy and flexible, so everything it moving with respect to everything else, particularly when it is new and all the stresses are still present. Running it relieves all these stresses and lets everything go to whatever shapes and alignments it wants to go to. There is no guarantee that this will be in the right direction or the changes are good, but you can't realistically do anything about it.

   There's no substitute for just running it. Lapping is necessary or desirable only until it's safe to run, once you start to run it, there's no point to it any more. If you somehow mount it in a Sunnen hone without distorting it, then hone it to a perfect round shape, piston too, and then stuff it back in the case, it won't be round any more. You can screw up a perfect running engine by changing the head screw torque. That's why once it is running, you generally want to *leave it alone* because even innocent changes might cause a wild variation.

   The topic of Fox 35 break-in and operation is about as well-documented by as many experts as you want to see, there aren't a lot of remaining questions or secrets about it. There is, unfortunately, lots of utter nonsense as well, but that's true of most things in stunt.

    You have it running, just keep running it, it will loosen up eventually. I would strongly suggest that if you don't like the power, more nitro is the cure. The compression ratio is extremely low, you will NOT go over the top on compression, just leave it alone. The Fox *is not* weak compared to other similar engines of the 50's and 60's. It's very ineffective compared to much more modern engines (both from a power and stunt effectiveness standpoint - which are assuredly NOT the same thing) but there's a reason it won all those contests  - 50+ years ago.

    I have the quart can because I in fact *did* get it partially broken in, and then did fill the tank and walk away. It's my personal opinion that it doesn't get right until you have 5+ hours of total run time, the plain bearing section is *very loose*, and a hundred or more flights. Its safe to fly long before that, but if you are not willing to have 6 months of half-assed flights, you need to do something to get that sort of time on it. *As long as you don't let it get too lean*, wear-out is not much of an issue, certainly not in any reasonable number of flights for sport or casual competition. I have individual engines with, conservatively, *40 gallons* or more fuel from them, with no apparent harm or consequential loss of power. You aren't ever going to fly that many flights with it!

    Brett

Offline Dave Hull

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1902
Re: Fox 35 Hop Up?
« Reply #25 on: January 19, 2019, 04:53:36 PM »
Peter,

One other thing to check, if you have not already: if the engine does not turn over freely, take off the muffler, if installed. Check again. The Fox case will easily distort if things are not flat or if too much load is applied. And the muffler/case interface is not always accurately machined.

This is not unique to Fox, by the way. A friend had pretty much the same symptoms with a Brodak 25. It didn't feel too bad before the first flight. After flying, it felt more and more bound up. To the point that it quit in-flight. So I took a look at it. Once we got the muffler off, it ran perfect. After we lapped the muffler face and reinstalled it, it still ran perfect. We caught the distortion early, and I don't think the life of the engine was degraded much.

One other thought:  check to see if the liner is glazed up with a castor varnish. On "break-in flights" I have experienced a gradual engine slowing simply because the engine was too tight, and the heat build up, which caused the castor to get stiffer and stiffer until the engine slowed down. It needed more break-in, and it would have benefitted from some synthetic oil in the overall mix. If yours is all varnished up, take care of that before you fly it again. A few engine runs with a castor/synthetic mix might clean it right up. I would not take it apart to clean it unless the alternate fuel doesn't take care of it.

A decent Fox should have enough pull for a smooth pattern with a Nobler that isn't too porky. At least when the wind is not a factor. But it probably doesn't come close to the power reserves of anything else more modern that folks are running. And because the power is more limited, the selection of prop is more important. Poorly shaped/finished props need not apply....

My 2 cents,

Divot McSlow

Offline Al Ferraro

  • 24 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 576
Re: Fox 35 Hop Up?
« Reply #26 on: January 19, 2019, 07:32:08 PM »
   The Stock Fox 35 has plenty of power to fly a Nobler, you just have to use 58’ .015 lines. I like the APC 10.5x6 prop and 10 nitro 25 all castor fuel. Every Fox 35 back plate that I removed had mark from the Rod hitting. I even tried the stuffer backplate on a old engine and that got chewed up real bad and sending the aluminum though the engine killing the piston and liner. Now when I set up a Fox 35 I cut a piece of .015 spring steel to the shape of the backplate and JB weld on to it. Rough up the backplate and the mating side of the spring steel with 80 sand paper before gluing, then I polish the side that faces the rod. I tested this backplate mod in my Fox Speed engine that turns over 18000 rpm with over 50 flights on the engine with no signs of metal wear, even after swabbing the crankcase with a Qtip looking for it. I also heard the old folklore story about the Fox 35  adjustable backplate test, but from what I heard there was no improvement in performance.
Al

Offline Air Ministry .

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 4980
Re: Fox 35 Hop Up?
« Reply #27 on: January 19, 2019, 09:23:46 PM »
 

Offline Air Ministry .

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 4980
Re: Fox 35 Hop Up?
« Reply #28 on: January 19, 2019, 09:28:15 PM »
Quote
The Fox 35 can still do what it has always done - serve modest size/weight stunt models with CONSISTENT performance for a long time. Not that the RC-based high-production stunt engines are Kleenex items, but you do have to spend a lot more to get one that will last for a few seasons of serious practice and competition.

Id thought the Brodak 40 was said to be a ' drop in ' Fox 35 replacement .



Is it a ' Modern Fox 35 ' or does it have differant run characteristics . Wot % mass increase .  :(

Online Dan McEntee

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6823
Re: Fox 35 Hop Up?
« Reply #29 on: January 19, 2019, 10:04:46 PM »
  I'll tell you something interesting about the Brodak .40. It has a muffler that is restrictive beyond belief. I have one mounted in a modefied Twister, my TwisterShark, and the engine has performed well. Later on I got a coupleof the Brodak .25s and tried one on a Ringmaster after breaking it in according to the factory instructions. The thing was absolutely gutless and wouldn't pull your hat off your head. I tried everything. The engine ran OK, needled fine and was well behaved, it just had NO power compared to a LA.25.
   I got a email from Tom Hampshire asking me ti send him the engine, as he had something to do with the development of it. I got it back from him some time later and he had modified the muffler into a straight through style, much less restrictive. The B-40 and B-25 share the same muffler, if you did not know that. I put it back on the Ringmaster and it was much improved, about the equal of the LA.25s I had been using. I thought that if the muffler improved the power of the .25, I wonder what it would do for the .40? So I tried it, and it turned the .40 into a hard breaking monster! The engine needled fine, and never ran away, but when using the same prop I had always used before, when I went into a maneuver,  it would break what felt like an extra 2000 RPM and just take off. I had a hell of a time keeping up with it in consecutive loops and such. When the model got back to level flight, it would just pure like a kitten. I put the stock muffler back on, and it was it's old self again. I should have put up a flight with no muffler to see what that was like, but didn't think of it. The Fox .35 will show you some performance changes with and with out a muffler, and with different styles of muffler, but in my opinion, not what I would call a drastic change. I like the Fox .35 just for what it is and get along with them OK. I have seen them fly 50 ounce airplanes when treated right and proped accordingly, such as Mike Gretz's Fierce Arrow that I have in my possession. Yeah, it won't be a NATS open class winner, but is certainly a viable power plant for the Joe Bellcranks of the world.
   Type at you later,
    Dan McEntee
AMA 28784
EAA  1038824
AMA 480405 (American Motorcyclist Association)

Offline Al Ferraro

  • 24 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 576
Re: Fox 35 Hop Up?
« Reply #30 on: January 20, 2019, 06:50:06 PM »
The engine needled fine, and never ran away, but when using the same prop I had always used before, when I went into a maneuver,  it would break what felt like an extra 2000 RPM and just take off. I had a hell of a time keeping up with it in consecutive loops and such. When the model got back to level flight, it would just pure like a kitten. I put the stock muffler back on, and it was it's old self again
   Type at you later,
    Dan McEntee
[/quote]
 The Fox 40 muffler runs great on Brodak .40 and looks 1000 times better especially on an upright engine. It sounds like you have one of the early Brodak .40 that has a smaller combustion chamber, I had the same problem. You can soften the break by adding a head shim or use a head from a new Brodak .40.
Al

Online Dan McEntee

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6823
Re: Fox 35 Hop Up?
« Reply #31 on: January 20, 2019, 07:23:09 PM »
  Hi Al;
    That's interesting. I had no idea that a stock Fox .35 muffler would fit on anything else. I got a bunch of them so will try that just for giggles and grinns when it warms up. Thanks for the tip.
   Dan McEntee
AMA 28784
EAA  1038824
AMA 480405 (American Motorcyclist Association)

Offline Al Ferraro

  • 24 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 576
Re: Fox 35 Hop Up?
« Reply #32 on: January 20, 2019, 07:27:42 PM »
The Fox. 40 fits, not the .35.
Al

Offline Steve Helmick

  • AMA Member and supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 9920
Re: Fox 35 Hop Up?
« Reply #33 on: January 23, 2019, 12:01:09 PM »
  Hi Al;
    That's interesting. I had no idea that a stock Fox .35 muffler would fit on anything else. I got a bunch of them so will try that just for giggles and grinns when it warms up. Thanks for the tip.
   Dan McEntee


K&B .40, OS .35/.40FP, .40/.46LA, Magnum/ASP .32/.36 will work with Fox .35 muffler. Bolt pattern isn't exactly the same, but close enough when using 4-40 screws instead of 3mm. The Fox mufflers are a bit odd, being available in up-sweep or down-sweep. That can be a good thing, but in general, it's likely the muffler you find at the swap meet is the opposite sweep from what you really want...one of Murphy's Laws. And of course, now that MECOA is the official Fox factory parts source, what they have is probably not what you really want.  LL~ Steve


 
"The United States has become a place where professional athletes and entertainers are mistaken for people of importance." - Robert Heinlein

In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.  In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

Offline Air Ministry .

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 4980
Re: Fox 35 Hop Up?
« Reply #34 on: January 24, 2019, 05:17:50 PM »
Theres these FOX 19/25/35 mufflers too , and Flow Thru ones , same case . NOTE : Theres Dual Pattern , Vertical only & horizontal only
Bolt Hole Ones , so BEWARE THERE ,


Offline Fredvon4

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2099
  • Central Texas
Re: Fox 35 Hop Up?
« Reply #35 on: January 25, 2019, 01:13:26 PM »
NECRO THREAD CANDIDATE ?

last post by Lou Crane....July 10, 2007----Jan 2019...12 years ----several contributors are long passed on to the circle in the sky...RIP

Yes I am grumpy today...deal with it
"A good scare teaches more than good advice"

Fred von Gortler IV

Offline Mark Mc

  • 24 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 718
Re: Fox 35 Hop Up?
« Reply #36 on: January 27, 2019, 10:42:22 PM »
Yes I am grumpy today...deal with it

What about the other 364???    n~

Offline Dennis Toth

  • 2020 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 4210
Re: Fox 35 Hop Up?
« Reply #37 on: March 04, 2019, 04:46:08 PM »
Peter,
There is some very good advice on getting the Fox 35 to perform in this thread. You did not indicate what type of ship you have it mounted in. I have been running the Fox 35 in my OTS El Diablo for the past four years and had pretty good results. The Fox works well with a smallish light ship say 48" span and about 34 -38 oz. If your ship is a profile the stick in the bypass is important. In an upright/inverted mount the stock Fox is OK. There are a few things that will make your life a little easier: stuffer backplate is good if you can find one; when mounting the engine on mounts with aluminum pads either have the engine mounts machined flat or make "crush" pads from 1/32" plywood that will fit under the mount lugs and form to the draft angle and prevent bending the case as you tighten down; use fuel with at least 25 -28% oil (50/50 syn/castor ok after break-in, all castor for break-in); use a Randy Smith PA Fox needle valve (it is like the ST NVA but has the center venturi section reduced to the same diameter as the stock Fox) or if you use the stock Fox NVA put a piece of short fuel tubing on the needle between the block and the index wheel to seal the needle; for the tank the Fox likes unpressurized uniflow tanks with the uniflow vent positioned in the free air stream on the circle inside side of the fuse about 3/16" off the side and pointing straight into the air stream (this is one of the most important tips to get the Fox to run consistent and steady); RSM 10x6 is a very good prop for the Fox 35.


Best,    DennisT

Online Larry Renger

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 3996
Re: Fox 35 Hop Up?
« Reply #38 on: March 04, 2019, 10:58:18 PM »
The setup sounds good, but with what line length and diameter?
Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

DesignMan
 BTW, Dracula Sucks!  A closed mouth gathers no feet!

Offline L0U CRANE

  • AMA Member
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1076
Re: Fox 35 Hop Up?
« Reply #39 on: March 05, 2019, 02:45:45 PM »
To Fredvon4, reply #36...

Still looking down to the grass...

One thing I didn't mention in my first long post a dozen years back: the mating surfaces.

If much else is decent, I try a slight touch of an abrasive compound between  1) the head and sleeve surface, 2) sleeve and top of casting and 3) sleeve and case. Tooling marks were often visible on the sleeve and head surfaces, and I've seen some cases or sleeves that seemed bent and/or out of true.

The sleeve should be a  "fairly smooth" fit in the case - not loose (rare) but not binding for part of a rotation in the case.
Gentle lapping with a fairly thin LAVA bar soap paste (about consistency of whole milk, color medium gray) on the outside of the sleeve usually corrected this. If actually bent,  not much can be done.

Bar soap paste, btw, also washes off completely...

The lapping of the sleeve flange to the crankcase top almost always showed areas where there was no contact. A slightly grittier LAVA paste usually worked to level the aluminum case to the flange underside.

The sleeve upper side was flat 'sanded' on a flat glass plate. Starting with 360 wet or dry and light oil, and care to keep pressure square to the plate, tooling marks soon vanished. When the upper surface looked smooth (no tool marks or missed areas) THEN I lapped the head to the sleeve, again with care to keep pressure even.

Once they "agreed" I further polished the iron sleeve surface to not quite a shiny chrome appearance. Ideally, these surfaces would not need gaskets to assure sealing of pressures.

The sleeves also often had sharp edged (?) burrs on the ports. from the probable broaching process which cut the ports. These can be felt from the inside of the sleeve. I touched them out with a small triangular Arkansas stone - careful NOT to mar the interior surface. Consider, those burrs were in close contact with the piston side surfaces, and may have actually cut against them... Of course, I may have done other cutting on the ports to set my preferred  timing  - also carefully de-burred...

Loosely fit the parts together and check how freely they turn through. The goal is to get the engine 'torqued in" so that it is just that free.

Assembling the engine at this point could provide a basically box-stock engine that will assemble true and free. Rushed factory assembly often left the box-stock 35s with distortions that can harm the beast. Get rid of them and many problems are avoided.

Head torquing procedure? "Star of David" sequence, small steps. ? OK, consider the front head bolt 12:00 o'clock. Run all six bolts in until they touch, then back them out 1/2 turn. Turn the the 12:00 o'clock bolt in 1/4 turn, then jump to the 4:00 o'clock bolt and do the same. Then the 8:00 o'clock bolt, THEN across the head to the 1:00 o'clock bolt and follow around in the same skipping manner. With a light touch, you can bring the head down squarely and evenly faster than it takes to describe it. Allen head bolts help, here. The position of the key helps keep the increments closely alike. 

After each six bolt sequence, check for freedom of motion - any bind or noticeable difference calls for backing the bolts out about 1/2 turn and sneaking back with small incremental angles.

I did my shaft mods, of course, and also "de-burred' the shaft intake hole, to include a small radius inside the sharp corners left by broaching the port. Not to the exterior of the shaft!

BTW, backplate fit was also lapped to minimize need for a gasket, (but use one anyway.) A full form gasket! Easy to cut out of thin gasket material by punching holes for the backplate bolts, bolting the material on, tracing around the opening with a sharp #11 X-Acto, then re- bolting that piece with the backplate on so you can trace around outside it with that #11 blade. Simple, quick and effective. It also supports the 'ears' on the backplate, which can bend when the bolts are tightened.

Starting the beak-in, the first run sloppy rich enough to almost require the battery to e kept on the plug. Plenty oil!! Run about one minute, or until the exterior of the case is almost hot to the touch. Several minutes cooling. Another two or three such runs, then check the head bolts - often they need a small increment of tightening. Use the same sequence and don't overdo it.

It may need another snugging down after 5 more such runs, but then should be nearly settled. Do check occasionally while still  on the bench. I prefer a bench break-in because you can shut down immediately if anything goes sour. After these first few minutes, go to the lighter load prop and try RPM nearer to what you expect for flight. If the load is light enough, such in-flight-like RPM will still sound like a fat 4-cycle!

Occasionally pinch the fuel line to get some 2-cycling time on it. It should drop back to 4- quickly when you release the pinch. If all is well, the engine should hold a steady - but NOT fully peaked out - 2-cycle steadily for extended time.
\BEST\LOU

Offline proparc

  • 2015
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2391
Re: Fox 35 Hop Up?
« Reply #40 on: March 06, 2019, 07:51:04 AM »

Randy sells a Super-duper Fox with all the above for $200. See    aeroproduct.net   - - and click "Engines"

I have a Randy Smith Super Duper Fox. The Fox I got when I was 12 years old I sent to him, and it came back all purdy like. I hang on to it because, if the word got out that Proparc doesn't own a Fox 35, I would be immediately banned from this forum, possibly banned from Pampa, and more than likely NEVER be allowed to fly stunt anywhere in the United States again.
Milton "Proparc" Graham

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1710
Re: Fox 35 Hop Up?
« Reply #41 on: March 21, 2019, 02:08:10 PM »
There is an article in Australian Control Line News issue 97 that shows how to fettle a Fox 35 without replacing  anything and most noted was grinding two lightening holes in the piston below the deflector and inline with the transfer port
This obviously lightens the piston and reduces vibration, and since the holes are not on the thrust face and mostly run in the empty space where port is it doesn't seem to affect wear. 
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Online Dan McEntee

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 6823
Re: Fox 35 Hop Up?
« Reply #42 on: March 21, 2019, 07:51:16 PM »
There is an article in Australian Control Line News issue 97 that shows how to fettle a Fox 35 without replacing  anything and most noted was grinding two lightening holes in the piston below the deflector and inline with the transfer port
This obviously lightens the piston and reduces vibration, and since the holes are not on the thrust face and mostly run in the empty space where port is it doesn't seem to affect wear.


   Hi Chris;
   If you can ever get a scan of that article and post it or email it to me I would be interested in reading it. I wonder how that would affect the infamous "Fox Burp" ??
    Type at you later,
  Dan McEntee
AMA 28784
EAA  1038824
AMA 480405 (American Motorcyclist Association)

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1710
Re: Fox 35 Hop Up?
« Reply #43 on: March 21, 2019, 08:10:07 PM »
Hopefully this attachment sticks!
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline BillP

  • 21 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 513
Re: Fox 35 Hop Up?
« Reply #44 on: March 21, 2019, 08:30:08 PM »
  Hi Al;
    That's interesting. I had no idea that a stock Fox .35 muffler would fit on anything else. I got a bunch of them so will try that just for giggles and grinns when it warms up. Thanks for the tip.
   Dan McEntee

The Fox 35 muff also fits the old plain bearing Fox 25 with bolt on venturi or rc carb. I'm flying that setup on a Flightstreak
Bill P.

Offline Peter in Fairfax, VA

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 1101
Re: Fox 35 Hop Up?
« Reply #45 on: March 21, 2019, 08:36:48 PM »
The pdf file of the newsletter with the Fox mods is quite legible.  Interesting mod, cutting holes in the piston skirt and the cylinder to both lighten and allow flow.

Offline Christopher Root

  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 129
Re: Fox 35 Hop Up?
« Reply #46 on: March 22, 2019, 09:20:32 AM »
If one blocks the bypass port with the piece of hardwood, is the stuffer back plate still necessary? Is there any harm in performing both mods?

C R

Offline Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13717
Re: Fox 35 Hop Up?
« Reply #47 on: March 22, 2019, 09:44:52 AM »
If one blocks the bypass port with the piece of hardwood, is the stuffer back plate still necessary? Is there any harm in performing both mods?

   The stuffer backplate is optional either way. The two things are independent of each other, and unrelated. The bypass modification prevents the "burp" and should probably be done regardless of the application. The purpose of the stuffer backplate is to keep the connecting rod pushed on to the crankpin, which reduces wear on the rod that would otherwise occur when it tilts. Some engines need this more than others, depending on how close to square the cylinder bore is with the crankshaft axis (and it apparently varies all over the place from case to case). This is not a big problem in stunt, but matters in the Fox racing events. Marvin Denny invented it.

 The stuffer backplate exists entirely for durability reasons, it isn't intended to change the performance of the engine (although it might, maybe better, possibly worse).

     Brett
« Last Edit: March 22, 2019, 10:18:30 AM by Brett Buck »

Offline curtis williams

  • 21 supporter
  • Trade Count: (0)
  • Lieutenant
  • *
  • Posts: 52
Re: Fox 35 Hop Up?
« Reply #48 on: March 22, 2019, 01:20:46 PM »
Another choice is to find a O.S. 35-s on Ebay.  I see new ones often.  They are about the same weight.

Online Lauri Malila

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1629
Re: Fox 35 Hop Up?
« Reply #49 on: March 22, 2019, 02:25:59 PM »
[quote author=Brett
 The stuffer backplate exists entirely for durability reasons, it isn't intended to change the performance of the engine (although it might, maybe better, possibly worse).

     Brett
[/quote]

I’m slowly ending up in a conclusion that we don’t want the smallest possible volume of bottom end.
It does make sense in racing classes like FAI combat, with quite restrictive silencer and intake regulations. I will, when I have time, try to figure out if the dominant behaviour of volume is related to resonance or flow dynamics. It’ll be quite easy to test by integrating an adjustable Helmholz type airbox in crankcase.
At the moment it’s clear that minimizing sharp turns and other sources of turbulence helps.
Also, better workmanship, material and fit never hurts in eliminating leaks and unwanted deformations. My backplates go in place with a very light interference fit. L

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1710
Re: Fox 35 Hop Up?
« Reply #50 on: March 22, 2019, 02:34:43 PM »
My understanding of the Fox burp is that the massive cast bypass port does not clear it's sinuses with each scavenge cycle.
One way to promote this is to reduce its volume and increase the charge speed by 'stuffing',  another way to increase the charge speed  and turbulence is outlined in the attachment to the ACLN article.
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Christopher Root

  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 129
Re: Fox 35 Hop Up?
« Reply #51 on: March 22, 2019, 03:01:10 PM »
Looking at the schematic for the transfer port block, I get the feeling that the sliver of wood does not cover the entire port (in a front to back dimension).  Is this true?  If so, why?

C R

Offline Chris Wilson

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 1710
Re: Fox 35 Hop Up?
« Reply #52 on: March 22, 2019, 03:58:15 PM »
The wooden 'stuffer' should be in intimate contact between the outer radius of the liner and the inner radius of the bypass port.
That of course leaves the left and right channels free to allow flow.
An unexplored option is to 'Vee' or wedge the wood - thick end at the top.
MAAA AUS 73427

You have enemies? Good. That means you've stood up for something, sometime in your life.
 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required

Offline Christopher Root

  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 129
Re: Fox 35 Hop Up?
« Reply #53 on: March 22, 2019, 04:07:34 PM »
Does the piece of hardwood block the entire port, or just part of it?

Offline Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13717
Re: Fox 35 Hop Up?
« Reply #54 on: March 22, 2019, 06:01:27 PM »
Does the piece of hardwood block the entire port, or just part of it?

  No, if you did that, it wouldn't run at all - no other way to get charge to the cylinder. Make it as wide as it shows in the drawing, all it does it take up space in the bypass, to increase the velocity of the flow.

    Brett

Offline Christopher Root

  • Trade Count: (0)
  • Commander
  • ****
  • Posts: 129
Re: Fox 35 Hop Up?
« Reply #55 on: March 23, 2019, 06:12:23 PM »
Saw the following in an old thread. Are more details available? Such as: to what diameter can we ream the venturi? Can sandblasting be sufficient to roughen up the inside of the case?   What size hole to drill the old port in the con rod?  How to true up the mounting lugs.   Still cold in N.E. and I have access to a machine shop . ...

C R

"All of the comments in this thread are interesting. Since Larry Foster made over 1800 L&J Fox .35 over the years he and I worked together on them I think I can outline fairly accurately what he did to these engines.

Crank case:
1) machine bottom of mounting lugs to keep case from twisting when bolted to engine mounts.
2) ream out venture to create larger volume due to increased diameter of ST spray bar
3) run a broach groove just inside the bronze bushing and attach to oil spillway to stop the fuel from blowing out the front end due to wear on the bushing. Creates suction pulling fuel back from front end and stops the normal "Fox .35 wet front end".
4) partially fill bypass port to reduce volume.
5) rough up inside of crank case to improve fuel air mixture.

Con Rod:
1) drill small hole in con rod end to properly lubricate rod connection to the wrist pin on top and bottom

Crank shaft:
1) balance shaft to reduce vibration
NOTE; both Larry and I agreed that the Hi Zoot was a better unit since it was heat treated after machining where the standard Fox crank became a little bit weaker after he balanced it. Only problem with the Hi Zoot was the diameter was more consistent that the bushing in the engine so fit could sometimes be a problem.

Piston/Liner:
1) never use a Fox re-run piston/liner assembly.
2) swap out pistons and liners to insure proper fit (this is where the over sized pistons came in)
3) Hand lap the pistons to the liner
NOTE: when the AAC and Ceramic P&L's came available use these instead of the stock P&L's.
4) Do not change the porting just clean up any rough edges with polishing.

Install ST.51 needle valve assembly.
Install Fox Stuffer back plate and use a custom laser cut gasket.
Install L&J designed and RSM manufactured hemi head
Replace soft fillister head bolts with Socket heads in both head and back plate. Properly torque the bolts to insure proper fit.

This is what was done on all of the L&J Fox .35 engines.

The final thing was to use only fuel with a minimum of 27% castor oil and 10% nitro. For higher elevations use 15% nitro. Larry and I found that if you used 5% nitro fuel you could not get sufficient lube and this resulted in burning out the con rod ends. Going to 10 or 15% nitro meant running more fuel for a flight but thereby getting sufficient lubrication into the engine to reduce wear.

I hope this data is of help to all interested parties. Although I no longer have a stock of the L&J hemi heads I certainly have the AutoCAD drawing and if there is sufficient interest in obtaining these I can have a new batch run up. Just be aware that in order to keep a reasonable price we will have to get a minimum of 500 of these made up.

Regards
Eric Rule"

Offline Al Ferraro

  • 24 supporter
  • Trade Count: (0)
  • Captain
  • *
  • Posts: 576
Re: Fox 35 Hop Up?
« Reply #56 on: April 08, 2019, 07:14:41 PM »

3) run a broach groove just inside the bronze bushing and attach to oil spillway to stop the fuel from blowing out the front end due to wear on the bushing. Creates suction pulling fuel back from front end and stops the normal "Fox .35 wet front end".
Regards
Eric Rule"
  Can anyone explain in detail and show a photo of the broach groove and oil spillway modification?
Thanks A


Advertise Here
Tags:
 


Advertise Here