Plugging boost port on fox .35

[Matt Curtis]

Is there info on this site on how to do this? Would like to find a full size pattern to copy so I could make the part the correct size and make it out of hardwood like oak or maple?

[Dan McEntee]

Is there info on this site on how to do this? Would like to find a full size pattern to copy so I could make the part the correct size and make it out of hardwood like oak or maple?

  There are no boost ports on a Fox .35 Stunt, only a main bypass port. It is too large, even compared to something like an ST.51!! Go to the engine section of the forum and search "stuffing a Fox .35 bypass port" or something similar. There has been much posted about it. I think the Tulsa Gluedobbers club has something posted on their web site also. The last time I looked at that I kind of questioned the dimensions of the wood block. Something like 14" to 5/16" wide will do and about 5/32" thick. It needs to be sanded to fit well in the case, and then concaved so the liner drops in easily like it would if it wasn't there. The cylinder and case are very flimsy so you can't force fit anything or it will distort things and never work. The cylinder should hold it in place but I generally wipe on a ;layer of JB-Weld to help position it all the way forward. it's not difficult but is tedious and must be done correctly.
  Type at you later,
  Dan McEntee

[Perry Rose]

A popsicle stick is the thing to use. Once cut to length, trial and error, the bottom end must be tapered so the crankshaft weight clears.

[Dennis Toth]

Matt,
There are a couple ways to do this. One is the stick in the bypass (see attachment for details) the other is to use JB Weld and fill the bypass about 1/3 full. I have used both and it works fine (not that critical).

If you use the JBW make sure you clean the bypass with degreaser and a little 400 sandpaper to give it better grip.

Best,    DennisT

[john e. holliday]

Are you really having trouble with the so called Fox burp?  In my experience through the years the burp wash caused by the wrong fuel formula and plug.

[Colin McRae]

I have a Fox 35 stunt engine on a Ringmaster (profile model). In an attempt to quiet down the burp, I tried a couple of mods on one of my engines as a test. I installed a Fox angle plug hemi-head from Lee Machine, and also a stuffer backplate. I did not do anything to the ports within the engine.

I must say these 2 mods did have a positive effect on the burp. Although the burp is still noticed, it is ever so slight when going inverted and almost eliminated.

I have been running 5% nitro and 29% all-castor fuel. And I have been operating the engine such that on the ground it is a wet 2-cycle so a very small break in the air. I may be wrong, but I have heard that the burp is more pronounced if the engine is run richer to obtain a solid 4-2-4. Also, an engine mounted profile contributes to the burp, but the burp is not as severe if the engine is mounted cylinder up.

[Brett Buck]

Are you really having trouble with the so called Fox burp?  In my experience through the years the burp wash caused by the wrong fuel formula and plug.

       This is a serious/disastrous problem with a straightforward fix, known for 30 years, why are we still debating it?

     I note that using the "wrong fuel formula and plug* can cause other issues but not this one, and time this "fix" works it is a coincidence.

     Brett

[Dave_Trible]

I use Foxes quite bit in my Classic airplanes (mounted vertically ) and never have a problem.   However my flying partner was having a fit with it yesterday on his Banshee.   It has not really been any issue before but the weather got hot yesterday and all the engines were running lackluster.  It finally totally quit at the intersection of the horizontal eight turning outside.  A broken prop was the only damage do mostly to the soft sandy loam of the flying field.  It wanted to do this at the same spot in the pattern for the whole session.  He will be investing in the new head from Lee Machine.

Dave

[frank williams]

Matt .... plug the bypass  ......Dennis's pictures are excellent ..... it takes a little effort but it works for a profile Fox ..no power loss at all

[doug coursey]

Flew the fox 35 in stunt in the late 50's and 60's and never had the burp on upright and inverted engines...dont know if it was the fuel mix..we mixed our own fuel for all our different classes of events

[Dave_Trible]

Flew the fox 35 in stunt in the late 50's and 60's and never had the burp on upright and inverted engines...dont know if it was the fuel mix..we mixed our own fuel for all our different classes of events

The problem persists when the engine is mounted horizontally, ie., on a profile airplane.  In certain spots an over-large fuel charge gets thrown into the cylinder and drowns the plug apparently.  That flow pattern doesn't occur usually when the engine is mounted in another position.  Mounting the engine inboard rather than outboard may also solve the problem on a profile but add other complications.  An idle bar glow plug may help some.....otherwise the stick or the new head.

Dave

[Joseph Lijoi]

       This is a serious/disastrous problem with a straightforward fix, known for 30 years, why are we still debating it?

     I note that using the "wrong fuel formula and plug* can cause other issues but not this one, and time this "fix" works it is a coincidence.

     Brett

It is a serious/disastrous problem. Note that some are citing upright/inverted installations. The problem occurs on profiles. I gave up on Fox 35's after trying hemi heads, stuffer backplates and whatever. Next was going to be an inboard mount. Trying to get a Fox 35 to run on a profile is a hobby in itself and if you enjoy that kind of thing then go for it. The reality is that compared to modern engines the Fox is a real dog. 

[Rusty]

I have an Ohlson and Rice I will sell you if you want a better engine. 

[Motorman]

Do I remember some pictures of what happens when the stick pops out?

[Brett Buck]

I have an Ohlson and Rice I will sell you if you want a better engine.

   It's a bad engine *for a profile*, even if you do the fix to keep it running, because of the vibration.

    On a full-fuse airplane, it has some flaws, it does not give currently-competitive power, but it *was no accident that it won all those contests*.

   As Frank notes, just do the fix as described, it doesn't hurt anything as far as power *for stunt* goes, or slightly improves it. Properly done, the "stick" does not and cannot "pop out".

     Brett

[Colin McRae]

There is another option no one ever talks about.

For those that like to run vintage engines, switch to a McCoy 35!

Popsicle stick placed in the engine not necessary! 

[Brett Buck]

There is another option no one ever talks about.

For those that like to run vintage engines, switch to a McCoy 35!

Popsicle stick placed in the engine not necessary! 

    It's much more likely to work without knowing formula. But it is nowhere near as durable or consistent as a Fox. For sport flying, I have suggested the McCoy as an alternative many times, but I think it would drive you nuts if you tried to compete with it. I don't think we are any smarter than they were at the time, the obvious Fox alternatives at the time were the Forster 35 and a variety of Johnson 32/33/35s.

    I again note that the era we are talking about went from about 1952 to about 1960, when Lew McFarland showed what you could do with a Veco 45 on a larger model.

     Brett
   

[Matt Curtis]

Thank you for all of the replies. I  think I will use the fox .35's that I have for the fox .35 speed event and use o.s. engines like LA or fp .25 instead. I just recently did use a fox .35 on a vampire that I just bought but that is a small airplane that I think will fly better with the constant speed from the .25 engine. Not much wing area on a vampire. No mods needed on the fox if I want to fly the speed event unless I want to but I do not know how to mod it for that with a  lathe as some have done to get up close to 100mph. I also have a fancy pants and ringmasters that would be better served with other power plants

[frank williams]

Joseph .... interesting you mentioned the inboard mount for a Fox 35 ..... I did that .... the interesting thing is that the 'burp' now occurs on the hard inside maneuver.

I don't think that the phenomenon is a hard influx of richening gasses.  I think its the opposite, at the time of the acceleration, the bypass gases are pushed back into the bypass, resulting in a momentary starved condition.  (but I may be wrong)

Frank

with the overly large bypass, the gas velocity isn't strong enough the overcome the acceleration
the reduced bypass area helps create a higher velocity to help the gases get into the combustion chamber

[Brett Buck]

Joseph .... interesting you mentioned the inboard mount for a Fox 35 ..... I did that .... the interesting thing is that the 'burp' now occurs on the hard inside maneuver.

I don't think that the phenomenon is a hard influx of richening gasses.  I think its the opposite, at the time of the acceleration, the bypass gases are pushed back into the bypass, resulting in a momentary starved condition.  (but I may be wrong)

   The idea that the plug was somehow getting fuel blurped on it and "going out" was the mistake we all made trying to diagnose it. That was the basis of the "plug and oil" theories, and why they always failed.   When I read your article, it was one of those "forehead slap" moments, so obviously right that I knew it was going to work even before I tried it.

     Brett

[Lauri Malila]

What is interesting that the problem doesn't depend much if the engine has a side- or rear exhaust, it kind of confirms me that the reason is in the bottom end of crankcase, most probably an oil accumulation on either side of the backplate bore.
Franks comment about reversed problem with inboard mounted engine is very interesting, I think it falls into same category than my experience with reversed burp when the engines running direction is reversed. Reverse cranks for Fox anyone..?
Basically, you have a slug of oil in the crankcase, probably held in place by the corner at cylinder bore/backplate bore intersection, and it stays there under static conditions. In violent positive or negative G, the slug is removed from its place and it richens up the mixture for a moment. That's why lower viscosity oil, leaner setting or higher operating speed helps but never fully cures the issue.
The popsicle sticks does 2 things; increases flow speed, which is good for mixing and scavenging, and it also separates the bypass & port to 2 narrower channels that aim to both sides of glow plug. So, the plug is somehow shielded from the most oil-rich mixture.
Probably the oil slug takes place on the left side of crankcase (seen from front). That would explain why I got completely rid of the problem by tilting the case so that cylinder points 40...50 degrees down from horisontal. L

[Joseph Lijoi]

Joseph .... interesting you mentioned the inboard mount for a Fox 35 ..... I did that .... the interesting thing is that the 'burp' now occurs on the hard inside maneuver.

I don't think that the phenomenon is a hard influx of richening gasses.  I think its the opposite, at the time of the acceleration, the bypass gases are pushed back into the bypass, resulting in a momentary starved condition.  (but I may be wrong)

Frank

with the overly large bypass, the gas velocity isn't strong enough the overcome the acceleration
the reduced bypass area helps create a higher velocity to help the gases get into the combustion chamber

I am no expert on the subject but I think that you are right and it is an issue of gas velocity or more like gas velocity at a given RPM. A tuned pipe might be giving you a form of adjustable gas velocity. There is probably an ideal formula for RPM/gas velocity that creates a stable run with power at a given RPM. Art Adamissin used to block off a port on the OS FPS to increase gas velocity but I think he did this to get a stable run at lower RPMs than the engine was designed for.