What year did the Fox .35S develop the burp?

[frank mccune]

     Hi All:

     I have two older Fox .35 Stunt engines.  One is a 1949 model and one is a 1951 model and both are like new.  If use these on profile airplanes, will I still be able to enjoy the Fox burp or were these engines devoid of the burp.  I am looking to avoid the burp issue!

                                                                                                                        Tia,

                                                                                                                        Frank

[Ron Cribbs]

Frank,

From what I have read and experienced 1954 is the magic year. I intentionally seek out 51-53 versions for that very reason. That and part interchangeability.

The bypasses are too large in all versions, just you have a better chance of avoiding the burp with the earlier ones. With Fox .35's it's hit and miss. You could get a later one that doesn't burp or one that does. It's a crap shoot.

Tank height, plumbing and  engine mounting all play a small role as well. Always use an RC long plug with an idle bar. I prefer the Thunderbolt.

Hope this helps.

Ron

[Phil Krankowski]

Inserting a plug in the bypass is an option too.
http://www.tulsacl.com/Engines.html

Phil

[Ron Cribbs]

Phil,

I intentionally left that out!

But, yes it's an option. It's just a last resort option for me. I have a 1954 Fox that I am really considering doing that mod to.

Ron

[Brett Buck]

    Hi All:

     I have two older Fox .35 Stunt engines.  One is a 1949 model and one is a 1951 model and both are like new.  If use these on profile airplanes, will I still be able to enjoy the Fox burp or were these engines devoid of the burp.  I am looking to avoid the burp issue!

    As far as I can tell, any of them with a die-cast case is a potential offender, but sand-cast versions are so rare that there's not a good database. The worst versions are those that wind up with larger bypasses, and the best are those with shallow bypasses, just as a tolerance stackup issue. I think it probably has something to do with how the raw casting is mounted when they machine it, sometimes it is nice and straight in the fixture and other times, the machined surfaces are sort of cocked off WRT the cast part. It also appears that they move it from fixture to fixture to machine the cylinder bore and the crank bore, since many times, they aren't exactly 90 degrees to each other. This is one of the items that the reworkers screen for, they measure it and reject the cases that are too crooked.

   The late GMA and I had a very long thread on the topic back when he was the moderator on the old RCO site where I took a bunch of measurements of the bypass and other things on various good or bad burpers. That was also the thread where he said it was entirely mythical and never happened, but then gave 2-3 different methods for "fixing it"!

    Don't worry about it, use the bypass stuffer - that solves the problem entirely. I wouldn't waste any time trying to screen for it or see how bad it goes, just put in the fix and then forget about it.

      Brett

[Phil Bare]

Put a 11 X 3 or 4 prop on it and run it in a good 2 cycle and be done with it.

[George Hostler]

One venerable CL flyer in another forum mentioned he replaced the Fox spray bar with 2 holes with another manufacturer (Enya, OS, etc.) single hole NVA, which solved burping and gave a smoother run. Should I dump the 2 hole spray bar and replace? Somewhere else I read that the Fox .35S ran better in an upright position than side winder. Should I target it for a plane that uses an upright engine position, like the DeBolt All American Senior?

[Phil Krankowski]

Fox did away with the two hole spraybar long ago.  The new ones are single hole, with much better fit on the threads.  A piece of fuel line over the threads to prevent air leaks is still a reasonable idea. 

Problem is Fox is not currently in the engine business, as of last fall, and I hear reports that they are not keeping common spare parts either.  Good luck finding a new Fox NVA.  (I guess calling any you find "NOS" or "new, old stock" would apply.)

[George Hostler]

Thanks, Phil, I'm going to replace that NVA with a single hole one. There are several alternates suggested to the Fox hard to get one, so I think I'm in good hands. Curious, but I may check and see if a Testors McCoy .19-.40 spray bar will work. Previous owner had some run consistency problems, not bad and he used the engine extensively. I'm planning on using it in my Sterling S-46 Hellcat profile build. Balance wise it shows an unmuffled Fox .35S in it, so should do a lot of good there.

[Brett Buck]

One venerable CL flyer in another forum mentioned he replaced the Fox spray bar with 2 holes with another manufacturer (Enya, OS, etc.) single hole NVA, which solved burping and gave a smoother run. Should I dump the 2 hole spray bar and replace? Somewhere else I read that the Fox .35S ran better in an upright position than side winder. Should I target it for a plane that uses an upright engine position, like the DeBolt All American Senior?

    I tried that back in the day (late 70's) by soldering up one hole on a conventional spraybar, but it didn't make any difference.

     You are absolutely correct that it runs much better upright or inverted. There's a very good reason it won all those contests, but they weren't won on profiles. I have often suggested saving it for full-fuselage models. Profiles are the problem, otherwise it's OK. A full-fuse airplane also handles the vibration much better.

    I couldn't recommend a "11-3 and crank it up" approach, unless you have a ready supply of crankshafts. A 25LA will still eat it alive, but won't self-destruct in 100 flights. Even an 11-5 is pushing it on a Fox, and that is a lot less stressful (since it can turn much more slowly).

    Brett

[Phil Bare]

Running the Fox .35 S with an 11 x 4 in a two cycle and comparing it to an OS .25 ran in the same manner with a 9 x 4 quickly shows that the Fox .35 S is more powerful. As for breaking cranks, I have been running Fox .35 S engines since the early 1950s and have yet to break a crank................ But then, it might happen tomorrow when I go flying....but I will have my S1 Ringmaster powered with an OS .25 LA along for backup. Maybe it won't peel the plating off the liner.

Phil

[Ron Cribbs]

 One thing I agree with Brett is that the NV has nothing to do with the burp. It's a Bypass volume issue.

The ST needle is far superior to the old fox one. I have a couple in mine. I also have the latest version Fox needle installed on a couple and they have 2 holes in the spray bar. It is completely redesigned and incorperates a large  knurled knob to adjust the needle. It also has an O-ring under the knob that completely seals the needle from outside air leaks.

It does have one drawback in that it can only be mounted one way. Apart from that it is a great design, but probably too little too late.

Ron

[Mark Mc]

Inserting a plug in the bypass is an option too.
http://www.tulsacl.com/Engines.html

Phil

Ahhh!  Now I understand.  I've been reading about the plug for a while, but just couldn't grasp it.  Now I see.  Not that I plan to do it, my Fox isn't burping on my Flite Streak.  But I did just score a nice .35 at the local hobby shop for $5.00, so who knows.

Mark

[Phil Krankowski]

Ahhh!  Now I understand.  I've been reading about the plug for a while, but just couldn't grasp it.  Now I see.  Not that I plan to do it, my Fox isn't burping on my Flite Streak.  But I did just score a nice .35 at the local hobby shop for $5.00, so who knows.

Mark

I needed pictures too on this one.  I also have not needed to use this as my Fox 35 is upright and partially faired in. 

Phil

[George Hostler]

but I will have my S1 Ringmaster powered with an OS .25 LA along for backup. Maybe it won't peel the plating off the liner.

My Testors McCoy .35 Red Heads have yet to peel a liner, let alone burp.  

[Ron Cribbs]

George,

Yep, they burp. Some not, some a lot. Thing is cutting out/burping usually does more damage to the airframe then the engine. The thing is you can have a Fox that is a dream and then another that is a royal pain. It's been happening for 60yrs. Fox engines last a long time and are very tolerant of bad fuel or a bad needle setting.

Red heads have their own quirks, they don't burp, but they don't always last. It's really the lesser of evils thing. One can cure a burp, but you can't get around bad metallurgy. You can always add castor to protect the metal, too much though will affect the run.

Many chose the Fox because it lasted and run problems could be solved, others chose the McCoy because it ran good out of the box and were cheap to replace when they wore quickly.

That's why there is loyalty to both. I think... Just my ramblings!

[Phil Bare]

My Testors McCoy .35 Red Heads have yet to peel a liner, let alone burp.  

Yeah George, that's the same experience that I have had with my McCoys also......................:-)

[Jim Kraft]

If the McCoy Red Heads would have had the metalurgy of the Fox's, they would have been the best of the best of the era where such engines were run. The numbers in the McCoys were near perfect, but, they built them to sell, not to last forever. Lets face it. In the 50's and 60's most of the flyers were young guys without money. The McCoy filled a need for an engine that would run great right out of the box if you happen to get a good one, and was cheap enough for anyone to buy. I have seen some that had no compression right out of the box, but many were great runners also.

They were also eye catchers with their red black and silver, and packaged in bubble packs for that reason. When I bought my first 35 the McCoys really looked great hanging on the rack, but I knew of the reputation of the Foxes and so went with the Fox.

 I still fly my first Fox 35 that I bought back in the 60's that is still running on all of the original parts. It has been on more planes than I can count. Fox Superfuel was $3.00 a gallon, and as long as I could get my kids to be a stooge I would fly as much as I could when not working. I am sure there are still castor soaked places in the city park I use to fly in. Matter of fact, I use to fly in two different city parks and on several ball diamonds. People loved to watch us fly and no one complained about our unmuffled Foxes. Fond memories for sure but those days are gone forever.

[George Hostler]

IMHO, McCoys are what they are. Back then, a lot of kids ran them at fastest 2-cycle some with lean runs. Testors offered their "39" fuel as the perfect choice, which it was not and ruined a lot of engines others included. It was short on oil and lacked sufficient Castor.

Now with better understanding we are able to get decent life out of these, adequate Castor and running in rich 4-cycle.

Regarding run inconsistency, my McCoy's had ACME or what ever make (older modelers would know) flex needle extension. This was a first attempt at getting the needle away from the prop before remote needle assemblies became popular. No matter how much I attempted to straighten the adapter's alignment, it wanted to rotate back to a set location. Of course I could have attempted to resolder and better align it.

Instead I went with the Evo .40/.46 RNVA, which with its fine thread and perfect fit, which solved my running problem.

Regarding the 1950's 3-bolt back Fox .35 Stunt that I received from you, Ron, that I haven't forgotten. It has considerably more vibration than the McCoy. Rather than fix the burp problem. I'm planning to put it in a full fuselage in upright position, something with more wing area than the Ringmaster S-1, like a DeBolt All American Senior. Upright in a full fuselage will eliminate the burp problem and with more wing area and its thicker wing will make it a much more pleasurable stunt engine.

My McCoy's are my stunt training engines. They will do me fine especially in these profile fuselage airplanes I got. And I believe they will have decent life on them, doing what they do best.
Who knows? May be one of these days I might win old time stunt with them. 

There is a lot of love / hate sentiments toward these McCoy engines. I just use them, not pay serious attention to how others feel and best of all have fun with them. 

[BillP]

Don't get your expectations up too much about an upright mount. I tried four different vintage Fox 35s (50s,60s,70s,80s) with and without hemi and stuffer in the upright config. All burped same as a profile side mount...virtually no difference. That was using both flat and tapered nv. All spray bars were 2 hole. 5% & 10% nitro/25% & 29% castor + various hot plugs. I don't think any of those have anything to do with it though. Like said already, stuff the bipass with a stick and be done with it.    

Also was with uniflo and standard vent tanks.

bp

[Phil Bare]

Wow Bill, I find that rather amazing. What mode were you attempting to run the Fox engines in?

Regards, Phil Bare

[Steve_Pollock]

George Aldrich wrote about the burp in his "Up and Around" column in the July 1994 Model Aviation ...

[George Hostler]

Perhaps then ditching the 2 whole spray bar and going to a single hole spray bar with enlarged hole to 0.055 inch dia. if not sufficient already is the trick?

George Aldrich mentioned 1968 base line. What about earlier Foxes?

[BillP]

Wow Bill, I find that rather amazing. What mode were you attempting to run the Fox engines in?

Regards, Phil Bare

In the last 25yrs or so...Stearman bipe (inverted), Super Ringmasters, T-Bird, AA Sr, TomTom. They all (TBird is the only survivor) flew fine and the only way I found to work around the burp is to do slow outsides.  I actually find it amazing that others say upright takes care of the burp because my personal results are totally opposite. Could be my setup but I don't see it. I get perfect runs otherwise.

bp

[Andrew Hathaway]

There isn't a year the Fox 35 developed a burp.  There's a year Fox changed the case casting, and the exterior of the bypass became flush with the cylinder fins, but how does that equate to an internal change in bypass volume?  How can anyone say that the internal volume with Fox manufacturing tolerances is any different, early case vs later case, or two earlier cases machined slightly differently?  Personally I've never seen, or experienced any significant difference between early and late Foxes, they all run about the same. 

The Aldrich snippet from Model Aviation seems a lot more applicable to a fouled up fuel system than having anything to do with the "burp".  He doesn't state that the problem was a burp, only that the engine couldn't be test run.  He also doesn't say that modifying the needle valve fixed a burp.  Within the established context of "pressure fuel systems for stunt", it seems the "burp" mention really doesn't apply at all to the topic.

The burp has nothing to do with the spray bar.  All the proof you need is that so many people have used different needles in Fox 35s for the last half century, that if it was the magic bullet fix, it would have been well established by now. 

This thread illustrates very well that some, but not all Fox 35s burp.  It's virtually random, and you won't know if you have a burp problem until the engine is in the plane you built for it.  Sometimes a Fox will burp on one model, but not on another model of the exact same design.  People have clearly gotten away with using Fox 35s on profiles for years, sometimes it works fine.  If you want to run a Fox, go ahead and try it.  Just be prepared to fiddle a little to get it running right.  If you can't handle taking the risk of building a plane around an engine that might be a bit finicky, there's always OS.

[Brett Buck]

Yeah George, that's the same experience that I have had with my McCoys also......................:-)

    After roughly 3-4000ish flight on ABN engines, I haven't ever had any liners peel, either. You didn't get 3000 flights out of a McCoy 35. Even Aldrich admitted it almost never happens on stunt engines.

    Brett

[Phil Bare]

OS engines are well known for liners peeling as well as main bearings pounding out.

Phil Bare

[BillP]

Wow Bill, I find that rather amazing. What mode were you attempting to run the Fox engines in?

Regards, Phil Bare

Sorry Phil, I just noticed you said mode and not model...it's fuzzy without my glasses!  Run is typical 4/2,  breaks going verticle, and 9/6 wood or 9.5/6 mas nylon. I do a lot of prop swapping (about 200 props in my stash) and 10/6 isn't as good to me on the Fox but is my go to prop for the McCoy 35RH. I seem to be the only one on the planet that doesn't fly 10/6 on a Fox 35.   

bp

[Brett Buck]

OS engines are well known for liners peeling as well as main bearings pounding out.

  Not in stunt. I have a single engine that I would venture a guess has more total flights than you have ever flown in your entire life.  The last time I ran that particular engine, it pulled the airplane to a 7th place finish at the nationals. It *was* getting worn out at that point, but the liner is not peeled and the bearings are not worn out.

    Brett

[Phil Bare]

Sorry Phil, I just noticed you said mode and not model...it's fuzzy without my glasses!  Run is typical 4/2,  breaks going verticle, and 9/6 wood or 9.5/6 mas nylon. I do a lot of prop swapping (about 200 props in my stash) and 10/6 isn't as good to me on the Fox but is my go to prop for the McCoy 35RH. I seem to be the only one on the planet that doesn't fly 10/6 on a Fox 35.   

bp

Hi BillP, The thing to remember is that load is the only thing that makes an engine go from a '4 cycle' to a '2 cycle'.  Prop load and weight of the airplane when the nose is pulled up.
When the airplane is launched with the engine running in a cackle it is running rich. We make a couple laps and the engine settles into a steady 4 cycle (centrifugal force makes it even richer). The reason that it runs in a '4 cycle' is because the fuel air mixture is too rich to ignite on the first revolution, the second revolution, part of the cylinder charge has been expelled and is now lean enough to ignite.
When the nose is pulled up the load on the engine is increased and the engine slows down giving the plug time to ignite the rich mixture.
As the plane starts down the back side of the loop, the engine unloads and goes right back to trying to run on a too rich mixture.
I think that a big gulp of the too rich mixture is what causes 'the burp' That is why a hot plug seems to help.
I learned years ago to run an 11 x 3 or an 11 x 4 in a strong 2 cycle and not be bothered with all the glitches and hiccups associated with the 4-2 business.

Regards, Phil Bare

[Phil Bare]

  Not in stunt. I have a single engine that I would venture a guess has more total flights than you have ever flown in your entire life.  The last time I ran that particular engine, it pulled the airplane to a 7th place finish at the nationals. It *was* getting worn out at that point, but the liner is not peeled and the bearings are not worn out.

    Brett

What ever you say, Brett................:-)

Regards, Phil Bare