Fox burp?

[John Rist]

When did Fox make the change that created the Fox burp?  I have a Fox 35 I think I bought new around 1961.  Would it be pre burp?  Just added a muffler and I am planning to use it in a Nobler.

[phil c]

If you run it upright or inverted in a full fuselage there is no burp.  the Infamous burp only occurs when mounted on a profile model and particularly when you are trying for a classic 4-2 run.(Run a Fox 35 on profile, with a chicken hopper tank and run it in a two cycle.  It will rarely if ever burp.  It is very sensitive to tank height though. The tank needs to be ~ 1/8 in higher than the engine.  I don't know the exact dates of production, or the possible various batches. but apparently the depth of the bypass varied quite a bit.  Putting a stick of bass or spruce about 5/16in wide in the middle bypass(no distorting the liner!) generally does the trick.  Look up the previous threads.

[big ron]

Neat muffler bracket first one I've seen is the muffler the same as the ones made for engines with the muffler tabs

[Dan McEntee]

   There are those with more intimate knowledge of the engine, but as far as I know, while the exterior of the engine has changed over the years, the internal "numbers" haven't changed. The bore, stroke, timing and port sizes have remained the same. It is the size of the port from the crank case up the side of the engine that is supposed to be responsible for the burp when mounted sideways. If one burps, they all should burp. Bob Arata had one that was mounted upright in what was basically a Flite Streak with flaps and full fuselage, and it would sound like it completely shut off on outside squares! I don't think I have ever experienced it, but I was schooled to run higher pitched props than what was considered normal. Tank placement on a profile should put the pick up even with the center of the bypass port area as a starting place, and that has always worked for me.
  Type at you later,
   Dan McEntee

[Brett Buck]

When did Fox make the change that created the Fox burp?  I have a Fox 35 I think I bought new around 1961.  Would it be pre burp?  Just added a muffler and I am planning to use it in a Nobler.

  Virtually all of them do it to one degree or another. Whether the owners recognizes or is willing to admit there is a problem is entirely another matter. Even when it quits outright and causes an inverted crash.

   The one that do it least seem to have (due to the Brownian-motion quality control) the shallowest bypasses and the worst have the deepest. But it wasn't by design, it was piece-to-piece variation.

  If you put it in a Nobler, you are probably fine because the issue shows up only when the acceleration is perpendicular to the cylinder axis. But does the fix takes a few minutes and either doesn't hurt, or slightly increases, the power, so no reason not to do it.

   Brett

[EddyR]

Your Fox looks like a pre 1957 model.No bosses are showing between the fins on the case.

[John Rist]

Neat muffler bracket first one I've seen is the muffler the same as the ones made for engines with the muffler tabs

Yes I started with a stock Fox 35 tong muffler.  The top plate controls the vertical location.  The strap was made from licence plate aluminum.  I plan to get longer bolts and elliptical  lock nuts.  Don't want to over tighten and warp the case.

[John Rist]

Thanks for all of the good info.  I probably won't add the Popsicle stick unless I have trouble.  I had Fox burp destroy a Ringmaster but as you pointed out it was side mounted.  Mounted in a Nobler it should be good.   By the way, my first serious stunt ship was a Nobler.  I built it in 1961.  It had a warped wing (my bad) and was heavy as a brick.  Didn't fly worth a hoot.  Got destroyed over the years (mostly hanger rash) and chucked.  All I have left is the Fox 35.  Some one gave me a ARF Nobler (un-built with missing parts).  SO that is my current project - See how a properly built Nobler flies powered by my trusty Fox 35 from the original Nobler.

 

[Terry Caron]

Hi John -

According to Bill Mohrbacher, the burp was introduced in 1954, with the deeper bypass:
http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=323889&mesg_id=323889&listing_type=search

Terry

[Dennis Moritz]

Fox 35 is marginal power for an ARF Nobler. Too heavy. I've seen these set up for the classic 4-2 falling out of the sky. Be prepared to run the Fox on the hard side. If your Nobler isn't light.


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[Dennis Moritz]

Look at previous posts about stock Fox 35s. Philly Fliers have won and placed high in many many contests flying profiles with the Fox 35. Stock. The Fox 35 casting is on the thin side easily distorts. Stuffing sticks in the port can cause an out of round liner that binds. Cinching the head bolts - tricky to get that right. Also a cause of binding uneven wear poor compression.



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