I was digging through my box of Fox 35s this morning, and I noticed that one of them has a narrower exhaust port than usual.  I checked 5 other Fox 35s, and several loose liners in my junk parts, and all had a wider exhaust port.  The other Foxes I compared span from the "sandcast" era up to the 40th anniversary models.  The engine with the narrow exhaust is an early 70's example, matte gray case, undrilled muffler mount bosses above and below the exhaust stack.  Just to be sure, I pulled the head and looked, the cylinder is installed correctly, the higher port is lined up with the exhaust stack.  Up until a couple weeks ago, it was on a plane, and seemed to run fine aside from an incredible Fox burp on the outside maneuvers.  I'm wondering if someone at the Fox factory was having an off day, or if there's another reason this engine is different from the rest.

I assume you are talking about the port in the liner. I have one from about 66 or 67 that has the narrow exhaust in the liner. It's on my Bislob and still runs great.

Yep, the port in the liner is maybe 1/4" narrower than normal.  From looking at my other liners, the intake port in the liner is normally narrow, which is why I thought maybe it was 180* out, but it's just an intake sized port at the normal height for the exhaust.  The intake port opposite the narrow exhaust port, is the normal size.  Makes me wonder if someone goofed when cutting the liner, or if Fox made them with narrow ports for awhile intentionally.  Either way, it seems to run decent.  I'll probably just plug the bypass and put it back into service.

The burp on this one is exceptionally bad.  I'm sure part of it was tank height as it still burped when I changed to a different Fox 35 with the bypass plugged.  I'm down to two Fox 35 planes at the moment, and one is a BiSlob so I haven't had a chance to run it on anything else.

No sadness here.  I'm pretty sure I paid less than $20 for the engine on Ebay and it's nicely broke in with great compression and seems to run fine in spite of the narrow exhaust port.  Out of about 25 Fox 35s, only one has what appears to be a manufacturing goof and it still runs good.  The only trouble is I can't build planes as quickly as I can accumulate engines. 

See if you can get a peek at the exhaust port of a RO-Jett. Shockingly small exhaust port. I'd bet that Duke knew by the middle 1960's that a big exhaust port was not important. An undersized exhaust port on a Fox .35 might actually be a good thing, because the liners are so darned thin. If you find a cylinder with a smaller than intended intake port, you'll have a real weak running engine. I doubt Duke would have shipped that.

I suspect that the Fox .29 Stunt is a nicer (stunt) engine than the .35 version, because the liner is so much thicker. But it would probably need a bigger popsicle stick in the bypass to fix the flow problem causing the burp. Anybody flying a Fox .29 Stunt? It should burp worse than the .35, because bypass flow should be even slower.  Steve

Steve,

The case for the .29 is identical to the .35, the thicker cylinder is a result of less material being removed due to smaller displacement.

I'd imagine if it was prone to burping then the burp would be the same as the .35 counterparts

Ron

I had one.  It wanted to run fast(FF engine) and did not burp noticeably.  It's long gone, many moves ago.

Phil C