First off, no one asked the gentleman which Fox35. Was it a 40th anniversary, or 50th or 60th. Or older. He also said it burps when not flying level. If the bypass was an issue it would be burping on outsides. Period. Probably isn't a burp. Tank, vibration, piston/cylinder fit. Prop out of balance, dirt in the filter, dirt in the needle. A miss-aligned needle. Leaking coverplate. What fuel? What prop? Frustrating to hear experienced folks recommending mods to an engine when the universal issues of IC motors on model airplanes aren't looked at first. How does the engine run on a test stand? Will it run at an even rpm? Does the motor lock up when you go to a fast 2-stroke? Fox 35s vibrate, create harmonics, each one an entity unto itself. Different model airplanes, even of the same design, coming out of an same brand kit box, will vibrate differently. Is this a bad vibe issue? Are the engine bolts cinched up tight? What muffler? Any muffler? On and on.
If you are having an irregular run, look first to the fuel system. Dirt in needle, filter, bad fuel tank, pin hole in fuel tubing.
Is there anyone around where you fly who runs Fox35s so they work reliably? Or for that matter, is there anyone around who gets model airplane engines of any brand to run reliably on U-Control planes? Consult them first. Most bad run IC issues are generic.
Fox 35s vary greatly. Part of this is variations in machining tolerances. All kinds of tolerances can be off. Often engines run well anyway. Sometimes they don't. Issues like that are not forum solvable.
First thing I would do is take engine off plane and run on test stand. If engine will keep a steady needle setting you're at 1st base. If it doesn't keep a steady needle setting look for dirt somewhere in fuel system. Reverse flush needle by removing needle putting finger over fuel inlet blowing fuel from threaded side of needle, if fuel flows through outlet, you're needle is probably clean. Better to take needle out, reverse flush, then examine in good light. Check tubing for pin holes.
If the engine runs reliably on test stand without locking up when leaned out to a fast (not screaming) 2-stroke engine is probably ok. Mount solidly to plane. Check tank. Many hard tanks need to have their backs popped and insides looked at. If you don't know how to do this, do not know what the tubes placements are suppose to be, find someone who does. Check tank for leaks. Pressure test tank underwater by capping all openings except one. Use syringe to blow air into tank. Push plunger hard. Any tell tale bubbles. Seal any leak with solder. A well set up clunk tank can also work. These are setup slightly differently for UC then they are setup for RC. One thing, pick up clunk tubing needs to curve towards outside of circle. On profiles tank needs to be mounted to some kind of cushioning material, velcro works well. Tank needs to be mounted solidly. Tank height needs to be adjustable. And. And. See if your engine works now. If not. Check for excessive vibration. If it's bad vibes. That's a tough one. Solutions are various. Bad vibes can effect full fues planes as well as profiles. Not possible to diagnose and solve those issues at a distance.
Fox35s from 40th anniversary forward were set up with tapered cylinders, pinch at the top. When engine is at operating temperature, combustion chamber heat straightens out the taper. Better seal. If. Many of the 40ths especially were set up (in our experience) with too much taper. Too much pinch. Pistons often requiring lapping. Breaking in does not do it. Lapping needs to be done by someone who knows how. This might be an issue with your engine. If that's the case your engine will lock up when leaned out on a test stand.
If you are ever near Philly PA, send a PM. We enjoy trouble shooting Fox35s and sharing yuks.