I HAVE A 0S 91 FX WITH A BISSON MUFFLER THE PROBLEM IS THAT FOR THE FIRST PART OF THE FLIGHT IT RUNS STONG AND GREAT, UPLINES ARE GREAT. BUT AROUND 2-3 MINUTES INTO THE FLIGHT IT STARTS TO BOG DOWN AND RUN HOT/LEAN. USING MED FUEL TUBING AND FUEL FILTER INLINE. TANK IS WITHIN 1/8 INCH OF BEING LEVAL WITH CARB. ALSO USING A GREAT PLANES EASY FUELER FOR FUELING. MY PLANE IS COWLED BUT ANOTHER FLYER AT THE FEILD IS HAVING THE SAME PROBLEM WITH A UNCOWLED ENGINE WE ARE. I'M RUNNING A 16-4 APC NOT SURE WHAT PROP HE IS USING. WOULD IT HELP IF I CLOSED OFF ONE OF THE EXHUST EXIT ON THE MUFFLER? IT WOULD BE CLOSER TO THE SAME SIZE OF EXIT HOLE ON THE STOCK MUFFLER. USING 15% NITRO AND 20% OIL WITH SOME CASTOR 4-5%IN THERE. HE TOOK A READING ON HIS ENGINE TEMP UPON LANDING AND IT WAS OVER 350 DEGREES.

Your engine is going lean and overheating. That's what would make it quit a few minutes into the flight. The going lean can be caused by many things. First off, though, get rid of the Easy Fueler. It's making a restriction, and could also be leaking air. You'll also need to check for any cracks and holes or other air leaks in the fuel tubing. Vibration can cause air to get agitated into the fuel, which also leans the engine.

Your Bisson muffler may not be giving the fuel tank enough pressure, so you are getting leaning due to low fuel pressure. Restrict the exhaust outlets to see if you get some improvement, or try the stock muffler to see if that helps. Many after-market mufflers just don't generate enough pressure to enable the fuel to move fairly well.

You may also find that the engine will run better and cooler if you go with a smaller prop, such as a 13 x 8 or so. Run the engine close to 12,000 RPM when leaned to just rich of peak RPM, and you'll find it has better performance, though louder.

If I may make a suggestion. Try flying with the cowl off. If the problem goes away, you have a cooling problem. I've seen it time and again.
Paul