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.