I have five flights on the 120 now. Here is my set-up: GP Venus II with O.S. 120. APC 16x8. Green Omega Cool Power 15%. Factory fuel tank in factory position. Engine is mounted 90 degrees with a Bisson Pitts style muffler. Here is the issue: The engine seems to run great on the ground and initially during flight. It will pull straight up after take-off. After several minutes (less than ten) the engine seems to lose power, because it doesn't seem to have as much vertical as it does on take-off. The difference between 50% throttle and 100% throttle is less, even though I don't fly at 100%. Is the Pitts muffler a problem? Is it not keeping adequate pressure in the tank?

It is cooling off in the evening here in Georgia, so I went two clicks rich on the high speed needle. That still didn't help.[:@]

If I would have known during construction what I know now, I would have mounted the engine so that the stock muffler exits out the area factory-cut in the fuse. It took alot of cutting to make the cowl fit over and around the Pitts muffler.