I have a 91FX engine that has never ran exactly as it should. I can't seem to get it leaned out enough to hit peak rpm without it going too lean and stalling. If I set it rich it leans out and dies after about 2 minutes of running. It does this on the plane or the test stand. I ordered the suggested replacement needle assembly and this has helped some but not cured the problem. Is there supposed to be a baffle in the muffler. The engine just acts like it doesnt get enough fuel flow at full throttle. I am using the medium sized fuel tubing. Would a fuel pump help correct this problem? Does OS have any other suggestions or replacement parts to cure this issue? Thanks for any help.

If you cannot get enough fuel flow at full throttle, you'll need to see if you're getting enough pressure to the fuel tank. What exhaust system are you using? Have you tried larger fuel tubing? (Larger tubing will require ties to hold it to the tank and fuel nipples).

Trending lean almost always means a fuel delivery problem. Overheating could be fuel delivery, exhaust system, or cooling airflow.

Using the stock muffler that came with the engine and medium size silicone fuel tubing. I will try the large size and see what happens. Thanks for any help.