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.