I have an OS 91 4 Stroke Inverted with a Slimline Compact Pitts Muffler in Hanger 9 60 Size Spitfire; the engine starts without a problem, low end runs clean with a smooth transition, but once in the air it starts to surge a little at mid to full throttle, what I mean by surging, the throttle goes up and down on its' own; throttle linkage and servo are solid, no free play.

Since then I have made some changes, I thought heat was not venting out properly or sufficiently, so I built an air dam in front of the exit vent to create some negative pressure to draw the heated cowl air out.

Also, I read somewhere, that the exhaust pressure nipple may not be in the best place to properly pressurize the fuel tank, not sure if this is the problem, the nipple was already in place when I received the muffler, I tried to upload a photo but it didn't work, basically, it is placed on the short header between the engine and the canister, also, would plugging one of the exhaust tubes help?

I will try uploading a picture later.

I have not ran the plane since I built the Air Dam, just thought someone may have the same problem and it was the exhaust pressure nipple placement, so while I have the cowl off, what better time to change the nipple placement.

Thanks.