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.

Some people, {{not me}} have had problems with the Pitts Muffler not producing enough pressure for the tank and they revert to plugging up one of the pipes. I have seen it said this cures the problem. When one of my .91s was surging I lowered the fuel tank and it cured the problem.
When I have a problem with any of my OS engines I go into the glow engine forum then to factory support and ask Bill Baxter what he thinks. He is my go to guy when I have an OS problem.

A couple years ago the good folks at Slimline sent me a pair of restrictors for one of my pitts type mufflers. They simply slide into the pipes of the muffler and have orifices that are about half the size of the original pipes. So by using these things you now have the equivalent of one pipe plugged. I don't know if they actually helped but they sure look cool.

An air dam in front of the exit? any resistriction to air flow once past the motor will be counter productive. The exit must be free.

Actually, an Air Dam produces outside negative or low pressure which draws more air out of the Cowl, does not restrict any airflow it improves it, not counter productive at all, read this thread, makes total sense on how effective air dams work on enclosed cowled engines

http://www.rcuniverse.com/forum/m_11...m.htm#11083204