I have mounted a O.S. 91fx engine on a GP Extra 300s .60. Due to the cowling I am using a Pitts style muffler. The only way I can get to run strong is at approx. (5) turns on the needle valve. I first tried the Slimline brand and then the B.C.M. version with smaller chamber and exits, and its very lean and runs hot. Consequently on my engine stand with the stock muffler it runs perfect all the way through the rpm range. I have never had problems with the smaller fx engines with pitts stlye mufflers. Thanks in advance for any feedback as I am a believer in the O.S.
Doug