TOYCANNON

My Feedback: (18)

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

Bax

My Feedback: (11)

Most likely, your engine's just not getting enough fuel. The muffler's probably not providing enough pressure to the tank. This is indicated by the fact that the engine runs very well with the stock muffler.

The other types of exhaust systems you tried may have been too restrictive, which is why you got the overheating tendency. You may have to use a less restrictive exhaust system and then start to block the exhaust exits to bring the pressure to the tank up to where the engine will run with the needle at 1-3/4 to 2 turns out.

We're sorry we don't have a certain answer for this.

adrian-RCU

lots of info on the 91fx - go to search and punch in 91fx read all about it. the best tip is bigger fuel tubing from the clunk to the carb - best solution ever now our fx burns the skies, upside down or whatever. best