Bob, I am running an APC 15x6, OS F plug, and PowerMaster YS 20/20 fuel. The engine has the stock muffler and I am leaving the cowl on (with appropriate cut-outs, of course!). I am using the stock tank, mounted at the CG.

I had a *slight* problem when breaking-in the engine (which I did on the plane-- 20 minutes of running no more than 4K RPM). It would run fine and then start to surge. Turned "out" (richened) the regulator 1/4 turn and that fixed it! Needles are set at about what it recommends in the manual.

I hope that you get yours working. It really is a nice engine!