You have an old OS .61FS ? Cool, I have one of them as well. Quite old, and not nearly as powerful as "modern" 4-strokes, but still great engines.

I run an APC 13x6 on mine. I've tried up to a 14x8, but that was a bit too much. I've also run as small as a 12x6, but I think that it was a bit too small for the engine.

Anyway, you want an OS "F" plug in that engine. If you're using a 2-stroke plug, it will do what you describe. This engine is a second generation 4-stroke, so it acts just a little different from the current stuff.

Btw, use a tach. Mine really doesn't want to run over 10k rpm, I usually set for about 9.5k with the APC 13x6, and get great runs. I suspect it gets in to valve float at about 11k rpm, so if you are taching around 10 on the ground, and the prop unloads in the air, you wind up with funky performance.

Oh, I run open stack, no muffler on mine. It's actually pretty quiet sounding, even with no muffler. (actually, it's loud on the ground, but the low pitch makes it not "feel" bad, and in the air, it seems real quiet).

I run Omega 10% or 15% fuel with no problems.