Rick,
I used Riches Brew for a winter and spring in my OS160 before changing to Cool Power. I have ran 2 OS160's with Cool Power 15% and they both run great (OS type F plug & 17X12, 18.1X10 props). Have you checked your exhaust system, engine-header interface, header-coupling, coupling-pipe? Over the past 3 years the only time ihave had to adjust the needle setting was because of a leak somewhere in the exhaust system.

Glen