I have the 91 of the same vintage, which I believe has the same carb. Had a hard time with it till someone recommended a head shim. At the time, Horizon still carried them. Turned it into an amazingly nice engine. I've run it on a 13x6 and a 14x6; it did well on both, but I did not tach it. Expect to fly it next year on a 1/5 scale T-M Scout. I'm using a JTec Pitts mufffler and it has a really nice growl.

Jim