AirGar, I use 15% wildcat no shim, and so far so good . I love the Rossi engines. I have a Rossi 40 Q-500 on a Viper, 45 on a Patriot, and a 53 on a F-20 Tiger Shark. The 53 is a monster, but a little heavy. I use Thunderbolt (under $4)long idle-bar plugs. They seem to really like them, and I have not had a single one to go bad this past season.