l07smitty

I just bought a Picco .28 team and it says to shim the head for 30% nitro. The LHS did not mention any shimming when he sold me the 30% r2r O'don. Do I really need to do this and what would happen if I don't. I plan to break it in with oily Traxxas 20% fuel then switch over. Should I exchange for 20%, .............I really don't need the added power or want to add the shim but if it runs that much better, than I'll stick with the 30% Thanks for any replies!!