adding shims under the sleeve is ok but the not the best way. really need to see where the transfers are in relation to the exhaust port. measure the blow down. for that engine a simple clean up of the crank, widen exhaust some and put an angle on the exhaust port facing the transfer port. run a Jett 25 size gold pipe and should hit 20,000 with no problem. an 8x8 is a better speed prop for this engine though.