I would say break it in with all four shims for about a gallon of fuel then take one or two shims out. We (the guys running it now) have come to the conclusion that you can safely take out one shim. Easyryder took out 2 I believe. Im runnning at .5mm head spacing (2 stock shims and another thin one from another engine) with 25% Byrons Gen 2 with no problems in my Clockwork modded JL.