Gx390 .I have a gx390 on a mini late running on a small tight dirt track built a torque motor ported head 50lb blue stripe springs 280-411 nr racing torque cam stock piston timming set at 28 deg long 2 stage header motor turns 52 to 5300 rpm changed gears 2 teeth didn't seem to help much from 51 to 5300 even put lighter drive in car no change checked tach pulls good just cant get rpms up .would like to at least get to 5800 unloaded rear wheels and it will turn 64 easily???