b3 spur will not fit a b4, slipper assembly is totally different. i used to use 20 tooth pinion with the standard kit spur on a 12 double with about 15 degrees advance on the motor for racing, if you are running in a small area you may need to go a bit smaller on the pinion, it depends a lot really on the motor, the one i was using was a real screamer, if yours is a bit less revvy you might need to go up a tooth or two on the pinion
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