I have a lawnmower with a 3 1/2 hp B&S motor.. will only start with a prime, new diaphram installed.. will run a bit then backfires a few times & quits.. could the shear pin be misaligned a bit causing this?? or??

Yes, it could be a partially sheared flywheel key. A bad spark plug will cause this as well as a valve problem.

Great, thanks for your reply.. I'll check them out starting with the plug..