Can we just clarify one thing, please? Have you replaced the flywheel key? The symptoms are consistent with incorrect ignition timing, which depends on the flywheel key being intact. You said "shear pin", and I'm not sure what you meant by that. The Briggs flywheel key is made out of zinc, not steel - be sure you replace it with another zinc one, or the next time he hits a water pipe he'll turn the crankshaft into a pretzel.
If the ignition timing is correct (that is, the zinc key is intact) I agree with Joe - the next most likely cause is a tight exhaust tappet, sticking or burned exhaust valve, or a loose exhaust valve seat. However mixture problems should not be discounted.
