leshby, I agree with Joe - if it puffs mixture back through the intake without firing, then the camshaft timing is wrong, the valve is sticking, the valve is not seating properly, or there is negative tappet clearance on the intake valve. You've checked the camshaft timing carefully, so you need to check that the valve isn't sticking, and check the tappet clearance. As Joe said, if there isn't enough clearance, the way that is corrected on side-valve Briggs engines is by grinding away the end of the valve stem. This must be done accurately, both in terms of getting the end surface precisely square to the stem, and in ending up with the precise clearance you need. Remember, you have to lap the valve into its seat before you finish-grind the stem-to-tappet clearance, because the valve will move downward in its seat when it is lapped in. If the valve or seat has been eroded by leakage, it might take quite a bit of lapping, and therefore move downward quite a way.
Before you grind the valve stem, check carefully that nothing is holding the valve open. Apart from sticking, it may be obstructed by some piece of garbage between the stem and tappet, or the valve retainer may be damaged or out of position.
