1. Excessive carbon buildup. Carbon has gotten stuck on the valve seat or valve face.
2. Valve seat has is popped out. This happens on engines that never get their fins cleaned ( excess heat makes aluminum expand too much). This is most common to happen on the intake seat. If the seat is not damaged by the valve, it can be re-seated into the block, but I HIGHLY reccomend that you take the engine in to an authorized shop to have them do this work. They will also re-cut the seat and valve face.
3. Ring wear/cylinder wear. This happens to years of hard service. The clearences get too wide, compression is blown into the crankcase. If the cylinder is not too worn or out of round, Chrome Rings (Standard size only Briggs and Stratton engines only) can be installed to re-gain your compression instead of oversizing and fitting the piston with steel rings.
4. Rings/cylinder scored. This happens when carbon falls off of the head or the cylinder starves for oil. If the cylinder or rings are in really bad shape, they must be replaced/oversized.
5. Head gasket blown. This happens after years of use or happens when the engine runs too hot. The cause of this is the head bolts relaxing.