My John Deere LA120 lawn tractor has a Briggs and Stratton 21HP OHV engine that quit suddenly. Bad gas was my first guess. Along the way, I discovered a leaky fuel pump. So far, I have now replaced: the fuel, the fuel pump, the fuel filter, the spark plugs, and I've disassembled and cleaned the carb.
I finally figured out it was running on one cylinder only. After removing the dead cylinder valve cover, it's obvious why: the exhaust valve push rod was bent badly enough to have disconnected from both sides. The exhaust valve itself is stuck CLOSED, which explains the bent push rod. On further inspection, there's what looks like a sleeve (is this the valve seal?) around the valve shaft, protruding out of the head far enough that the valve will not push in at all. Tapping on the valve with a hammer slightly pushes that sleeve back into the head, creating enough space that the valve now moves.
The intake valve seems to move normally. The intake push rod, however, also appears slightly bent, though still in place.
What could have caused the stuck valve? Any ideas for next steps? I don't think I can release the valve spring retainer, because I can't open the valve enough, without hammering the valve and its "sleeve" back into the head. I would be open to replacing the entire head assembly, but I don't want to do that if there's something wrong that's even deeper which would cause the same damage.
UPDATE: Removed head and everything looks fine inside: no visible damage. Ordered a new head assembly, installed (using torque and valve clearance specs), and everything runs great. Root cause: newbie mistake: air cooling fins and channels around this cylinder were stuffed with grass, leading to head overheating, leading to the valve seal coming loose and protruding, leading to stuck exhaust valve, leading to a bent pushrod and failure.