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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.

valves and rockers

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.

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It looks like the valve guide has move out of the head. It is as you have described a sleeve that the valve stem slides in.

The valve guides are pressed into the head and should stay there. Other than it being a badly fitting guide, all I can think is that the cylinder head has overheated causing the valve guide to become loose.

Is the head aluminium? The aluminium would expand more than the steel guide when overheating which could allow the guide move.

Another possibility is that the valve stem has seized in the valve guide and then the piston has hit the valve head and pushed the guide out. If it has happened then the valve will likely be bent and the piston will be damaged.

There doesn’t appear to be much oil in the photo. Had you cleaned it?

You really need to take the head off and have a look at the valve and piston.

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  • The valve stem seems to move easily in the guide, once I tapped with a hammer to press the guide a little bit back into the head, creating enough space for the valve to move. I did not clean anything. There is a very light coating of oil over most of this space. It did occur to me that this valve cover is the one that drives the fuel pump, which failed. Any chance the leaky pump could have leaked fuel back down the vacuum port and into this space, screwing up the lubrication or causing this problem some other way?
    – Peter
    Oct 10, 2021 at 20:23
  • "take the head off": roger that
    – Peter
    Oct 10, 2021 at 20:25
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    No quick fix here. At least you're going to have to have the head rebuilt but it's quite possible that the damage is so severe that you'll need a new head. The valve guide should not come out like that and whatever caused it to happen needs to be determined and fixed.
    – jwh20
    Oct 10, 2021 at 20:47
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    A lack of oil lubrication, oil filter clogged, fan shroud was removed or obstruction in fan shroud preventing cooling airflow causing this engine to overheat until the cylinder head valves seized. Was this lawn tractor serviced regularly?
    – F Dryer
    Oct 11, 2021 at 1:49
  • Oil & filter changed this season and oil at correct level. Fan shroud obstruction is a possibility. There was a lot of grass clippings stuffed in places it shouldn't have been when I started the serious inspection. Fuel in the oil from flooded cylinders?
    – Peter
    Oct 11, 2021 at 2:23

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