Normally I would agree with Merk about this but didn't you say the exhaust valve wasn't closing?
If that is the case the valve stem or guide could be full of crud, the stem could be warped, bent... or the tolerances could be too close and the guide may need to be reamed.
Check the key like Merk said, if it has even a slight mark on it, replace it. If that doesn't do it, I would look into a problem with the valve or guide.
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