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My guess is that all you will learn by pulling the valve cover off is whether or not you have a valve sticking or not. Which may be the problem but.... there are other things to eliminate before you start doing an overhaul. There is a possibility of a broken ring or two, or an excessively worn engine, that is allowing excess oil into the combustion chamber but that usually provides early indications long before you get to the condition you are presently in. Let me repeat, There is always some oily rersidue in the cylinder, that is how the piston gets lubricated. If the cylinder isn't firing, over time that will build up to where it could even foul the plug to where it won't fire at all. All your indicators point me to a carburetor issue. If you have a known good plug in the right cylinder, will it fire on the right cylinder with the choke partially closed? What happens if you switch plugs from cylinder to cylinder? If you remove the cast elbow to the inlet side of the carburetor and while the tractor is running on one cylinder, what happens if you partially or fully cover the right inlet on the carburetor? Is there vacuum being developed on cylinder #2 similar to #1? Is there blow back on one but not the other? Now to check to see if a valve might be sticking a compression test can be performed. Check both cylinders and compare readings. The absolute readings are not nearly as important right now as is how close to equal they are. Make sure this test is performed immediately following a session where the tractor has been running on one cylinder. Same temps, etc.
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