It sounds like you are getting oil in the spark plugs that is what causing them to foul and to black due to burnt oil residue.
This happens when oil enters the combustion chamber, typically due to worn piston rings, too much clearing between the valve guides and stems, or worn bearings.
In order to fix the spark plug oil fouling, you will have to fix the oil leak issue.
You can start by checking the engine compression.
Worn piston rings can cause the cylinder(s) to have low compression.
This engine should have 135 to 185 PSI of compression with the difference between the highest and lowest cylinder being not more than 28 PSI.
If you have low compression, you can do a oil test to see if it is due to worn piston rings. Pour a tablespoon of oil into the spark plug hole. Then check the compression if the compression rises then the piston rings are at fault.
Weather the piston or the valve guide leaking, this will require engine disassembly.
Please let me know of any questions.
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Friday, October 15th, 2021 AT 7:42 PM