From my understanding, and the pictures you've shown, it appears you have the wrong size spark plug - it's much longer (more thread) than the spark plug referenced in manual. Better double check.naimc wrote: ↑ FYI, that old spark plug indicates something wrong / off with your engine. It should not look like that after only one season. Looks like it's heavily coated in carbon and oil , carb is too rich or oil is getting in the combustion chamber. ( I would pull out the new one you put after if it got a few hours of runtime and look at it again. and check your oil level more often perhaps. )
For reference i just pulled the plug on my 721RC it must at least 6 years , notice that its dry and not covered in a wet oil gum.
vs your plug
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