Are you sure that there are no leaks on some oil seals (gear, sprocket..)?
I would try different oil brand, before everything else. Or if you are sticking to the same oil brand for long time, then do the compression test and maybe find some endoscope. Have you checked the spark plug? On the web you can find some useful pictures for reading spark plugs.
Last edited by cca; 23-10-16 at 22:01.
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