Just to put the blow by to bed for good I'd pull the air filter and if it's dust dry there is no blow by or very little...Looking at the plug to me it could just out of tune or the mower/engine doesn't get a good workout and needs a load and worked hard..Also O-Ring's can and do fail they get flat spots and go hard not easy to see with the naked eye not here but in high pressure hydraulic systems they can and do just blow out...All engines resonate at a certain frequency that's why steel parts break/crack and fall off an engine that oil filler tube sure don't just sit there doing nothing it vibrates like crazy wearing out the O-rings... And those O-Rings have been working as a wearable part from the 1st day you fired up your mower...Oh yeah how does the engine actually run? out of curiosity what makes you pull the plug to see the burn has it not run right from new? Hope that helps its got me thinking to
