firstly you need to be careful what you read and question what your read.
Black sooty plugs are a sign of rich running or a retarded spark in just about any internal combustion engine .
However mower engine have had fixed carb jets for over 20 years so the only way to have a rich mixture is a clogged air filter .
Changing the oil prevents wear it will not ever fix any problem other than not having enough oil .
Seafoam claims to "clean" fuel lines and a clean line will flow more fuel thus make things richer.
Unless your engine has an external spark timing module it also has fixed timing so that excludes a retarded spark provided that the flywheel is in its proper place .
The flywheel has a shear key in it to make sure the timing is correct .
So to check it you remove the blower cover then the nut / bolt that holds the flywheel on
The timing key sits in a pair of slots that must line up exactly to make a perfect square.
Get a good magnifier or take a photo of it and blow it up to check that the sides of the slots are strait lines where they meet
Just slightly off is a cracked key .
However the flywheel does not magically shear the key & move it requires an abrupt stop like hitting a tree root and stalling the engine .
We need the engine numbers so those with access to dealer service information can find the specification for your engine ,
For a while quality mower makers fitted engines with electronically controlled timing because they have significantly better power ( torque ) than fixed timing engines, perform better, accelerate better and use less fuel .
Unfortunately these units were not as reliable as the simple fixed timing and costed more to make so eventually they all got dumped because Joe Public has no idea how to evaluate a mower and in most cases cheaper wins even if it as low as $ 5 .
