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maintaining your small engine and also a good indication
of how your engine is running.A spark plug that is
brownish in color and dry is an indication of an engine in
good condition.Aspark plug that is black and oily is an
indication of a worn oil ring on piston and will eventually
need overhaul,as well as replacing spark plug more
frequently.Aspark plug that is black and sooty or dry and
black, means the engine is getting to much fuel,exhaust
will be black smoke with this condition,float sticking open
in carb or running on choke will also cause this.
Most times I SET THE GAP of the spark plug to .040 with a
guage you can get at any auto parts store.I PREFER
CHAMPION for the engines I use,unless its japanese, then
I USE nkg,since its japanese,but whatever your preference
the above guide on coloration works on them all.
SAFETY SWITCHES on any of the equipment your small
engine is powering will shut it down pronto, if they go
bad.A RIDING MOWER USUALLY HAS THREE,one on the
clutch brake,one on blade engadgement,and one on the
seat.They will keep spark plug from firing even if engine
spins over,if you can spin the engine and the engine will
not start, take spark plug out and leave it in plug wire,
while someone spins engine and watch to see if spark is
jumping across electrode on spark plug,make sure plug is
touching metal of engine,if not firing, then its safety switch
or bad plug.You can read more on safety switches in the
troubleshooting guide at the link 'small engine guide'.