If it backfires while you are driving, the intake manifold bolts may be loose or the gasket may need to be replaced. If it backfires when you turn off the ignition, the fuel mixture is a little on the rich side.
Its either smackdown, rideon, or keepitsteady
I think a electric lawnmower would be cheaper because current usage goes off the amount of amps and if you have a lawnmower that has a low voltage then it would cost you less.
to put the oil in the lawnmower's crankcase would be fine, so long as it is an appropriate viscosity.
I'm fairly sure you cannot put a light bulb in a lawnmower. You could lay it on the deck somewhere but it would never light up but your question was in a lawnmower not on a lawnmower,... on so the answer would be that this is not a valid question.
Having no catalytic converter installed is illegal and can cause a myriad of problems. Probably not a backfire though.
it's lean
You don't need a lawnmower. There is no law that requires you to need a lawnmower, but, if you have a lawn and want to keep it tidy you would be advised to use a lawnmower as a scythe requires much skill and a lot of energy.
It has moisture (water) in it.
timing or a plug not sparking
backfire timing off
from unburned fuel in the cylinder
A sparkplug misfiring.