"Doug Kanter" wrote in
:

I've momentarily forgotten what substance you add to a gas
tank that results in enormous damage, and a similar repair
bill. Does anyone else remember?

sugar.
or a ping pong ball... that works in cars. causes intermittant
stalling. you'd need something a bit smaller for a mower.

draining the oil from the mower does wonders for the engine
also.
lee