how do you set valves on 17.5 tecumseh lawn mower?

I hope that the timing marks are aligned--crank gear to cam gear inside engine. I would say that the spark timing is fixed by the crank and flywheel aligning key. Check to see if the flywheel is properly aligned with the key slots in crank and flywheel. If you hit a stump or somethng that stops the blades instantly, the flywheel will still move under its momentum. This will advance the spark timing. Since it seems to have good compression, I would say the valves are closing all the way. Valve timing has no effect on spark timing. I don't know what else to suggest. Good luck

Robert Brown
August 2008