Jules Richardson wrote:
(does this count as "home", or is there a better place for it?)

I think today's the last chance for getting our elderly MTD lawn
tractor running before I throw in the towel and go buy a
(long-overdue) replacement tomorrow. It's refused to start after
being set aside for the cold season, and the grass has got to that
point where it needs the first cut of the year (as soon as this wet
spell ends sometime next week, anyway)

The engine's a 10HP B+S with electric start. Fresh gas, fresh oil.
Fuel line's clear, carb is free of debris. It's got compression,
spark, and is getting gas to the cylinder. I pulled the head and
checked the bore and valve seats, and no obvious problems there.

I've got the service manual, and the best it's managing at the moment
is with the needle valve 1-1/2 turns out and idle valve at 1 turn out
(as per manual for "this'll at least allow it to run so you can
adjust the carb"), where it'll *almost* run - it'll fire and turn
fast enough to disengage the starter pinion, then immediately die on
me.

Any ideas what else to look at? I wondered about timing, but it seems
unlikely that's out unless the cam gear jumped a tooth. I could pull
the engine and take the bottom cover off to take a look (and check the
governor mechanism*), but it's something of a last resort job after
I've checked everything else.

* I don't think it's that, as I've tried manually holding the throttle
wide open at the carb, overriding what the governor might be trying to
do, and it doesn't help.

Buy a NEW spark plug before you do anything else.

Jon