(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.

cheers

Jules