This is an 18HP engine on a Craftsman mower, the one with the loose carb throttle (http://www.hobbytalk.com/bbs1/showthread.php?t=222972). I took the head off, cleaned up the carbon deposits, peened the worst of the scars down, cleaned everything up as best I could. Got new gaskets, put everything back together. Cleaned the carb out again, put some Loctite on the throttle screws and reassembled, cleaned it again, reinstalled carb. Charged the battery up, climbed aboard, took a deep breath and turned the key. Somewhat to my amazement, it started right up and ran like a top!

... for about 10 seconds, then began to gallop severely. Not the usual hunting and surging; it basically dies, then coughs and restarts (sometimes with a bang), runs up to full RPM, then dies again, repeat indefinitely until it dies completely. If I shut it off, it will restart again and run fine for a few seconds, then begin dying again.

Suggestions? I noticed the throttle control shaft seems quite loose in the carb body - is there maybe a bushing missing at the bottom of the shaft? Also the foam seal at the top of the throttle shaft is gone. Could those things be causing the problem? (he says hopefully, not wanting to mess with valves) Or could the ignition module be going bad? (he says doubtfully)

It does sound like an ignition issue to me. I would look for a wire that may be pinched in the cover or rubbing the flywheel causing intermittent spark.

Took the blower housing back off, didn't find any obvious pinched or worn wires, but the wire going into the coil looks as if it has maybe been tugged on a little. I shoulda taken a pic... Anyway I messed around checking to make sure all the connectors were properly seated, etc. and then gave it another go. This time it ran OK, in fact for several minutes it ran real well at WOT. Drove it around and mowed a little and it ran perfectly under load. But when I stopped and tried to idle, trouble returned... started missing, hunting badly, won't idle at all; and now it misses at WOT with no load, too. Still runs fine under load though.

Only way to get it to idle is to back out the idle mixture screw about another full turn or more - then it idles fast, and of course won't accelerate properly. If I set the screw back in where it should be, it won't idle, but it will pick up fine when the throttle is opened back up.

I still think there's something wrong with the carb. I've cleaned it so many times I'm sick of looking at the damn thing... but that loose throttle shaft bugs me. The bottom of the shaft, where it goes into the hole in the carb body, isn't even round, it's oval, so there's a lot of play in the throttle plate when installed. Maybe I need a new carb, or at least a new throttle shaft?