Yeah, kinda seems like you might be losing spark as the engine heats up…But, does the engine continue to run fine when hot? Kinda sounds like the engine runs fine until you shut it off, which makes a spark issue a little less likely. So, like Bill says, it is best to check spark when it is hot and won’t restart…Could be something as simple as a bad ground, or funky stop switch. But, you really need to confirm whether or not this is a spark issue before digging into the ignition system.
What happens if you shut the engine down, then restart it immediately, does it start up OK then? Keep, in mind that you may need to use the choke when restarting a hot engine that has sat for a bit…Seems counter intuitive, but today’s fuels tend to vaporize easily, especially in hot climates. Are you trying to restart at idle speed, or are you turning the throttle up to “start”? Keep in mind that the neutral lock out RPM is adjustable on those older models. There is a stop held in place on the shift detent that can be moved forward/back a bit to raise/lower max RPM in neutral, perhaps you need to advance the throttle more when trying to restart hot…

There are other possibilities also, suppose the carb could be flooding over…But, it wouldn’t run/idle very well if that was the case… One other thing to consider, how is the recoil rope, is it too short? Is the recoil engaging OK? That recoil is geared down quite a bit, so it takes a good yank to get the flywheel rotating fast enough…If the rope is too short, you won’t get the flywheel rotating very well, even though it seems like you have pulled the rope out far enough… Don’t get me wrong, I am not advising you to yank the rope out real far, and please don’t let go of it letting it snap back, or the spring will probably be damaged.

Keep in mind that the neutral throttle limiting “lock out” is adjustable on those early models…There is an adjustable throttle stop on the shift linkage, it is slotted and can be moved by loosening two small screws….
Funny this topic appears now…I noticed this same symptom on the 81 9.9hp that I just revived while at the AOMCI meet this weekend… The engine would start right up hot after sitting for a few minutes with no choke…After I let it sit for more than a half hour, I usually choked it, would start but immediately stall…Took a few pulls with no choke to get it going….I should have tried it with no choke first I guess…

jerryswiss wrote:

Ok, this is what I have learned. I got it to run in a tank and can start it consistently when warm. It has spark when warm, and will start, but has a somewhat inconsistent idle and is somewhat hard to start. If you pump the primer bulb on the fuel line it makes it start easier and idle smoother. Carb is clean, fuel pump rebuilt, tank vent is ok. I think there is an air leak somewhere.

What do you think? Where would the common place for an air leak be?

Well, I don’t really think there is an air leak…Especially if the engine’s running/idling quality is good….Did you advance the neutral lock out stop link? I don’t think I really understand what you are describing.
You say you can start it consistently when warm….But has inconsistent idle, then you say it is somewhat hard to start….
Are you saying it is hard to start when hot, or that it runs poorly after being restarted when hot?
Did you advance the neutral lock out?
Perhaps you can post a video of the issue you are having.