Talked with BoatDr the other day on this and have a couple things he told me to try to narrow this issue down. Will hopefully have a change this weekend to drop in the water and work on this.

Was hoping somebody might have had the same problem and could offer up their solution. Motor is a 35hp briggs. Only mods are Mikuni single carb, MM muffler and IM heads. Motor cranks fine at ramp and runs fine at WOT. When I run the motor for long enough to get it good and hot, then shut it down, it won't crank back up. If I only run a few minutes and don't get it really hot, it will crank fine. Get it good and hot, shut down and it will not crank. Turns over just fine but won't fire off. I can let it cool back down, hit it in carport at home 2 hours later and she'll fire right up.

Anybody had this issue before?

thanks!

Does it become flooded? Will it never fire despite you giving slight throttle / choke etc.

Have you checked your valves?

According to the Lawnmower websites, you might have a coil that malfunctions when hot.

If you seach "Briggs engine won't start when hot", you will see that a few people have had this problem with their small engine.

But this also might be one of those, "One Problem, but a dozen possible solutions". Those are always fun to figure out.

Just bring a rope, I'll pull you back to the ramp.

West, once hot it won't fire no matter what I do. Can choke it, feather throttle- none of it works. Have tried unplugging oil sensor as well. Thought it may be flooding. Used to have a lot of gas out of overflow. On the mikuni. I adjusted float valve and stopped that. Still have the same cranking problems when hot.

BBB, I was told chances are one in a million that both coils would be going bad at same time. Maybe i'm that lucky though and will win half a billion dollars tonight!

When it's hot pull the plugs and see if you got fire. Easy and cheap way to check it.

On my 16hp Vanguard, I lost a coil the other day, and it was hell to start. Last year I was working on a Kawasaki FH541V; a 17hp vertical twin, mounted on a mower, and he had a coil that was intermittantly cutting out. It was recoil only, and it wouldn't start when hot. It would only do it after it had been running for a while. It wouldn't start when hot, but would fire right up the next morning.
I see this a lot. It's similar to an old ignition control module in a small chevy with an HEI ignition. It runs fine when it's cold, then when it heats up, the circuitry inside the control module produces resistance, and the engine begins to run so poorly to the point where it dies, then it won't restart- that is until you let it cool down.

I see engines come in my shop, and folks complaining about hard to start when cold. First thing I do is plug in my inline spark checkers to the plug/wire. It's like an inline checker that has a LED bulb in it. It doesn't pull any current away from the coil, and it glows red when it's running. Lisle tool makes them, and you can find them at the autoparts house for about ten bucks. I have two, I plug in both, and just let it run until I make the determination that the coils are good or bad.

West end angler also had a good point about the valves. I wonder if you've checked your lash since you've had the heads off? A leak down tester is always great to check to see if your valves are sealing good. When the metal swells, usually it seals better, but you never know until you check.

You'll get it fixed. I'd love to hear back to see what the problem ended up being. Good luck with it.