TO30 stumbles when hot

My TO30 runs fine when cold. But it stumbles and sputters when hot. When warm the choke smooths it out, but when fully hot even that doesn't make it run "right". It will run indefinitely, but only very roughly and with full choke and decent throttle.

Spark seems good even when that's happening. It's strong and blue.

A small propane bottle held around the carb doesn't change anything (like it did the time I had a leak at the throttle shaft). But the propane at the air intake smooths it out.

I've cleaned the carb many times. Carb cleaner. Tiny wire to check the small passages. Bit of compressed air to see that the passages are clear. I haven't yet bought a full carb rebuild kit but maybe I should. But I'd like pointers on narrowing the problem to the carb before I spend the money.

Some weeks ago (see old post) I thought it was the sediment bowl assembly restricting fuel flow. I dissembled and cleaned it with no impact. I removed the inline fuel filter I had had for years.

Jets are adjusted as the manual says. Fiddling with the main jet will hardly impact the problem once it's happening.

I'm not convinced it's getting adequate fuel flow. How fast should it drain from the bottom plug of the carb? How many seconds to drain a pint. It doesn't "gush" like some mention, but it runs steady.

I know when this started--although whether cause and effect or just coincidence is another issue. I had bought a new fuel cap. Not vented but I didn't notice until too late. I ran the tractor just long enough to get it nice and hot (ran like a dream). I stopped the tractor briefly but forgot to shut off the sediment bowl. When I returned to my work it wouldn't run right. Removed the fuel cap and realized the tank was under decent pressure. Apparently the unvented cap had caused the gas tank to build pressure. The carb and air intake line were filled with with gas, presumably from the pressure overcoming the float needle. I drained it all, fixed the fuel cap problem, eventually cleaned the carb, but problems continue.

Sorry this got long. I assume the problem is something obvious that is staring me in the face, but I keep missing it.

Since it runs well cold, can assume the carb is not the problem.

I would look at 2 areas, first consider the possibility of vapor lock. Happens when the fuel boils in the gas line, pushes the gas back into the tank instead of allowing it to flow to the carb. Be sure the fuel line is routed as far from the exhaust manifold as possible, and use a steel line, not copper. You can also wrap some insulation around the line where it is near the exhaust.

The other thing to look at is the coil. Is it the proper coil? Does it have a resistor? A wrong coil will overheat, a bad coil can break down with heat. The choke can help because a richer mix is easier to ignite.