Ok. This is my story. When I would pre-heat my tuned .21 and try to start it, the fuel line looked like I had a huge air leak somewhere. Tons of large air bubbles in the fuel line. I checked the tank and seal many, many times. Nothing solved the issue. I checked the temp of the carb one time and it was near the same temp as the pre-heated engine. So I used a small toothbrush with some water to cool down the carb while keeping the engine warm.

Then when i'd put the car on the box Whalaah!!!, instant fire and purring like a kitten. The carb might be getting too hot and boiling the fuel before the fuel passes through the carb. So try to keep the heat off/away from the carb when you're pre-heating. I'll bet this solves your issue.

Let us know what you find.