There are a couple of things you can try. First, make sure that your gas cap is vented. It should have a couple of holes in it, or at least a washer underneath. This will prevent the engine from creating a vacuum. The other thing to look for, is that the gas line from the tank to the carburetor is routed away from the engine block and the exhaust since this can cause the gas to vaporize. Make sure that you have the right spark plug in. There is also a spacer that was/is placed onto the main nozzle to prevent this type of "mishap's" "Occasionally, on engines equipped with the dual system carburetor, some rich starting conditions have occurred when the engine is warm. This condition can be corrected by inserting a non-metallic spacer in the center leg of the carburetor, as shown (part # 632158). This spacer is designed to reduce the amount of prime charge in the main nozzle area for better starting under warm engine conditions. It can only be used on Dual System carburetors and does not lean out the carburetor mixture. This spacer must be reinstalled if originally equipped in the carburetor.") It is available at places like this. Of course, make sure that your valves are set properly as well since a hot engine will cause your valves to expand further and will make it extremely hard to start. You can do a compression test on your engine and see what it is when the engine is cold versus hot. This will give you a better idea what you are dealing with. Lots of ideas, but a little troubleshooting will get you there. Good Luck.