We've ruled out the coil, and it's unlikely to be the ignition. Could be a valve hanging open when thermally expanded. Sounds like it could also be the engine leaning out from fuel starvation before shutting off. For more information, check the spark plug colour and condition. Spark plugs never lie. When you drop the carb bowls, check for any sediment at the bottom. If you see any, try disconnecting the fuel line from the tank and running some petrol into a clean bit of white tissue paper, to see if you get any specks of dirt. If you get any, unfortunately you've got the horrible job ahead of cleaning out and sealing the tank. In any case, when you go through the carburettors look really carefully for any problems.
