ok os if it only does it when it is warming up how can it be anything to do with the coil ?? the coil and i0gniton system works the same when it first starts and when its at optimum running temp ,i think it has something to do with the timing being out abit or maybe the auto choke in the carby

not aiming it at anyone just not sure the ignition system has anything to do with it :)

good luck !