Are you running a 6 volt electrical system? If so, check the voltage that's available at the + side of the coil while the engine is cranking. If it's less than around 4.5 volts, find and fix the dirty connection somewhere between the ammeter and the coil, including the ignition switch contacts. 6V systems are VERY unforgiving of loose or dirty connections, including grounds. Drill a small hole (1/8" for instance) inside the gas cap to be sure it's venting.
Jerry
