Check the voltage at the coil + terminal while cranking. If it's much below 5 volts, start looking for loose, dirty wire connections from the ammeter, to the ignition switch to the coil. Dirty points can also drop the firing voltage enough to starve the engine for spark. Clean, tight wire connections everywhere are a must, especially with 6V electrical systems.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!