OK- - - -it's time for me to get ignored- - - -again! Low voltage to the coil primary winding can create a weak spark, one than can just barely ignite starting fluid and totally fails to make gasoline burn. Since diagnosing that condition requires a little bit of mental gymnastics and the ability to use a voltmeter, it's seldom, if ever considered. On a 6V system, there needs to be a minimum of 4.5 volts available at the upstream side of the coil. For 12V systems the primary voltage available at the coil needs to be 9.5 volts or so. Sources of low voltage can be loose or dirty connections at the ignition switch, excessive starter current draw, and/or something as simple as dirty battery cable connections. The KISS principle needs to be applied before more exotic theories get considered.
Jerry
