What's the cranking speed like with the ignition turned off? A slow-turning starter draws a huge amount of current due to not enough "counter-EMF" being induced in the starter windings. That, in turn, drops the battery terminal voltage low enough that the ignition system can't produce a hot enough spark to fire the plugs, even if a tiny spark is visible while cranking. A properly-charged battery should be able to crank the engine for at least 15 seconds with the ignition disabled without the voltage at the battery posts dropping below 4.5 volts.
Virtually all "slow crank" complaints with a 6 volt system are the result of loose or dirty connections, and/or too-small cables. There's no such thing as having too big a battery cable on a 6V system. Run the ground cable all the way to the starter housing- - - - -the "ground strap to the frame with a second conductor to the starter" is a disaster waiting to happen.
Jerry
