Does the engine continue to run after you release the starter pedal? If so, leaving the jumper in place will be at least a temporary fix. That fact reinforces the idea that you're losing at least some of the voltage between the ignition switch and the coil when the jumper is not in place. 6 volt systems really don't like even slight voltage drops! Just one loose or dirty connection, probably at the back of the ignition switch, can cause the exact starting problem you're describing.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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