Don't know if you have an alternator and electronic ignition. A friend of mine had this problem, seems the alternator provides the power to keep it running. He had to install a diode to keep the alternator from feeding back to the ignition system.

1954 Chevy Half-Ton
In the Stovebolt Gallery
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1958 Chevy 1/2-Ton Stepside
In the Stovebolt Gallery
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"Time's fun when you're having flies." ~~ OLD FROG