That's the classic symptom of an open ballast resistor. It gets bypassed by the ignition switch when it's in the "crank" position. That's why the engine runs as long as you hold the key there. If you hold it with one hand and shift to "drive" with the other hand, the neutral safety switch will cut out the starter and the engine will stay running.
You can verify this by turning the ignition switch to the "run" position, then measuring the voltage on the positive terminal of the ignition coil. The voltage will vary depending on whether the breaker points are open or closed. With them closed, you'll find close to 0 volts. With them open, you should find 12 volts. This should be tested with a test light. If you use a digital voltmeter, carbon tracking through the break in the resistor can allow enough current through for the voltmeter to pick that up and falsely show 12 volts.
You can rotate the engine by hand to make the points switch between open and closed.
Wednesday, August 10th, 2016 AT 11:15 PM