40 Ford won't start when after running

[Joe KCMO]

I have had the same problem with my 39 for the past several years. I replaced everything except the motor. I think I have finally solved the problem. It was the stock coil resistor. When cold supplied 2.9 volts to the coil. Once the resistor warmed up, the resistance went up and the voltage dropped closer to 2 volts stalled and would not start. After it cooled down the voltage went back up to 2.9 volts and started.

After a lot of experimental work, my solution was to replace the original wire resistor again ( many new old style resistors were bought and tried) except this time with a new solid state wire wound heat sink power resistor. The resistance is constant at any temperature and delivers about 4 volts. No more problem.

Not a solution for a 100 point show car. But for a third of the price of crapy original resistor at $5 and solves the problem. My engine always starts and even runs better with the higher voltage.

I replaced the carburetor, the distributor, the coil, the resistor, the points, the caps, the rotor and the wires. The final solution was the original equipment resistor which increases resistance when hot. The voltage is good enough to keep the engine running (barely) but would not start after the resistor warmed up. For $5, it is worth a try. Take the voltage readings when it will not start and if you find reduced voltage (less than 3 volts), I think you have found your problem.

Every car will be a little different due to parts being switched over the years. Coils have different resistance as does the wiring in the car. Take your voltage readings and you will find your problem.