No matter what I do, my Chevy “stumbles”

Question:

I have a 1938 Chevrolet that I grew up with and have restored. It has a stock 216 cid 6-cylinder engine with a stock 6volt ignition. When the engine is running at any speed it “stumbles.” It does it when it’s under load and when it’s not under load but, of course, it’s more noticeable when it’s under load.

When I put an induction timing light on the coil wire instead of seeing a steady pulsing light, every now and then it flashed and the engine missed. It also does this on every cylinder at random.

I had it scoped and they said my points were not making contact all of the time. I have put in three sets of points and three condensers but the condition didn’t change. I checked the distributor shaft play and even changed the distributor but this didn’t help either. I tried three different ignition coils and there was still no improvement. I put a jumper wire directly from the battery to the positive side of the coil, put a jumper wire from the distributor to the coil, and installed a ground wire from the distributor plate to the engine block but after all of this nothing changed. I checked the voltage to the positive side of the coil and it is a constant 7 volts.

As a last resort I installed a Pertronix Ignitor electronic ignition, which completely eliminated the points. Nothing fixes the problem. Any suggestions?

Answer:

You describe all of the parts you’ve replaced two or more times but don’t mention the distributor cap or rotor. A crack or other defect in one or both of these can cause the stumbling problem you’re experiencing. The same applies to the ignition wires.

Have you evaluated the function of the ignition switch? If the switch has corrosion or broken or loose parts it may be interrupting the flow of power to the ignition circuit, causing the stumble you see. Also carefully examine every inch of the ignition system wiring because a loose connection, corroded connection, or intermittent short caused by a break in the insulation can cause the problem you’re having.

Another essential element in the ignition system is the battery and its connections. A defective battery with an internal defect or a problem with the battery cables that causes power to the ignition circuit to momentarily cut out will cause an engine stumble. The same thing applies to the generator.

If you systematically go through every part in the vehicle’s electrical system and still can’t find the source of the problem then I suggest looking at the fuel system and the engine’s basic mechanical health.