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A few days ago, I noticed a sudden loss of power driving around town. It was intermittent, so I limped home to check things out. I live on a hilly driveway and barely made it to the top and in the garage.

Checked the fuel tank for red death, pickup tube, fuel line and filter. All clean. Measured voltage at the “+” side of the coil and got between 17V and 18V, so that rules out the RPM module. Checked the fuel cutoff solenoids for power, all click normally as they should.

I have an older Pertronix ignition with then orange distributor cap and flame thrower coil. The cap itself and associated mounting plate look solid with no cracks or gaps. The short coil wire to the distributor cap has a small section of insulation rubbed off from vibration. It’s rubbed down to the inner braided insulation. Some other folks have told me that this maybe the issue. I’ve also heard the the rotor itself could be the culprit and have too much play.

The truck will start right up and idle fine. But when it warms up and is driven, the loss of power starts. The coil wire insulation and excessive rotor play make the most sense for this power loss.

Any other ideas?

Honestly this is out of my realm. But anyways...

How do the contacts look on the cap? Maybe clean them up with sandpaper if you haven't done so already.

Have you checked the timing?

Whenever I've had similar issues with my Pinzgauer I would start with checking plugs for gap and condition, valves, timing, and syncing the carbs.

Good luck!

In may limited experience, I would replace the cap and rotor. For a couple years I experienced the same thing, Truck was running on less than 4 cylinders but seemed to idle fine. (I have limped home on two cylinders.) could never find a cause but a new rotor and cap along with plugs would fix the problem. The old cap and rotor would always look fine. The same thing would occur after another couple of months. Checking the temp on the exhaust headers would point me to which cylinders were not firing. I had the pertronix at the time. Changing to PinzSSI terminated my quarterly need to do all of that.

Check the coil spark. It should jump at least 1/4-1/2”
If coil is bad replace with any coil with MORE THAN 3.0 ohms.
Common problem.symptom.

Well....I replaced the coil, plugs and wires. Gapped plugs at about .032”. Truck ran great on two around the block test runs. This evening, after the second test run, I turned it off and let it sit for about 20 minutes. I started it back up and the hesitation/misfire returned. Barely made it up our hilly driveway and into the garage.

When I installed the coil, the idle voltage was between 18-19V. During revs, the voltage did not go above 20V. Also inspected the distributor cap and did not notice any cracks or wear.

It appears this issue is heat related when it warms up. It’s fine when it starts. Revs and idle are smooth. But after it reaches operating temperature, all bets are off.

Ballast resistor?
RPM module?
Distributor cap?
Distributor rotor?
Distributor shaft play?

Back to square one.

Might want to toss in the PerTronix controller in the mix as a possibility.

Check the timing, and confirm that timing is advancing correctly with RPM increase, and is set correctly at idle. Use an inductive light.

Also, I'm assuming you don't have any new exhaust leaks as that can cause issues, but not necessarily the ones you're experiencing.

Update:

For those that might be having the same issue, replace your distributor cap along with the coil, plugs and wires. The stumbling seems to have been resolved.

Thanks for the help!

I am having similar issues. My civ. ignition was newly installed 2 months ago but the stumbling just began recently.
What is the best way to check the spark at the coil?

Make sure fuel supply hose is not cracked, there should be fuel visible in the filter.

Thanks for the head's up. Fuel supply is good, filter full & flowing ok. Starts & idles fine but runs very poorly under any kind of load. How hot should the coil get during normal operation & do the Flamethrowers have a good reputation? Haven't done anything to the engine lately except install a new rubber starter mount.
Thanks for suggestions.

What civilian ign are you using? Is there an easy way to check to
advance versus RPM?

Obviously, at idle and start close to TDC it would start and idle fine,
but at RPM you would lose power versus RPM W/O advance and it
would be considerable and to the point were at higher RPM it would
run very badly.

Also check for loose or stranded wires at starter since you mentioned doing the mount.

I've never heard anything good about the flamethrower coils, found one that had been removed from the truck when I bought it, have read a few post that said they eventually fail.

You are miss-informed.
Flamethrower coils work for years.
They usually fail if voltage regulator is bad or over extended cranking. When you push starter button, coil gets full 24v.