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My trucks is a 1981 712 from swiss surplus....Like the subject says the engine began stumbling / backfiring not being able to get past idle. It has a civilian cap rotor and wires conversion which had been working great . The petronix conversion since I bought it in 06. I have replaced the cap and rotor, adjusted the valves, rebuilt carbs (they needed anyways) swapped out coils to see if it made any difference - none. My last thought is the distributor and the advance mechanism could be the problem. My question is would I be on the right path ? Did I miss something? and is there a trick to getting the distributor out?

Check the gap between the Pertronix pick-up sensor and the black-plastic-encased magnet on the distributor shaft -- it should be quite close, but not touching.
Check your timing. You'll need a standard timing light for this. Since you have civilian wires, it's super easy to use an inductive pick-up timing light. You'll also then be able to see if your advance is working correctly.

And remember...the firing order is 1-2-4-3 on these....a simple mistake that some make when changing the plugs

After checking the timing be sure to check the 4 solenoid leads 2 going into the drivers side of each carborator. One of mine came slightly loose & my vehicle backfired to the point I had to shut down on two trips. Found the issue after a bit of time troubleshooting. Runs great after tightening the connections. Best & hope you get the issue resolved. Keep us posted. Always learning. Bob V

BobV wrote: ↑Wed Oct 18, 2023 8:52 am After checking the timing be sure to check the 4 solenoid leads 2 going into the drivers side of each carborator. One of mine came slightly loose & my vehicle backfired to the point I had to shut down on two trips. Found the issue after a bit of time troubleshooting. Runs great after tightening the connections. Best & hope you get the issue resolved. Keep us posted. Always learning. Bob V

Yes, exactly this! One of mine was damaged and I soldered the connection back on until the replacement came. Ran great until one day I could barely get it started, ran like crap etc. The connection had broken off again and caused all kinds of issues.

Glad you were able to troubleshoot it. It's amazing what an adverse impact when they fail. Really recommend keeping at least one spare in the truck. This problem shut me down twice. Hope you're in good running order now.

Best,

Bob V