Here's the problem: misfire and backfire through the exhaust above 3,500 rpm, in any gear, "MOST" of them time (not always) typically 1x out of 20 3,500+ rpm run ups will it NOT backfire/miss.

Car's a 340 6 Pack automatic with all 3 Holley carbs running well. Car was on the dyno and is not running lean (before this all started happening).

Here's what I've changed or replaced:
Cap, rotor, plugs
Have checked the wires with a spray bottle
New Orange ECU box (have a Chrysler electronic ignition conversion)
Ballast resistor
tried a friend's known working distributor
Some even suggested the Sun tach be removed from the equation. Done.

Here's another thread before I heard the backfire and changed the distributor: https://board.moparts.org/ubbthreads/show...&PHPSESSID=

Thoughts?

No. I wasn't doing anything, working on anything, etc. Started it one day and had the problem...

Whats the voltage at the alternator when this is going on? Loose or dirty connections. Bad grounds. Just some other things to check.

check phasing on the dist cap see where the 'fire ' is actualy hitting on the electrods ?? just a outside thought
may run fine at or just above idle but when the advance stats working it could actualy 'blow' the fire off the electode??

i've had that exact same problem on 2 mopars and it ended up being 3 different things....
1. bad ecu/coil.
2. bad reluctor/pickup (make sure you use a brass feeler to set gap).
3. low coil voltage (supply at about 4VDC - which was a bad ballast)...

but the most persistent problem i had was the ecu, that one was exactly how you describe...tried 3 orange boxes and all eventually after a few weeks did it (including just dying after 40mins on hot days)...finally went to a chrome mp box, all gone for 5yrs now, and there really is a big difference between the orange and chrome mp boxes...chrome much much better, better idle, better power...and they work!!

hope that helps!

Check your grounds first.