I am trying to figure out why my 354 hemi is popping and backfiring. The timing is jumping around at around 2500 rpm. I can move the rotor back and forth quite a bit. (Twisting not rocking) . Is this normal ? It is a stock disti with pertronix.
The Rotor is supposed to do that. The Pertronix pretty much either works or it doesn't. Check the vacuum advance - applying vacuum to it should move and hold the advance plate in an advanced position indefinitely, if the diaphragm is defective, it will not hold, would show erratic timing, and also introduces a vacuum leak to the carb and can cause this condition (backfire). Could be other causes for backfire but this would be something to check and/or eliminate.
Check the slack in the timing chain. Rotate the crank back and forth and watch the rotor. Crank should only move a couple of degrees before the rotor moves. If it moves much more than that it could be the cause of timing variance at a steady engine speed like that.
There should be an internal spring in the vacuum advance unit that holds the breaker plate in the retarded position. The spring is behind the brass nut on the unit. If it is missing it could cause the breaker plate to move around affecting the timing. If the spring is present, a bad diaphragm would only prevent the vacuum advance from advancing the timing under a light load condition. It would also be the source of a vacuum leak. If the engine is popping and backfiring, it could cause the centrifugal advance to jump around at 2500 rpm. It sounds like you have an ignition problem, bad plug wire, coil, Pertronix pickup, etc. I don't believe either the vacuum advance or the centrifugal (what you feel when you turn the rotor) are the cause of your problem. A friend with an ignition scope could pinpoint your problem quickly. I once had a new Dodge that the factory forgot to put the pin in the reluctor wheel in the distributor. This allowed the reluctor to move around on the distributor shaft and move the timing all over the place. The engine didn't pop or backfire, it just surged like mad when you brought the rpm above maybe 1000 rpm. This was a Dodge police package and it ran fine for about the first month. After that it was almost impossible to drive.
I've posted this in a couple threads here. I dumped my (dual point) 331 (same as 354) distributor and installed a 340 late model electronic ignition. Direct drop-in except the tab on the shaft needs a 1/4" extension welded on. Wire in a Chrysler electronic module and you are off and running with a bullet proof ignition.
It has a Pertronix module. It is two-wire. It provides external ground. No grounding to the point plate is required.
Thanks for the advice. You all have given me some things to check. I will keep you updated. This thing is driving me nuts.
Fish out your intermediate shaft/drive gear and inspect it. Carefully check every single tooth, as well as the pin that holds it on. Double check that the Pertronix module is the correct gap away from the lobes.
Are you sure ?! Pertronix in their troubleshooting docs, specifically mentions to check for presence of that braided uninsulated wire. When using points the braided wire is what helps ensure an uninterrupted low resistance ground as the vacuum advance moves around the point plate. I "assumed" the same issue was a concern for the electronic replacement modules. That's one thing they mention, the total resistance from the points plate all the way to battery negative post should not exceed 0.2 ohms (also ditch the ballast and run full voltage).
I put an Accel electronic conversion (similar to Pertronix I assume) on my grandson's 305 and fried a couple modules. I had grounded it to the engine but tech said it has to be grounded to sheet metal, not the engine. Makes zero sense to me. A ground is a ground.
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