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Technical Hemi distributor play

Discussion in 'The Hokey Ass Message Board' started by BDUB77, Oct 28, 2021.

  1. BDUB77
    Joined: Nov 16, 2018
    Posts: 112

    BDUB77
    Member

    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) image.jpg . Is this normal ? It is a stock disti with pertronix.
     
  2. Truck64
    Joined: Oct 18, 2015
    Posts: 5,237

    Truck64
    Member
    from Ioway

    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.
     
  3. greybeard360
    Joined: Feb 28, 2008
    Posts: 1,843

    greybeard360
    Member

    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.
     
  4. jaracer
    Joined: Oct 4, 2008
    Posts: 1,316

    jaracer
    Member

    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.
     

  5. 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.
    Ignition article.jpg Hemi2.jpg
    upload_2021-10-28_14-34-33.jpeg
     
  6. I’m assuming that little braided ground wire is attached and in good condition ?

    Tommy
     
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  7. gimpyshotrods
    Joined: May 20, 2009
    Posts: 19,204

    gimpyshotrods
    ALLIANCE MEMBER

    It has a Pertronix module. It is two-wire. It provides external ground. No grounding to the point plate is required.
     
  8. BDUB77
    Joined: Nov 16, 2018
    Posts: 112

    BDUB77
    Member

    Thanks for the advice. You all have given me some things to check. I will keep you updated.
    This thing is driving me nuts.
     
  9. bchctybob
    Joined: Sep 18, 2011
    Posts: 3,347

    bchctybob
    ALLIANCE MEMBER

    It may take a couple more of those gold things in your avatar to real home in on the problem.....
     
  10. gimpyshotrods
    Joined: May 20, 2009
    Posts: 19,204

    gimpyshotrods
    ALLIANCE MEMBER

    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.
     
  11. Truck64
    Joined: Oct 18, 2015
    Posts: 5,237

    Truck64
    Member
    from Ioway

    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).
     
    Desoto291Hemi likes this.
  12. gimpyshotrods
    Joined: May 20, 2009
    Posts: 19,204

    gimpyshotrods
    ALLIANCE MEMBER

    I cannot remember the last time I left the ground strap in. Maybe I am just lucky.
     
  13. Truck64
    Joined: Oct 18, 2015
    Posts: 5,237

    Truck64
    Member
    from Ioway

    Right, it won't hurt anything to leave it installed.
     
  14. lippy
    Joined: Sep 27, 2006
    Posts: 5,815

    lippy
    Member
    from Ks

    Might want to inspect the distributor shaft bushings. JMO. Lippy
     
  15. gimpyshotrods
    Joined: May 20, 2009
    Posts: 19,204

    gimpyshotrods
    ALLIANCE MEMBER

    Never said it would.
     
  16. Truck64
    Joined: Oct 18, 2015
    Posts: 5,237

    Truck64
    Member
    from Ioway

    Right, I'm jus' sayin'.
     
  17. 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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