Fluctuating neg. to ground on ignition coil

Small Block Mopar Engine

  1. Stephan D

    Stephan D Well-Known Member

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    Well, I replaced cap and rotor a couple of weeks ago, so they should be OK.

    Pulling the distributor shaft sideways revealed NO play at all as far as I could feel with my bare fingers.

    Finding a workshop with a Sun machine in this remote part of the globe will not happen, I have yet to find a workshop that has an oscilloscope, chances are small to find one...

    DSC04628.JPG DSC04626.JPG DSC04624.JPG .
     
  2. 512Stroker

    512Stroker We are all here because we are not all there.

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    Very interesting I thought for sure you would find some play in the shaft.
    A lot of corrosion inside the distributor, I hope the mechanical advance is operating properly.
    This miss you have are you certain it is only at idle rpm?
     
  3. nm9stheham

    nm9stheham Well-Known Member

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    Check the gap between the reluctor on the shaft and the pickup. It should be .008" (0.2 mm). Check with a non-magnetic feeler gauge, like brass. (Just in case you need it, 3 thickness of very light printer or copier paper is typically .008" to .009" thick.) That gap needs to be stable and consistent.
     
  4. 67Dart273

    67Dart273 FABO Gold Member FABO Gold Member

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    Try to find some local amateur radio operators, and if one of them doesn't have a scope, they may lead to other electronics oriented folks
     
  5. Stephan D

    Stephan D Well-Known Member

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    Not sure if it´s corrosion or just dirt, but the advance is working properly when checking with the timing gun, and it revs smooth and freely without any problems if I push the throttle, the slight "misfire" problems is only at idle.
     
  6. Stephan D

    Stephan D Well-Known Member

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    I'll see if I can find a non-magnetic feeler gauge, didn't know such a tool existed, I learn something every day :)
     
  7. 512Stroker

    512Stroker We are all here because we are not all there.

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    Are you sure you are not hearing an exhaust leak.
    Most miss fires get worse with load and more throttle.
    Very very strange it miss fires only at idle.
     
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    • Stephan D

      Stephan D Well-Known Member

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      It´s not so much about hearing, rather feeling the small jerks that make the whole car vibrate significantly at idle, which is where we sadly spend most of the time in the congested Stockholm traffic.
       
    • 512Stroker

      512Stroker We are all here because we are not all there.

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      Its hard to tell from a video without putting my hands on it , but that did not look that bad. Maybe all you need is some new motor mounts
       
    • nm9stheham

      nm9stheham Well-Known Member

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      It is not a lot, but it could run more smoothly for sure.
       
    • 512Stroker

      512Stroker We are all here because we are not all there.

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      Have you run a compression test and/or cylinder leak down test on this engine?
       
    • nm9stheham

      nm9stheham Well-Known Member

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      One thing that cannot be ruled out is valve sealing. If it is erratic, then this will happen. The only easy way to possible see if this is the issue is to try to clean the valves, by using some Rislone or other valve treatment, to clean the intake valves at least. If it works, then good. If it does not work, then that does not completely rule out erratic valve sealing; it could be worn valve guides. Partial disassembly is needed for that: pull a few valve springs and manually check a few guides for wear.

      Ring sealing is another possibility.

      Since you are working on this, OP. Measure all the resistances of the spark plug wires. They should typically be in the range of 3000 to 6000 ohms and none should be over 10000 ohms. And check spark plugs for tight caps on the upper ends, and firm seating. How new are the plugs?
       
    • Stephan D

      Stephan D Well-Known Member

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      Yes, did a compression test with the following result:

      1 150 2 145
      3 150 4 140
      5 155 6 142
      7 140 8 150
       
    • Stephan D

      Stephan D Well-Known Member

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      First: Thank you all for you patience and keeping good ideas coming!

      Spark plugs and wires where replaced in 2017, I just drive a thousand miles each summer so they are fairly new I would say.

      With new spark plug wires I got rid of micro-jerks at highway speed, the old ones where from 1979...

      Got new valve stem seals installed a couple of months ago as it was smoking blue when warming up, sadly the change of valve stem seals did not solve that problem. :-(

      Is Rislone just added to the engine oil without any further action?
       
    • nm9stheham

      nm9stheham Well-Known Member

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      I have used Rislone in 2 ways:
      • With the engine revved up, very, very slowly dribble (pour a bit at a time) 1/2 can down the carb throat to help clean intake valve stems and valve faces. It will make a LOT of smoke. (That part is fun LOL)
      • Put the rest in the oil to help clean all the valve stems.
      It may help, it may do nothing.

      If your change in the spark plug wires helped the engine to run smoother at highway speeds, then that may indicate that your issue is in the ignition system. You still need to recheck the spark plug wire resistances. Don't rule anything out.

      Is the timing with a timing light steady, or does it jump around?
       
    • Stephan D

      Stephan D Well-Known Member

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      Ok, I saw there are different kinds of Rislone, I have used the Zinc additive at oil change.

      What kind should be poured into the carb, the "Valve seal repair"?

      I will check the spark plug wires resistance on thursday if I can find the time.

      Timing is steady as far as I can see with the timing light.
       
    • nm9stheham

      nm9stheham Well-Known Member

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      Well, this is how I used to buy Rislone LOL
      Vintage Shaler Rislone Engine Treatment Full 1 Qt Motor Oil Can FULL NOS | eBay

      But I think you want the Engine Treatment, not the Oil Treatment which has the ZDDP. Now I do not know if the Oil Treatment will do the same inside the engine oil. I don't think I would do both....but you need the ZDDP. So maybe just do half of a small container of the Rislone Oil Treatment down the carb. Just an easy thing to do, but a long shot to do you any good.

      BTW, my neighbor when I was a kid would do this about once per year on his old pickup trucks. It would clean off the valves and it would run more smoothly for a while. Us kids would love to run around in the smoke LOL
       
    • 512Stroker

      512Stroker We are all here because we are not all there.

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      OP have checked to see if you have not wiped out a cam lobe on the engine?
       
    • 512Stroker

      512Stroker We are all here because we are not all there.

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      Not perfect but respectable for an old factory motor
       
    • Tooljunkie

      Tooljunkie King of cobble/master of the broken bolt FABO Gold Member

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      In most decent feeler gauge sets. Two or 3 brass ones in it.
       
    • Stephan D

      Stephan D Well-Known Member

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      Wouldn't that show up in the compression test or create vibrations at other RPMs than idle as well?

      If not, how do I check if a cam lobe is wiped out, removing the valve cover to see if all rocker arms move in a similar way?
       
    • Stephan D

      Stephan D Well-Known Member

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      After reaching operating temperature AND being shut off for 10-30 minuter the idle is just perfect, if it run like this all the time I would be VERY happy:
       
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      • 512Stroker

        512Stroker We are all here because we are not all there.

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        You could have compression with a bad cam.
        Visual inspection of rocker arm movement is the easy method.
        Kill the ignition and rotate the engine using the starter.
         
      • nm9stheham

        nm9stheham Well-Known Member

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        I had a Ford 302 act just like that. It would start to run rough and miss erratically when hot. If I stopped at a store for 5 minutes and started it back up, it would be perfectly smooth. I replaced the intake manifold and carb base gaskets, thinking that it had an erratic vacuum leak, related to temperature, but that did not fix it. I never found the cause; I worked on the ignition system, replacing a lot of the parts, and that intake manifold area. I cannot recall if enriching the mixture made it better or not. I never replaced the carb or the distributor or coil.

        I sold the car as-is for $600, and the buyer drove it as a daily driver like that until he junked it.

        Honestly, with the compression readings and the smooth operation you see at some temps, then I do not think there is anything wrong with the cam. I suspect fuel-air mix or ignition.

        One thing to think about is that if the engine sits for 10-15 minutes, the temperature is going to go up as the coolant is not circulating. That might effect the vaporization of the fuel.... just another guess of what might happen!
         
      • 512Stroker

        512Stroker We are all here because we are not all there.

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        I agree with your cam statement
        Just trying to run a process of elimination.
         
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