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timing mark jumping around, why?

Discussion in 'The Hokey Ass Message Board' started by DOUBLEZO, Nov 3, 2010.

  1. DOUBLEZO
    Joined: Aug 5, 2005
    Posts: 204

    DOUBLEZO
    Member
    from SACramento

    the stats, 8ba with 400 miles. pertronix in a bubba gm dizzy, new epoxy pertronix coil, new wires, new ngk plugs, new stromberg carb. while setting my timing the timing mark will jump around a bit. the engine sort of stumbles when this happens. the mark always jumps to the advance side. i really dont know what to look for anymore and its driving me insane. any thoughts? thanks.
     
  2. 49ratfink
    Joined: Feb 8, 2004
    Posts: 18,030

    49ratfink
    Member
    from California

    I'd check the advance in your bubba gm dizzy
     
  3. 53sled
    Joined: Jul 5, 2005
    Posts: 5,818

    53sled
    Member
    from KCMO

    What does the vacuum gauge tell you? I've read on here that you can diagnose just about anything with a gauge and a spark plug reading. I'm no expert though.
     
  4. lowsquire
    Joined: Feb 21, 2002
    Posts: 2,564

    lowsquire
    Member
    from Austin, TX

    Mine did that when my plugs were dying.. I had my carbs set up too rich and it was killing plugs..I think what happened was the resistance in the plugs caused some sort of cross firing in the cap? might be wrong, could be the pertronix.. thats just a guess.
    I would have a chat to Bubba, he'll no doubt set it right.
     

  5. landseaandair
    Joined: Feb 23, 2009
    Posts: 4,411

    landseaandair
    Member
    from phoenix

    I had one (Pertronix) do that on me. Worked fine, removed it to swap motors, put it back in and when I tried to time it, the mark moved around like the sequential tail light on an old Cougar. I gave up and put the points back in.
     
  6. noboD
    Joined: Jan 29, 2004
    Posts: 7,543

    noboD
    Member

    I've seen timing marks bounce around with a bad harmonic balancer too, but that should not effect the way it runs.
     
  7. DOUBLEZO
    Joined: Aug 5, 2005
    Posts: 204

    DOUBLEZO
    Member
    from SACramento

    i just hooked it up to a vacume guage as was suggested. at idle which is set to about 500 the needle bounced between 17 1/2 and 19 1/2. hmmmm?
     
  8. crash11049
    Joined: Aug 31, 2009
    Posts: 136

    crash11049
    Member

    This is from a Stewart-Warner Motor Minder vacume gauge, dont know if it will help or not. [​IMG]
     
  9. Could be bad bushings in the distributor shaft.
    (this next one's a guess, as i know NOTHING about Pertronix or other electronic ignitions)....but maybe wrong air gap between the 2 doo-dads that make up electronic crap?
     
  10. Beebeebobby
    Joined: Sep 5, 2010
    Posts: 224

    Beebeebobby
    Member
    from Webb City

    How about tightening the distrib...just a smidge? LOL
     
  11. Timing chain slack??
     
  12. landseaandair
    Joined: Feb 23, 2009
    Posts: 4,411

    landseaandair
    Member
    from phoenix

    No chain in a flathead.
     
  13. vacuum advance influenced by vacuum fluctuation ?
     
  14. I'd call bubba and ask him, especially about how early the advance starts to come in. If it's already picking up the weights, that can cause the timeing to fluctuate slightly.
     
  15. Beebeebobby
    Joined: Sep 5, 2010
    Posts: 224

    Beebeebobby
    Member
    from Webb City

    IF, you had the distributor out...then you might be off a tooth..just sayin...could make it impossible to time correctly.
     
  16. studhud
    Joined: Jan 6, 2006
    Posts: 1,403

    studhud
    Member

    My Hudson does that cuz the drive gear is worn ie slack
    Dave Hitch
     
  17. junk yard kid
    Joined: Nov 11, 2007
    Posts: 2,719

    junk yard kid
    Member

    just cause it got me the other day for a minute, Its numbered different than a chevy
     
  18. Hot Rod Bob
    Joined: Mar 21, 2007
    Posts: 1,146

    Hot Rod Bob
    Member
    from T-ville Ky

    Could be a vacuum leak.
     
  19. Beebeebobby
    Joined: Sep 5, 2010
    Posts: 224

    Beebeebobby
    Member
    from Webb City

    Too bad he don't have one of those old Indian motorcycles with the throttle on one handlebar and the distributor adjustment on the other handlebar....LOL....Great little trick..You advance or retard the timing on the "fly" sitting at a light etc...nice little LOPE...LOL
     
  20. DOUBLEZO
    Joined: Aug 5, 2005
    Posts: 204

    DOUBLEZO
    Member
    from SACramento

    the dizzy is tight and timing is dead on. (except for the jumping around) ive been breathing gas fumes all day. is there any way to test the pertronix? thanks for all the help, i really appreciate it.
     
    Last edited: Nov 3, 2010
  21. 2manytoys
    Joined: Feb 24, 2009
    Posts: 225

    2manytoys
    Member
    from Fresno

    I'm not familiar with the term 'bubba' on the GM distributor. If it has the aftermarket Pertronix mounted on the old point plate it could be the worn point plate bushings moving erratically. Worn mounting plate bushings will cause timing issues visible with a timing light.
     
  22. Bubba is our resident expert on here for ignitions. Do a search on here (GMC Bubba). He is in Vegas this week and may be hard to contact. If you don't find a solution by this coming Monday contact him @ 317-487-9460 after 9am EST
     
  23. espo35
    Joined: Jul 16, 2010
    Posts: 310

    espo35
    BANNED
    from california

    Does it do it with the vacuum advance disconnected? If so, look to the mechanical advance.
    If you have a missfire, that will cause the engine rpm to fluctuate which will cause the timing to jump, rather than the advance causing the engine to stumble.
     
  24. 53sled
    Joined: Jul 5, 2005
    Posts: 5,818

    53sled
    Member
    from KCMO

    vacuum jumping, sounds like if you pull the vacuum advance, it should smooth out. let us know.
     
  25. RR
    Joined: Nov 30, 2008
    Posts: 105

    RR
    Member

    Check the clearance between the Pertronix module and the magnet or distributor lobe thru the entire rotation of the shaft. The only time I had this problem was with an Accel distributor. The reluctor was so eccentric that it was clear (maximum clearance) at one angle and would scrub the pickup 180 degrees later. (Accel was little help in solving this, saying I was basically wrong. Jegs took it back even though it was out of their return window- that is why I will buy Jegs..)
     
  26. american opel
    Joined: Dec 14, 2006
    Posts: 1,222

    american opel
    Member
    from ohio

    i had the same thing happen to me as RR.it was so out or round it wasnt funny.i took it back to summit and bought a msd.
     
  27. Larry T
    Joined: Nov 24, 2004
    Posts: 7,661

    Larry T
    Member

    How are your sparkplug wires run? If they are too close together you can get induction (is that right?) firing.
    Larry T
     
  28. 69fury
    Joined: Feb 24, 2009
    Posts: 1,419

    69fury
    Member

    This. Can you borrow someone's electric timing light with a digital rpm readout to confirm the idle is as dead smooth as possible. With vacuum unhooked the mechanical advance could be flinging out just a touch- check your springs, etc for snugness/tension to rule this out.

    I agree to check that the engine/plugs/wires are changing the timing instead of other way round

    rick
     
  29. DOUBLEZO
    Joined: Aug 5, 2005
    Posts: 204

    DOUBLEZO
    Member
    from SACramento

    thanks, ill have to give him a call and have it tested if it comes to that.

    no more vacume advane on this dizzy.

    i had them running through those aftermarket looms. tried to run them stock , still problems. ive tried 4 sets of wires everyway possible.

    i have a good light that shows rpm. its around 510 and goes to 525 the back down around 510 again and again give or take.

    i wish it was that easy, this thing is killing me!
    im starting to think its something else like maybe a valve, i dont know anymore. here is a video just for fun. its not the best quality but you can see the mark move. [​IMG]
     
  30. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,682

    Bruce Lancaster
    Member Emeritus

    Flathead curves start advancing right off idle, circa 500 rpm...likely your distrib is just starting to advance at your idle speed. Vac should be disconnected and plugged while setting initial timing.
     

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