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carb 308 tach and rpm stuttering

Discussion in '308/328' started by Sailawaysg, May 10, 2008.

  1. Sailawaysg

    Sailawaysg Rookie

    Apr 11, 2008
    47
    #1 Sailawaysg, May 10, 2008
    I just bought a '79 308GTS , it's been sitting for 2/3 yrs . It ran very rough when I looked at it and after fresh fuel,new plugs and running it a bit it's better but here is the question!
    Holding the throttle @ 3k rpm when it starts to run rough the tach falls to 0 and back to 2/3k just for a second. Is that normal or do I have a problem elsewhere to look for? It runs well at idle and well wide open (sort of).
    SG
     

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  2. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    19,897
    Texas
    Full Name:
    Steve Magnusson
    #2 Steve Magnusson, May 10, 2008
    It's normal if there is a problem with the current firing the coil primary (i.e., the tach signal is a measurement of the coil primary waveform so bad running and the tach signal going bad together is a clue that that's the problem) -- but I can't recall if the tach signal is taken off the 1-4 coil or the 5-8 coil -- check the schematic in the OM but I think it's a M (brown) or MN (brown/black) wire (if you want to check the wire connection at the coils). If you still have a points-based ignition system, you should measure the dwell and see if it's reasonable and not unstable or not. Also, wouldn't be a bad idea to put a timing light on it and confirm the ignition advance is quasi-working on both distributors. If you've got some other ignition type/modifications, please advise.
     
  3. Sailawaysg

    Sailawaysg Rookie

    Apr 11, 2008
    47
    #3 Sailawaysg, May 11, 2008
    The problem is in the 5-8 bank(I think). the plugs were soot black and dry when I replaced them. (running better now) Idles fine but missing a bit and comes on fairly well @ 5-7k full throttle, but still missing...and watching the tach while watching the carbs at 3k when it stutters the tach falls to 0 and back to 2.5k or so ,plus the left front carb wants to cough/spit at idle also. I think the carbs still have some crap in them from sitting so long but wonder about the points etc...
    Thx,
    SG
     
  4. thecarreaper

    thecarreaper F1 World Champ
    Silver Subscribed

    Sep 30, 2003
    16,377
    Savannah
    Full Name:
    I NEED A JOB
    #4 thecarreaper, Jun 7, 2008
    get rid of the 2nd set of points, or install the optical ignition boxes like i did on my 308's.

    i hate points.


    your miss sounds like its happening at the transition from the idle to main circuit when the 2nd set of points and mechanical advance weights are supposed to start pulling.


    did i mention i hate points....
     
  5. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,366
    NY
    #5 Steve King, Jun 7, 2008
    My 77 the tach wire (brown) comes off of the 5-8 coil.
     

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  6. bowserrh

    bowserrh Rookie

    Oct 16, 2007
    26
    Poway, CA
    Full Name:
    Robert Bowser
    #6 bowserrh, Jun 8, 2008
    I had a similar problem. Tach would go to zero for a second or so. Turned out to be the fuse box. I suspect the engine vibrations would produce harmonics in the fuse box that would cause momentary loss of contact in the circuit feeding the coils and/or the instruments.

    To test this, put some rubber bands around the fuses to hold them firmly in place (i.e., around the two prongs holding the fuse). If the probem goes away, it is the fuse box. Replace with Birdman version. Of course, you should do that anyway....

    If a carb car sits for 2 to 3 years, you really need to have the carbs cleaned. These carb cars do not like to sit, and get very cranky when they do.

    Bob
    '78 308GTS
    '01 Corvette Z06
     
  7. f355spider

    f355spider F1 World Champ
    Rossa Subscribed Owner

    May 29, 2001
    17,176
    USA
    Full Name:
    Hugh G. Rection
    #7 f355spider, Jun 9, 2008
    Bob, welcome to Fchat! :)

    To add to Bob's comment, I would recommend removing all the fuses, and using a bit of Scotchbrite pad and electronic cleaning solvent, clean each of the copper mounting tabs. Do the same on the fuse contacts themselves. Then, gently bend each of the tabs a bit closer to each other, and re-install the fuses. Should make a difference. Better yet, get the replacment fuse boxes offered by a couple sources here on Fchat.
     

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