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Technical Weird engine miss

Discussion in 'The Hokey Ass Message Board' started by T McG, Sep 14, 2021.

  1. T McG
    Joined: Feb 12, 2005
    Posts: 1,262

    T McG
    Member
    from Phoenix

    Well guys I have a weird one for you, working on a 57 T-Bird with a 312, rebuilt a few years ago with low miles. Cylinders 1-2 & 4 are not firing sort of. Pulling the wires makes no difference in the running, it's still rough, replaced those wires, swapped plugs, checked valve clearance, checked compression, 145 lbs. swapped carbs, it has a Pertronixs Ignitor II & matching coil, 12 volts at the coil air gap set, new plugs. When I said it's not firing sort of, it has spark at the plugs but to me it seems weak but still sparks and the timing is right on. There is a huge exhaust manifold temperature difference, the side that is the problem is around 300 degrees and the left side is around 500. I checked for a restriction but there is none. Out the tail pipes you have a definate fump fump fump witch to me typically indicates valves but again I checked them and I have a good steady vacuum reading. I'm at a loss, any ideas?
     
  2. Almostdone
    Joined: Dec 19, 2019
    Posts: 892

    Almostdone
    ALLIANCE MEMBER

    You’ll think this is crazy, but check the distributor cap sockets and plug wire connectors for corrosion. I assume you already did that at the inside of the distributor cap and rotor.

    John
     
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  3. oldiron 440
    Joined: Dec 12, 2018
    Posts: 3,299

    oldiron 440
    Member

    Put the points back in it...
     
  4. saltflats
    Joined: Aug 14, 2007
    Posts: 12,594

    saltflats
    Member
    from Missouri

    What do the spark plugs look like in those cylinders?
     
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  5. T McG
    Joined: Feb 12, 2005
    Posts: 1,262

    T McG
    Member
    from Phoenix

    They are new but slightly wet when you check them
     
  6. T McG
    Joined: Feb 12, 2005
    Posts: 1,262

    T McG
    Member
    from Phoenix

    I have considered that but typically Pertronix usually works or doesn't
     
    firstinsteele likes this.
  7. jaracer
    Joined: Oct 4, 2008
    Posts: 2,419

    jaracer
    Member

    Is it possible that you have bad intake lobes on the cam on those cylinders? Take a look at the valve action with the valve covers off. Better yet, measure the valve lift on the affected cylinders versus a good cylinder.
     
  8. oldiron 440
    Joined: Dec 12, 2018
    Posts: 3,299

    oldiron 440
    Member

    I've seen more than a few that will do weird chit...
     
  9. Rickybop
    Joined: May 23, 2008
    Posts: 9,606

    Rickybop
    ALLIANCE MEMBER

    You replaced the plugs and wires for the cylinders in question. And the spark is still weak on only those cylinders.
    Bad distributor cap?
     
  10. saltflats
    Joined: Aug 14, 2007
    Posts: 12,594

    saltflats
    Member
    from Missouri

    Put a vacuum gauge on it and report back.
     
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  11. T McG
    Joined: Feb 12, 2005
    Posts: 1,262

    T McG
    Member
    from Phoenix

    Cam action is good
     
  12. T McG
    Joined: Feb 12, 2005
    Posts: 1,262

    T McG
    Member
    from Phoenix

    Steady vacuum
     
  13. T McG
    Joined: Feb 12, 2005
    Posts: 1,262

    T McG
    Member
    from Phoenix

    Tried a new cap and rotor
     
  14. Deuces
    Joined: Nov 3, 2009
    Posts: 23,753

    Deuces

    I'm thinkin' bad cap also... But I could be wrong....:rolleyes::(
     
  15. gary macdonald
    Joined: Jan 18, 2021
    Posts: 308

    gary macdonald
    Member

    Are the exhaust valves opening ? You can have compression but no exhaust valve action will kill those cylinders. Was the intake manifold installed properly.
     
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  16. harpo1313
    Joined: Jan 4, 2008
    Posts: 2,584

    harpo1313
    Member
    from wareham,ma

    Sure sounds like a cap failure ,plugs set for 38 to 42 ?
     
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  17. Doublepumper
    Joined: Jun 26, 2016
    Posts: 1,535

    Doublepumper
    Member
    from WA-OR, USA

    A worn out distributor can wobble, sometimes causing some cylinders to get weak spark due to fluctuating rotor gap. I have dealt with this in the past.
     
  18. Mr48chev
    Joined: Dec 28, 2007
    Posts: 33,861

    Mr48chev
    ALLIANCE MEMBER

    I'd have to agree with T McG in that the Pertronix usually either works or doesn't work with little in between. That is the same with most electronic ignitions though. They either work or you start figuring why they don't.

    Pertronix distributor that is reasonably new or a conversion kit on an old distributor of unknown quantity?

    It might be that if the distributor is an old original one that the bushings or the plate the module /points sets on is worn out.
    By sticking the distributor in a distributor machine or hooking the rig to a scope you should be able see if the distributor it's self is sloppy inside and not maintaining the correct air gap on all eight cylinders.
     
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  19. From what I'm reading, shouldn't be anything wrong with it. Maybe your ears and thermometer are broke :D:D:D
     
  20. Wanderlust
    Joined: Oct 27, 2019
    Posts: 788

    Wanderlust

    Had a similar problem with my 292, installed a new cap and thought the fit was sloppy. Ran ok to begin with, the I started to notice a random miss, changed back to the old cap and miss went away
     
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  21. saltflats
    Joined: Aug 14, 2007
    Posts: 12,594

    saltflats
    Member
    from Missouri

    How many inches?
     
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  22. saltflats
    Joined: Aug 14, 2007
    Posts: 12,594

    saltflats
    Member
    from Missouri

    Check the fazing of the rotor to cap if this is a conversion.
     
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  23. Wanderlust
    Joined: Oct 27, 2019
    Posts: 788

    Wanderlust

    I have since gone back to the” new” cap but I cut a narrow strip of electrical tape and applied it to the inside of the outer mounting lip to take up the slop, as in lateral movement, seems to be working ok so far.
     
  24. Truck64
    Joined: Oct 18, 2015
    Posts: 5,325

    Truck64
    Member
    from Ioway

    If the engine has a miss the engine manifold vacuum will most definitely not be steady. This would be very evident. Ordinarily I'd mention to double check that there aren't a couple spark plug wires out of firing order.

    Since you've a steady vacuum (how much?) this is unlikely. A 57 T-Bird 312 should have the optimum Y-Block camshaft installed. Do you happen to know which one it is?

    Much wanking around can be completely avoided by utilizing an ignition oscilloscope.
     
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  25. Rickybop
    Joined: May 23, 2008
    Posts: 9,606

    Rickybop
    ALLIANCE MEMBER

    Wankin' Around.
    Wasn't that by Tommy James?
    Wait...
    That was Draggin' the Line.
     
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  26. jaracer
    Joined: Oct 4, 2008
    Posts: 2,419

    jaracer
    Member

    Like Truck64 said, a scope check of the secondary will quickly tell you if the problem is ignition related.

    The problem is related to one cylinder head. I thought about a problem with the intake, but the fuel distribution doesn't work out to 1, 2 , and 4. Cylinders 2 and 4 are fed by the same side of the carburetor, but 1 is fed by the other side. You would have to have a blockage to two separate passages.

    I have seen valves with heavy carbon deposits that block air flow on some cylinders much like bad intake lobes. If a scope check doesn't show an ignition problem, I might think about pulling the intake and maybe the head.
     
  27. Tow Truck Tom
    Joined: Jul 3, 2018
    Posts: 1,879

    Tow Truck Tom
    Member
    from Clayton DE

    I'm feeling like I need to be certain of manifold restriction. How was this checked?
    I would be busting the rusty plumbing away from the head to see no evidence of Rodent invasion. Nasty work I know.
     
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  28. Deuces
    Joined: Nov 3, 2009
    Posts: 23,753

    Deuces

    That's a great song.....:cool:
     
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  29. lucas doolin
    Joined: Feb 7, 2013
    Posts: 541

    lucas doolin
    Member

    Same symptoms on a recent repair. Spark plug wires were reversed on the 2 problem cylinders. Double check firing order and maybe switch the 2 cylinder plug wires and see if problem is solved. Really quick fix if it works.
     
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  30. Moriarity
    Joined: Apr 11, 2001
    Posts: 30,778

    Moriarity
    SUPER MODERATOR
    Staff Member

    a friend of mine had a 65 riviera that acted the same way. It had a pertronix, we put a points dist back in it and it ran great after that
     
    saltflats and Truck64 like this.

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