1973 z28 350+.030 strange engine shake !

Discussion in 'Troubleshooting & Diagnosis' started by Lou6t4gto, Jan 24, 2018.

  1. Lou6t4gto

    Lou6t4gto Veteran Member

    258
    8
    Aug 18, 2010
    Ocala, Fla
    I think I remember that I Have put a timing light on it and checked that the advance was smooth. also, I've tried 3 different HEI' s in it in the past few years.
     
  2. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

    9,295
    225
    Nov 4, 2004
    Ringgold, GA
    Bonzo has a valid point. Pop the hood when it's dark, have someone (or take the air cleaner off and do it yourself) bring the rpm's up to where the vibration is and see if there is any arcing or cross firing on the spark plug wires. Also check the inside of the distributor cap for arc tracing.
     
  3. Lou6t4gto

    Lou6t4gto Veteran Member

    258
    8
    Aug 18, 2010
    Ocala, Fla
    just installed new plugs, new wires and new rotor and cap ! no difference. had already changed coil and ign module.
     
  4. cadillac_al

    cadillac_al Veteran Member

    588
    36
    Apr 22, 2015
    Maine
    Is the engine still running smoothly while it's vibrating or is it breaking up/missing?
     
  5. Lou6t4gto

    Lou6t4gto Veteran Member

    258
    8
    Aug 18, 2010
    Ocala, Fla
    pretty smoothly, hard to tell with the shaking. can run the rpm's up,& down and it runs smooth, but stop and keep it at 18-1900 and it shakes every time.
     
  6. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

    9,295
    225
    Nov 4, 2004
    Ringgold, GA
    Ugh, sounds like that's going to be a bear to find. Do you have a mechanics stethoscope? Crank the idle up or have someone hold it there while you probe around the front and rear of engine and listen for anything unusual.
     
  7. bourbon_scotch

    bourbon_scotch Veteran Member Lifetime Gold Member

    2,017
    52
    Jan 10, 2003
    USA
    take the flex fan off and try it, or just take your belts off and try it with no accessories turning
     
    Last edited: Jan 26, 2018
  8. djorgensen3

    djorgensen3 Veteran Member

    344
    27
    Oct 15, 2009
    Peoria, Az
    Double and triple check the firing order. I mistakenly crossed a couple plug wires once and was super frustrated cuz it had similar symptoms as yours. Don't just go by doing one at a time, actually verify hand over hand with the wires and firing order. Just a thought.
     
  9. frankz

    frankz Veteran Member

    2,068
    5
    Aug 24, 2009
    Illinois
    Are you certain that both the harmonic balancer and flexplate are zero balanced?
     
  10. Lou6t4gto

    Lou6t4gto Veteran Member

    258
    8
    Aug 18, 2010
    Ocala, Fla
    firing order absolutely correct. Flex plate , definitely neutral. ( hard to mistake a 400 plate for a 350.)and is for a 1st gen 350 2 part seal engine. The balancer is the stock 6 3/4" 0 balance, but Is 43 years old. Seems strange, "Most Sites" say a 350 should have a 8" balancer, but this ones always been on there and when looking at replacement balancers they do come up 6 3/4". But WHY, ONLY at 18-1900 RPM ??? at idle you could put a glass of water on the air cleaner and almost have no ripples). if it was balancing ( the rods were balanced within a gram a long time ago (20 years) along with the pistons. crank was replace when "freshened up" (about 4 years ago). Wouldn't "out of balance" get progressively worse as the rpm goes up ? No, It did not start with the crank replacement. the shake /vibration can be felt in the steering wheel(Standing still, in park).
    unfortunately 18-1900 rpm is right where cruising speed is around town !
    thanks for the ideas.
     

Share This Page