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Sbc engine vibration

Discussion in 'The Hokey Ass Message Board' started by chop&drop, Sep 30, 2013.

  1. chop&drop
    Joined: Oct 11, 2006
    Posts: 369

    chop&drop
    Member

    I've got a '71 vintage 350 Chevy in my deuce sedan and I've got a vibration that starts about 1800 rpm. The engine was professionally rebuilt and supposedly all of the rotating assembly was assembled with matched (by weight and dimension) but not dynamically balanced. It is definitely rpm related (not vehicle speed). The flywheel, the clutch and transmission have been changed with no effect on the vibration. I'm thinking I may have a bad harmonic balancer. I don't know for sure but I doubt the balancer was replaced with a new unit in the rebuild.

    Any ideas? Other possible gremlins I haven't thought of?
     
  2. by chance do you have a sbc 400 balancer installed?
     
  3. chop&drop
    Joined: Oct 11, 2006
    Posts: 369

    chop&drop
    Member

    Don't think so. I think the builder would have known better and it looks to be the correct balancer.
     
  4. I agree that it sounds like something with the balance. Does the vibration stay constant after 1800 rpm or does it get worse or phase out as the rpm goes higher.
     

  5. chop&drop
    Joined: Oct 11, 2006
    Posts: 369

    chop&drop
    Member

    It stays pretty constant. I haven't taken it too high because of the vibration but it's still there up to around 4000 rpm.
     
  6. Did you have the drive shaft balanced? HRP
     
  7. Fenders
    Joined: Sep 8, 2007
    Posts: 3,922

    Fenders
    Member

    Does it vibrate when stopped with the trans in neutral?
     
  8. chop&drop
    Joined: Oct 11, 2006
    Posts: 369

    chop&drop
    Member

    Yes, the driveshaft was built and balanced when I went from a 4-speed to a 5-speed.
     
  9. hard to diagnose but I would start with checking the harmonic balancer and timing index and see if they line up or mebbe hub shifted? just keyboard guessing
     
  10. chop&drop
    Joined: Oct 11, 2006
    Posts: 369

    chop&drop
    Member

    I'll have to check it tomorrow but I'm pretty sure it does vibrate stopped with the trans in neutral. I do know that it did it before the clutch, and trans were replaced. At one time I suspected the input shaft of my old Muncie but the new 5-speed didn't affect it.
     
  11. chop&drop
    Joined: Oct 11, 2006
    Posts: 369

    chop&drop
    Member

    I'm suspecting a shifted balancer hub but don't know how much or how little it would take to create a vibration.
     
  12. well, real world if your using a used one and on a motor you like, history and others misfortunes teach us to just get a new one and be more assured.
     
  13. davidbistolas
    Joined: May 21, 2010
    Posts: 962

    davidbistolas
    Member

    It wouldn't be the driveshaft if it's doing it at idle too.

    What about the flywheel or flexplate?
     
  14. Fenders
    Joined: Sep 8, 2007
    Posts: 3,922

    Fenders
    Member

    Some other things to check that can cause vibration:
    bad motor mount
    misfiring
    fan or loose belt
     
  15. chop&drop
    Joined: Oct 11, 2006
    Posts: 369

    chop&drop
    Member

    Flywheel is a new GM piece. Not being speed sensitive should eliminate the driveshaft too. I can make the vibration cone and go at the same vehicle speed by shifting to a different gear.
     
  16. chop&drop
    Joined: Oct 11, 2006
    Posts: 369

    chop&drop
    Member

    I don't think it's misfire. It doesn't "bang" or "pop" and a change of plugs, wires, cap, rotor and a switch to a Pertronix kit didn't affect it. Belts are pretty new as are the water pump and fan. Haven't looked at motor mounts. Nothing has more than 3000 miles on it.
     
  17. scrap metal 48
    Joined: Sep 6, 2009
    Posts: 6,072

    scrap metal 48
    Member

    I too developed a vibration changing from a 4 spd to a 5 spd.. I've been told the pilot bearing is different for the 5 spd and did not change mine.. I have not confirmed that the bearings are different but didn't have the vibration before.. My vibration is most notable when in 5th gear...
     
  18. chop&drop
    Joined: Oct 11, 2006
    Posts: 369

    chop&drop
    Member

    The vibration has been the same with the 4 speed and the 5 speed. I did change the pilot bearing with the trans change. Vibration is the same in any gear above 1800 rpm.
     
  19. 57Chevypilot
    Joined: Nov 3, 2012
    Posts: 6

    57Chevypilot
    Member
    from Seattle

    I had the same issue with my 57, it was the flexplate bolts, they were tight but the bolt holes were slightly elongated, allowing a vibration. I used some grade 8 washers and torqued them down properly and the vibration went away.
     
  20. Christom
    Joined: Nov 3, 2011
    Posts: 217

    Christom
    Member

    I had a similar problem on a 283 Impala engine years ago. In the end what gave it away was under the hood I saw some shaking at the alternator. Believe it or not a new fan belt fixed the problem! The old one had a stretchy spot that shook shit out of things as that bit went round the little pulley. It might not be your problem it does show the need to think outside the box - so to speak. Also check the balancer isn't moving around due to a bad keyway. That will also show up with an erratic timing light signal off the timing mark.
    Vibrations are a real bitch to find - really takes a lot of elimination work.
     
  21. my car does the same thing,350/350.sitting in nuetral and rev it up it vibrates in a pattern,if you hold the gas constant,the vibration goes in and out with a pattern..new balancer,i still dont know what the hell it is..my car shakes and rattles so much i dont notice it much anyway
     
  22. okiedokie
    Joined: Jul 5, 2005
    Posts: 4,031

    okiedokie
    Member
    from Ok

    Have you had the engine builder take a look at it?
     
  23. drdoom
    Joined: Mar 18, 2007
    Posts: 65

    drdoom
    Member
    from new jersey

    two ways to check your balancer,first put a timing light on it,see were your mark is,also bring the rpms up to the vibration and watch the balancer with the light,second bring engine up to tdc pull the cap and check the rotor should be pointing to number one wire,also remove drive belts and see if it goes away,hope this helps
     
  24. T McG
    Joined: Feb 12, 2005
    Posts: 1,255

    T McG
    Member
    from Phoenix

    The 400 balancer has a big notch cut out of it and the 350 is smooth all the way around.
     
  25. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 25,272

    Jalopy Joker
    Member

    another thing to check is the angle of trans to pinion angle.
     
  26. BobsurUncle
    Joined: Dec 9, 2010
    Posts: 12

    BobsurUncle
    Member
    from Tracy, Ca.

    Out of balance pressure plate. One of those things forgotten about in manual shifts.
     
  27. I am thinking a bad flywheel or a plate, New dont mean correct.
     
  28. henryj1951
    Joined: Sep 23, 2012
    Posts: 2,304

    henryj1951
    Member
    from USA

    just being silly here----- check the NUT behind the wheel.
    hope you find the problem , and be sure to let us know the fix...


    :cool:
     
  29. 56sedandelivery
    Joined: Nov 21, 2006
    Posts: 6,360

    56sedandelivery
    ALLIANCE MEMBER

    Had a similar issue with a 305 to 334, balanced, stroker kit I bought years ago. I bought the kit, then found out the 305 block I had, had spun main bearings real bad, and was junk. Put the stroker kit on the shelf for a couple/few years. Got a good 305 block, took everything to my machinist, and he clearanced the block, bored it .030 over, and did the short block assembly. Engine sat on a stand for a couple of years, then wound up in my DD truck. It would idle fine, but any kind of driving, and there was a dullness to the engine, it would't rev up, and it was a dog. I could't figure it out, and parked the truck. It had a 400 dampener and flexplate, they were new, they had to be right. I took the pan off to confirm it was a 400 crank, it was, and it was obvious the rods had been balanced, and I could see differences in the pin bosses of the pistons suggesting that they were also balanced. I even stuffed a lot milder cam in the engine thinking maybe the one in the engine was too big, and causing a vacuum problem; I was lost. Then one day I read an article in a car magazine that had photos of the different flexplates and flywheels, their diameters, and related starters.......................and a big light went on in my head. Comparing the photos to my flexplate I decided what I really had was a 454 flexplate, not one for a 400. Because it had been so long that I had bought the stroker kit, contacting California Discount Warehouse was pretty much out of the question. I had paid for a balanced rotating assembly, and I figured what they did was a "target balance" type of balance job, not a "gram accurate" balance job, and some warehouse employee just tossed a 454 flexplate into the shipping order. Bought a new 400 flexplate, and installed it; pulled the pan AGAIN to check the main bearings, and they were fine. Put it all back together, started it up.................................smooth as glass!!! Make sure you do have a neutral balanced flywheel, be sure there is't a balance pad like a 400 or 454 flywheel would have. The only other possibility I could see, in your case, is IF it's a LATE model 350 that has a neutral balanced dampener AND a weighted, balanced flywheel ALSO; that's how they balanced those engines. In that case, if you had a neutral balanced flywheel, the assembly would't be balanced correctly, and it would shake/vibrate/etc. I still have that 454 flexplate if anyone needs one; I surely have NO use for it. Butch/56sedandelivery.
     
  30. Danshotrod
    Joined: Dec 7, 2006
    Posts: 213

    Danshotrod
    Member

    I had once purchased a 350 small block Chevy rebuilt from a machinist. It was one of the last ones that he had rebuilt and his health was going down. When the Motor ran
    from idol on up to about four grand it had a bad vibration and I never could really figure out what it was. So I took it to another machinist and had the rotating assembly balanced. Come to find out he put a 305 crank in at 350 block. The cranks are the same but they balance differently


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