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SBC has a no load miss?

Discussion in 'The Hokey Ass Message Board' started by RMR&C, Jul 28, 2013.

  1. Been chasing this problem for a while.....when running in neutral at any speed the engine has an erratic miss. Under load (feathering clutch or driving) it runs great, nice and smooth.

    Things I have tried; rebuilt heads, new Bubba's points distributor, rebuilt Q jet carb including throttle shafts, all new tune up parts like plugs, cap, etc. Have checked for vacuum leaks multiple times.
    Have adjusted the valves several times. When I had the heads off, I pulled all of the lifters to check for weird cam wear, all looked fine.

    I'm at a loss....anybody got any ideas? Am I missing something simple?
     
  2. saltflats
    Joined: Aug 14, 2007
    Posts: 12,594

    saltflats
    Member
    from Missouri

    Were the heads just rebuilt?
     
  3. RayJarvis
    Joined: Oct 11, 2010
    Posts: 209

    RayJarvis
    Member

    one plug not firing? does it miss at lower rpm? if so its most prob a plug or wire or cap corrotion on one contact . itll straighten up at higher rpm . check your spark with a spare plug till you find the offender. youy might find no rpm change when removeing plug wires with engine running. that's most likely a bad plug on tjhat cylinder . hows your compression?
     
    Last edited: Jul 28, 2013

  4. SMOG_GUY
    Joined: Jun 28, 2011
    Posts: 388

    SMOG_GUY
    Member
    from Dinuba

    One thing I see at work is an over active vacuum advance causing this.
    Does this distributor have vac advance? Try disc temporarily.
     
  5. gas pumper
    Joined: Aug 13, 2007
    Posts: 2,957

    gas pumper
    Member

    Along with this^^^, you might have just a tad too much advance at idle.
    If this is the case then using the timed vac port for the advance might cure this. Then as soon as you crack the throttle open it will bring in the vac. under load.
     
  6. Like this! What's your initial timing? Too much could cause a no-load miss. As mentioned already, try disconnecting the vac advance temporarily. Or if it's connected to full manifold vacuum try connecting it to ported vacuum. Or back the initial timing down a few degrees and see if it make a difference.

    Hasn't got an idle circuit plugged up on it, does it?
     
  7. Thing is, the miss is not just at idle, it misses at all rpm's (no load)......timing is set at 8 degrees btdc. When I disconnect the vac can it runs worse. I haven't checked the compression since I swapped the heads. Will do that. Before it had 160ish.

    Does a Q jet have a ported vacuum outlet? Will try about anything...
     
  8. OK, did a compression test, all cylinders 165-175 which is better than before the head swap.
    Set timing at 6 degrees, no change. Hooked vac advance to ported vacuum, ran worse.
    Attached engine ground, no change.
    As for the carb, I have swapped on a known good carb with no change.

    Gonna pic up some new plug wires and see if that does anything.
     
  9. fedsrule
    Joined: Sep 5, 2011
    Posts: 27

    fedsrule
    Member

    6 degrees initial, it would be lazy. If this is a typical stock-type SBC with 76cc heads like 882s you'd want more like 12-16 initial. with 20 in the can in by 2700 or so.
    Make sure idle mixure screws are equal side to side.
     
  10. saltflats
    Joined: Aug 14, 2007
    Posts: 12,594

    saltflats
    Member
    from Missouri

    Does this thing have a EGR on it by chance?
     
  11. No EGR. Motor is a early 70's 350, bored 030, flat tops, 76cc heads, mild cam(smooth idle).
    As for timing, seems to run best at 8-10 initial.
     
  12. tfeverfred
    Joined: Nov 11, 2006
    Posts: 15,791

    tfeverfred
    Member Emeritus

    I had a similar problem and Hoop98 bumped my initial to 14/16 and I'm running timed vacuum. Try it. Nothing to lose.
     
  13. saltflats
    Joined: Aug 14, 2007
    Posts: 12,594

    saltflats
    Member
    from Missouri

    Have you checked for vacuum leaks internal.
    Pull the pcv and block that hole in valve cover start it and thake off the breather tube and see if you are building pressure or if its sucking could be the intake leaking on the bottoms of the runners.
     
  14. ANDEREGG TRIBUTE
    Joined: Jan 1, 2008
    Posts: 1,385

    ANDEREGG TRIBUTE
    Member
    from Bordertown

    I know this a realy stupid question, but have you clipped your timing light to each wire in the hopes of seeing a erratic flash that corresponds to the miss?
     
  15. fedsrule
    Joined: Sep 5, 2011
    Posts: 27

    fedsrule
    Member

    OK I think I have something. you hooked it up to ported vacuum with 6 degrees lead. ran worse. It would run worse with that little initial on ported, because you were pulling alot of lead from the can at idle on manifold vacuum. Try going to ported vacuum source and upping initial to 12-16.
     
  16. Thanks for all of the help! Still haven't got it cured but it is running much better with every little tweak. New plug wires helped a bunch, plus I got rid of those ugly yellow things.
    It does not seem to be an ignition problem. All plugs are firing. I still think it's a vacuum leak/ slightly lean condition. Checked for internal leak, nothing, plus the gaskets are all new.
    Will keep playing with it. Thanks again.
     
  17. Have you checked for timing chain wear?
     
  18. RayJarvis
    Joined: Oct 11, 2010
    Posts: 209

    RayJarvis
    Member

    heres one thing you might try. take a section of vac hose and go straight from manifold vac to advance vac on your distributor. this should bypass a lot of extra vac line which could be bad and let you know if its a vac leak in your hoses. sometimes you cant see a small rubthrough on a vac line
     
  19. shemp
    Joined: Dec 16, 2006
    Posts: 512

    shemp
    Alliance Vendor

    My guess also.
     
  20. hoop98
    Joined: Jan 23, 2013
    Posts: 1,362

    hoop98
    Member
    from Texas

    If you mark your distributor at 0-90-180-270 and put the timing pickup on the coil wire you can be sure the cylinders are firing at 90 degree intervals.

    Also I think Autozone rents Smoke Leak detectors, best way for vacuum leaks.
     
  21. When you are getting the misfire hold the rpm steady and take a look at the timing marks with your timing light to see if it stays steady. If you have a bad chain you'll see the timing marks jumping erratically.
     
  22. Fleetliner
    Joined: Aug 4, 2006
    Posts: 103

    Fleetliner
    Member
    from Oregon

    Also try putting a vacuum gauge direct to manifold vacuum to check valve seating condition. Look for a fluctuating needle for a bad valve. Rebuilt heads doesn't mean it was done correctly.
     

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