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Customs SBC Stumbles/Sputters above 2500-3000 RPM's...Any input appreciated!!

Discussion in 'The Hokey Ass Message Board' started by Motoracer11, Jun 21, 2015.

  1. Motoracer11
    Joined: Jun 21, 2015
    Posts: 2

    Motoracer11

    Good morning gents-let me start off by saying I have searched ALL over the place in an attempt to find a thread discussing the same issue I am having and I've had no luck. Similar issues, yes, but not the same. If I inadvertently missed a thread that I should've read before posting, I apologize [​IMG] also, sorry for the short book that follows...trying to be as descriptive as possible.

    I'll start with some basic specs: 1971 SBC with about 80k miles when pulled from donor car...if I remember correctly, 150ish psi per cylinder with no more than maybe 10% difference between any 2 cylinders. Engine is a 4 bolt main with stock internals, Edelbrock Performer intake, Sanderson headers, 1" carb spacer, and a Holley 600cfm carb that was purpose built for off-roading by a local shop in Santa Clarita.

    Now for the issue: the truck fires up no problem, idles smooth around 600rpm's, and seems to run very well up to roughly 2500 rpm's. Right around that point, the truck starts to stumble, stutter, da da da da da da...and here's where it differs from other threads...this issue doesn't clear up. Much of what I've read says stumble off of idle, stumbles when punching it around this rpm and then clears up and pulls, etc.

    My total timing is right around 35* at 3000 rpm's, upgraded to HEI ignition and replaced all ignition components, new plugs gapped at .045...no change. Added fuel pressure gauge and only had about 2.5 lbs after the reg...rebuilt reg and can adjust psi again...adjusted to 5psi (and 6psi) at idle...idles smoother than before, but issue at 2500ish rpm's still exists, float levels appear to be set correctly, accelerator pump squirts 2 good, strong streams, fuel system is new, so no clogged filters or old gas. Truck also runs the same with or without air cleaner on the carb. Sprayed carb cleaner around carb base and manifold to check for leaks...all sealed. Zip-Ty on vacuum secondary rod was pushed down after a test run, so I believe they are opening.

    So I think I've determined that I have good spark, plenty of air, plenty of fuel up to the carb, and no vacuum leaks, etc. I'm a bit unfamiliar with automotive carbs, although I've rebuilt plenty of motorcycle carbs and I am mechanically inclined, so I'm willing to give about anything a shot at this point. I THINK this is carb related, but I'm not sure.

    Any help, input, etc is very much appreciated!! Thanks for reading all of this!!

    -Kevin
     
  2. oldrelics
    Joined: Apr 7, 2008
    Posts: 1,727

    oldrelics
    Member
    from Calgary

    What has been done to the carb that makes it "purpose built for off-roading"?
    Other than that, round cam lobe syndrome maybe?
     
    40fordtudor likes this.
  3. I'm thinking bad cam also Check that.
     
    40fordtudor likes this.
  4. Pop-Rodder
    Joined: Oct 6, 2011
    Posts: 325

    Pop-Rodder
    Member

    I think, before you do anything rash, replace that carb with a, oh lets say, stock quadrajet that you know to perform well and see if you still have the problem. You may just have a "purpose built" carb built for the wrong purpose, or wrong engine specs. Sounds to me like you have a basically stock sbc that would work considerably better with a tamer carb. Give it a try, it only takes a few minutes to change them out and if nothing else it will eliminate the carb as being the problem.
     
    firstinsteele likes this.

  5. onetrickpony
    Joined: Sep 21, 2010
    Posts: 501

    onetrickpony
    Member
    from Texas

    It would be real easy to disconnect the secondaries and see if anything changes.
     
  6. What year truck? Do you have 12 volts to the distributor? Older chevies had a resistor wire to give only 6 volts.
     
  7. jetnow1
    Joined: Jan 30, 2008
    Posts: 1,803

    jetnow1
    ALLIANCE MEMBER
    from CT
    1. A-D Truckers

    Also check your fuel pump- a tired one may give enough fuel at low rpm but be unable to supply
    enough at higher rpm's.
     
  8. 40fordtudor
    Joined: Jan 3, 2010
    Posts: 2,503

    40fordtudor
    Member

    I'm with Pat59 and Oldrelics in thinking there is a lobe or two missing on the cam--I ran into a similar situation years ago.
     
  9. blowby
    Joined: Dec 27, 2012
    Posts: 7,503

    blowby
    Member
    from Nicasio Ca

    Is it load related - will it rev higher going downhill than uphill? If it is load independent I would pop the valve covers and check for broken valve springs. See if you can push any of the valves open with a stick. Also check valve lifts.
     
  10. Motoracer11
    Joined: Jun 21, 2015
    Posts: 2

    Motoracer11

    Wow, thanks for all the posts!!

    I'm not entirely sure, but I believe only a spring loaded needle/seat and instead of individual vents for the fuel bowls, it has a single vent tube shaped like a "U" with holes drilled in it to vent and limit flooding.

    I absolutely agree with this...I'll try to find/borrow one that I can swap out and test!

    I'll check this out as well and report back!

    94' ranger cab, 71 engine/trans. I have good voltage to the dizzy...no stock wiring exists in the truck as it was all rewired when initially built into a strictly offroad/"race" truck/prerunner...that being said, I did check real quick before work this morning to confirm!

    Gave this a shot as well...tried the old Holley blue and a new one. No difference.

    The stumble/stutter seems to be consistent regardless of load...revving in the garage, uphill, downhill...she doesn't like anything above 2500-3000rpms

    Thanks again for all the replies! I'll check everything you guys suggested and report back. I'm really liking the HAMB so far!
     
  11. 656565
    Joined: Jul 2, 2016
    Posts: 1

    656565

    What did you find out? Having same problem with mine
     
  12. Baumi
    Joined: Jan 28, 2003
    Posts: 2,743

    Baumi
    ALLIANCE MEMBER

    Try another coil! I had a pertronix coil that would stumble over 3000rpm.Sounds like that could be your problem.My money is on the ignition, especially the coil.
     
    31Vicky with a hemi likes this.
  13. 1 When did this happen ^ ?
    2 Did the above coincide with the appearance of the stutter ?
    3 Did it ever run correctly ?

    From your description it's sounding very much like an Ignition problem.
    Just forget the fact that parts are new, more importantly the fallacy that the new parts are good and actually doing their job.

    I recently had a brand new coil fail after 45 mins drive time.
    I've seen "new" ignition parts cause well seasoned mechanics to chase their tail.
     
    Baumi likes this.

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