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carb problem--I think????

Discussion in 'The Garage' started by gmcman, Nov 10, 2006.

  1. gmcman

    gmcman Registered Member

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    I have a 67 gmc 1/2 ton.It has a 305 v8 and i believe the proper 2 barrel(2gc) carb for that engine.My problem is that it has a hesitation when you mash the throttle.I rebuilt the carb myself.The accelerator pump appears to be working properly.When driving slow and you mash the throttle it hesitates then takes off or stalls.It seems that maybe its not getting enough fuel at that time.If i close the choke plate some or hold my hand over the carb it will rev up everytime but if I don't restrict the air it will bog out. Does the power valve take affect in this kind of situation? It seems to have good vacuum when idling.It idles good and drives good but even if you quickly step on it,and even half throttle,it hesitates.Its got me stumped.Any info would be greatly appreciated. By the way ,not that it matters,it has a turbo350 tranny. Thanks.
     
  2. 73k5blazer

    73k5blazer Unplug the matrix cable from the back of your head Premium Member

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    These are rochester carbs, I don't believe they have power valves, that's a Holley thing.
    But it sounds like it's leaning out during heavy acceleration. Did it not do this prior to your rebuild?

    My 1970 Impala (2bbl carb SB400 engine.350 trans) does this.
    I've not looked at it in depth, as the car is only a sunday driver and I usally have the family with me, so I'm not really mashing the gas anytime.

    I'd kinda like to know where to start looking too. I *think* the trick is to adjust it somehow so the bottom butterfly opens quicker than the top, making the mixture richer. But I'm just guessing here.
     
  3. diesel4me

    diesel4me 1 ton status Premium Member

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    2GC blues..

    Yes,the 2GC carbs DO have a "Power Piston" located inside the carb,mounted in the bowl area..its contolled by vacuum,has a passage drilled in the carb body that goes to manifold vacuum..

    If you use the incorrect gasket for the bowl,or base gasket,this passage may get blocked off,disabling the power piston ,and a bog or flat spot results..of course the piston could just be sticking, or gummed in the "off" position..it has a spring that holds it open,vacuum holds it shut until you boot the throttle and vacuum dies off,then it opens and enriches the fuel mixture so the motor wont bog or starve for fuel..it will cause a noticeable power loss if its not working..

    The accelerator pump can cause the same symptoms too,if its cup is scored,or the bore it rides in is all scratched up,its stroke not adjusted properly,or the little steel BB sized check balls are not sealing in the bottom of the well the cup rides in,or missing completely--they get lost often during a rebuild--or the ball in the passage to the venturi's where it dumps the fuel needed to stop the engine from bogging is not sealing or missing..

    A low float level can cause bogging and lack of power too..

    Advancing the timing may help if the carb checks out OK.....try hooking the vacuum advance to manifold vacuum rather than ported..some distributors are set up to be run that way on some 70's smog era motors..:crazy:
     
  4. gmcman

    gmcman Registered Member

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    carb problem

    When I rebuilt the carb everything appeared good.The power valve rod was free and the passages for the valve itself were clear.I believe all the gaskets had the proper holes for the vacuum to operate the power valve.The only thing is that you can't accually see the valve if its working during operation.I have a q-jet and manifold i can swap and i plan too but for now this carb is driving me crazy and i want to learn from this and figure it out.Just wondering if i was overlooking anything.Low vacuum,vacuum leaks,timing,egr troubles,low copression,etc-would any of these be a possibility?It runs great.Starts great.I installed new rotor,cap,wires and plugs and i know the timing isn't perfect but should be close.Just thought someone may have had this same carb and problem and figured it out. Thanks for the feedback so far.
     
  5. diesel4me

    diesel4me 1 ton status Premium Member

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    thats me!

    I've had several 2GC carbs over the years,and a few I never could get to accelerate correctly,despite the carbs looking perfect,and having been carefully rebuilt..I guess they were flawed internally or something..

    But all you listed CAN cause hesitation--timing off,vacuum advance not working or hooked to the wrong vacuum source,EGR opening too soon or not closing completely,etc..some claim weak valve springs can cause hesitation,but I've never found any on my motors that bogged on acceleration..putting a Carter AFB or Edelbrock Performer on made my problem go away!..:crazy:
     

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