Carb woes

Discussion in 'The Venerable Q-Jet' started by BYoung, Nov 5, 2018.

  1. BYoung

    BYoung Stage me

    Original QJet 7041242 is whistling at idle but will cease when I place my hand over it. RPM will also rise slightly with hand over carb. This tells me vacuum leak but I've checked over and over and can't find any. A new base gasket and PCV grommet were installed as of this weekend to no avail. Yes, the old original grommet is sitting peacefully on the intake valley pan. :D It broke into two pieces when removing it. Additionally, the car will crap out when shifting the transmission into reverse or drive and the car drops 100 or more RPM when the air cleaner is installed so I have to bump up the idle screw to keep it alive.

    Timing is set using Larry's thread and I've also spoken with other well known experts to be sure I have it correct. Manifold vacuum reads 16-17 at idle. It is slow to start when cold, acting like it is starved for fuel but I can see a steady stream of fuel when depressing the accelerator. The engine has to be fully warmed up to act somewhat normal but is still iffy at idle in gear. I am running the Lectric Limited breakerless ignition.

    The carb has been to two rebuilders in the past year but the problem has yet to be resolved. I'm tired of throwing money at it without results. Ideas are welcome.
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Is this an otherwise stock engine? Does it have a cam? Have you tried spraying carb cleaner around the outside of the carburetor? It could have an internal leak from damaged casting or incompatible parts like in a remanufactured unit.
     
    BYoung likes this.
  3. pbr400

    pbr400 68GS400

    Sometimes they leak air around the throttle shafts. Less common (but mine does it) is a warped base, top, or body. Mine has a leak between the base and throttle body; I change that gasket about once a year and retighten the base to body screws a couple times after the swap. A straight edge shows the bottom of the body is slightly bowed.
    Patrick
     
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  4. BYoung

    BYoung Stage me

    Stock engine, stock original carb. Sprayed all over and no sign of a leak.
     
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    It's leaking somewhere, a stock engine should pull close to if not a full 20" of vacuum. Have you owned this car since new? Is there a possibility the engine has been rebuilt? Maybe the intake doesn't fit right? If the carburetor is whistling, sure sounds like an internal leak. Try another carburetor if you can.
     
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  6. johnriv67

    johnriv67 Well-Known Member

    Could be as simple as a leaking air horn gasket or as much as a cracked baseplate. Total agreement with Larry
     
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  7. Brett Slater

    Brett Slater Super Moderator Staff Member

    Following.
     
  8. BYoung

    BYoung Stage me

    Larry, all indications and paperwork I have say the engine is bone stock and never rebuilt. I’ve got some gaskets on the way and will replace this weekend. Fingers crossed it’s that simple. I appreciate all the replies.
     
  9. Taulbee2277

    Taulbee2277 Silver Level contributor

    Let us know how it works out!
     
  10. TORQUED455

    TORQUED455 Well-Known Member

    Someone needs to smoke test your intake.. bathtub style intake gaskets weren’t known for leaking vacuum much, especially on an unmolested engine. It’s very difficult to find an intake manifold gasket leak using a smoke machine, but it is very easy to find external leaks such as split vacuum lines, carb mount gasket, etc. Does your car have power brakes? Have you pinched-off the power brake booster if it does? How about some pictures of the engine sans air cleaner!
     
    BYoung likes this.
  11. BYoung

    BYoung Stage me

  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Is that TCS solenoid near the carburetor booked up to anything? Can’t tell from the pictures.
     
  13. BYoung

    BYoung Stage me

    The TCS is unhooked. Left it on there so I wouldn’t lose it and to retain the factory look. The pics are from six months ago and only change is vacuum can is now on ported vacuum. Since the most recent carb rebuild it wanted ported instead of manifold vacuum.
     
  14. TORQUED455

    TORQUED455 Well-Known Member

    Not sure I’m following the logic on ported vs non-ported vacuum advance. Did you try pinching off the vacuum line to the booster with the engine running? You shouldn’t notice any type of change in the way it’s running if the booster is ok. Also, if you are indeed going to remove the intake, post pictures of the carb mounting gasket you used, as well as the intake manifold plenum w/o the gasket on, and the bottom of the carb. DO NOT turn the carb upside down - hold it high and upright to take pictures.

    You can fish the grommet pieces out w/o removing the intake. It takes some patience.

    The other thing you can try is to take a piece of heater hose about 3’-4’ and use it as a stethoscope. With the engine running and whistling away, one end goes to your ear, and the other end goes to everywhere on and near the carb, including the intake manifold gasket.
     
  15. BadBrad

    BadBrad Got 4-speed?

    Spotted it. These green dots are well known leakers.

    upload_2018-11-6_18-13-7.png
     
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  16. BYoung

    BYoung Stage me

    In speaking with another member, it could very well be an internal carb leak. Gaskets are on the way and I hope to get to it this weekend. I'll post results.
     
    Brett Slater likes this.
  17. shiftbyear

    shiftbyear Well-Known Member

    Is there fuel dripping out of the primary nozzle at idle? You may need a flashlight to see. Do the mixture screws have any effect on idle when turned in or out? Would a thicker base gasket screw up the choke adjustment? I prefer a thicker 1/4" base gasket, not really important right now. Last, when you take the carb off view the primary throttle plate position, if the transition slot is way exposed something is wrong in the idle circuit, make sure fast idle cam is off. Good luck
    Holley_t-fer_slot.jpg
     
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  18. Stage 2 iron

    Stage 2 iron Nice Chevelle LOL

    Brian, did you hit the secondary throttle shaft with carb cleaner at idle if the idle goes up found your problem I had this problem when it NOS 71 stage one carburetor drove me nuts had intake off twice
     
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