ThermoQuad Installation/initial tuning?
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Thread: ThermoQuad Installation/initial tuning?

  1. #1
    Full Member 64ragtop's Avatar
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    ThermoQuad Installation/initial tuning?

    o[ I kinda feel like that just now. Got a really pretty rebuilt ThermoQuad from a well known rebuilder,
    but canNOT seem to get it to idle properly. The engine "surges", racing then falling off - about once per second.
    I've been assuming a vacuum leak, but can't find one with the rubber hose "stethoscope" trick. Tried just starting
    over, resetting the idle mixture screws to 1.5, 2,and2.5 turns out from the seats, but can't seem to find a sweet
    spot. if I adjust the curb idle screw to raise the idle speed, I can make it whistle like on old Navy bosun's pipe,
    but have to crank it up to where the surge is almost 1500rpm so it doesn't die in between when it warms up.
    Any ThermoQuad experts out there

    If it ain't broke, fix it 'til it is!

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  3. #2
    BBD LEGEND Capt_Jack's Avatar
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    Re: ThermoQuad Installation/initial tuning?

    the only time Ive created that sort of scenario is when Id hooked the vacuum advance to a metered port on the carb.* the vac would advance the timing in surges and create just about that sort of reaction.


    so...is the vac adv hooked up?* if so, try ditching and plugging that once to see if she settles down. if so, you might have the wrong vac port.

    its how I roll

  4. #3
    Full Member 64ragtop's Avatar
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    Re: ThermoQuad Installation/initial tuning?

    The tube to the vacuum canister on the distributor is connected to the port just below and to the right of the right side idle mixture screw, that the 1974 Factory Shop Manual identifies as "To Distributor OSAC valve" What's an OSAC? Is it different from the vacuum advance device on the distributor?
    If it ain't broke, fix it 'til it is!

  5. #4
    BBD LEGEND Capt_Jack's Avatar
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    Re: ThermoQuad Installation/initial tuning?

    not sure on what that lable represents. however, I would suggest removing the hose from the vacuum canister on the distributor itself and just blocking the hose coming from the carb. then try starting it and see if it makes a difference in the surge. as far as where the right port would be....uh.....

    well...not exactly sure on a T-quad

    its how I roll

  6. #5
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    Re: ThermoQuad Installation/initial tuning?

    OSAC (orfice spark advance control valve). It is an emission control device and delays vacuum advance signal to the distributor. It goes between the vacuum advance port on your TQ and the vacuum advance on your distributor. Just get rid of it and run a vacuum hose straight from the vacuum advance port on your TQ straight to*the distributor. I think the port you are using is the correct one, you can verify for sure, it should not have vacuum at idle but should have vacuum when the throttle is opened.

    Does your engine have an EGR (exhaust gas recirculation) valve? If so, this could be the cause of your problem. It is only supposed to open when the car is cruising at part throttle. If it is hung open it will recirculate exhaust gas at idle and make a smooth idle impossible. The engine will jump and surge alot. If you have one it will be located on the intake manifold on the drivers side just below the carburetor. Block off plates are available so you can get rid of the EGR valve.

    I have a Edlebrock performer with a TQ on my 400 and it runs well, as good as any holley.

    Hope you can find the problem.

    Roger





  7. #6
    Full Member 64ragtop's Avatar
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    Re: ThermoQuad Installation/initial tuning?

    HHhhmmmm....There is an egr valve in the floor of the manifold, but it was there with the Edelbrock, and didn't seem to cause any problem. It's not connected to anything, should it be blocked anyway??
    If it ain't broke, fix it 'til it is!

  8. #7
    Full Member 64ragtop's Avatar
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    Re: ThermoQuad Installation/initial tuning?

    (bump)
    If it ain't broke, fix it 'til it is!

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    Re: ThermoQuad Installation/initial tuning?

    what you running for a cam?

  10. #9
    Full Member 64ragtop's Avatar
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    Re: ThermoQuad Installation/initial tuning?

    Previous Owner said the car has a "mild" cam. It's a late 70's 318, with 360 heads and intake. Didn't have this problem
    with the Edelbrock 1406, but all I changed is the carb and fixed the kickdown linkage.

    If it ain't broke, fix it 'til it is!

  11. #10
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    Re: ThermoQuad Installation/initial tuning?

    Is your intake a stock one? From what you say it seems some intake swapping has been done with the EGR valve swapped also. If the EGR gasket is bad a vacuum leak could occur causing a lean condition. Also the EGR valve could be stuck open or leaking. If it can be done easily, block it off just to elimenate it from being possible trouble.

    Another possibility could be the TQ secondary plates are not closed completely and this would produce a lean condition similar to a vacuum leak.

    There are several ways to check for a lean condition. If the accelerator pump lever has sufficient slack in it you can manually pump it and the accelerator pump will squirt small fuel charges into the engine. If the engine speeds up you are lean. Another way is to take a squirt can of Berrymans B12 or any carb cleaner in a squirt can and spray a small shot into the carb. The engine will speed up if its lean. Does the engine run better hot than cold? If so it is lean. Does it run best with the mixture screws all the way out? If so, it is lean. If none of these tests seem to affect engine performance then I would look more closely at the EGR valve.

    If you can't find the problem is it possible to try another carburetor or go back to what you had that worked before?

    Good luck,

    Roger



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