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Thread: SU carb idle changing

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    Default SU carb idle changing

    I'm new to this forum, and looking for some help. I have a '69 roadster 2000 with SU carbs (rebuilt about 7 years ago). They were working fine until the other day. When I first start the car, the idle is fine, around 800 rpm. After driving for several miles, the idle moves up to 2000 rpm. After driving further, it is up to 3000 rpm. It is not a linkage problem, and I cannot lower it by manually operating the throttle. Everything looks clean, and I haven't found a vacuum leak. Dash pot oil looks OK. The next time I drive the car, the process repeats. Any ideas?

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    Return springs are where they are supposed to be? None broken/missing. Had that happen once allowing one carb to do do just what the heck it wanted to do.

    I'd say you need carbs but that would be too self serving.... nyuck nyuck
    Bruce Palmer
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    Yes, the return springs are present and in good shape. Manually moving the linkage doesn't have an effect on the idle speed. Could the needles be hanging up and causing this issue?

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    I doubt it, but the best way to check would be with the air cleaner off and with the dampers removed just lift and drop the pistons several times. They should fall unhindered.

    Without something happening to bend a needle, I can't think for the life of me what would have gone gunnybag in that department.

    With the choke off do you have some slack in the cables at the carbs. With the cables tight and the nozzles pulled down some you might get that racing in engine speed.

    Are our nozzles (or one) hanging down after the choke is off? Reach under there and see if one pops up.

    Just shooting in the dark....
    Bruce Palmer
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    Thanks for the comments Bruce! I'll double check the choke cable, the pistons and the nozzles tonight and let you know what I find!

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    Registered User tlorber's Avatar
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    Also check the cable where it connects to the throttle linkage bar (just under the hood latch). Sometimes the ball at the end can dance around and the cable can get tighter. I twisted a wire around mine so that it stays in the detente.

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    I did some more checking, still no joy. Choke cables are frayed and need to be replaced, but that isn't the problem. Pistons move freely, nozzles were in position. I tried messing with the slow speed screws, but could not get the idle to drop. The service screw is not making contact, so that's not the issue. I'll check out the cable at the linkage bar,and play with it some more this weekend and see what happens! Thanks for the help!

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    If :

    1. your mechanical linkage from pedal to carb is not binding or loose
    2. your carbs are functioning
    3. choke/jet and throttle valve are not binding
    4. blipping the pedal at initial "cold" idle stays at 800rpm
    4. the problem rpm rise increases as the car warms up

    Then the problem may be a thermal expansion of the manifold/components causing intermittent vacuum leak at temperature or maybe you have one of the servo diaphragms connected and it is malfunctioning?

    Try tightening all of the paraphernalia plugs on the balance tube as well tightening the manifold bolts holding the manifold to the head.

    Disconnect choke cables and servo diaphragm to also rule them out.
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    Thanks Blue, I haven't check the manifold yet, so I will do that!

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    I'm sure you already checked this, but I just gotta ask...

    While the idle is in it's abnormally high state, did you manually pull the throttle shaft with your finger in the direction that would lower the idle? Not at any part of the shared linkage... but right where the throttle shaft enters each carb body?

    Haha!! I know, I know... but I'm Captain Obvious!!

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