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Thread: High Idle on cold start up

  1. #1
    Join Date
    Oct 2009
    Posts
    4

    Default High Idle on cold start up

    We replaced a fuel pump on a '78 scout ii, and we noticed it now has a very high idle when we start it from a cold start. The idle calms down after about a 1 1/2 mile, but until then the idle is so high we do not need to put any pressure on the gas pedal to get her moving. We took it back to the mechanic who replaced the fuel pump, (and the carb 4 weeks before that), but because the mechanic is 20miles away, we could not show him the high idle as the engine was very warm. He went ahead and adjusted the carb to a lower idle, but, I'm wondering if this was the right thing to do. Also, she has a cold start problem, i.e., she will start and die on the first attempt after sitting for a while, but will start correctly on the 2nd attempt. Are these issues related?

    Thanks so much!

  2. #2
    Join Date
    Feb 2009
    Location
    Aiken, SC
    Posts
    2,423

    Default

    The issues are related. The high idle is one of the features of a carb'd engine with an automatic choke. The high idle is adjusted when the engine is cold, and is specifically designed to keep the engine running long enough for it to warm up.

    The choke setting is pretty obvious once you find it. On the IH engines, it's a coil of metal that expands and contracts with temperature. You set it when it's "about" as cold as it is likely to "generally" be in winter so that the choke is just "barely" fully closed. As the engine warms up, the metal coil spring expands, and there is less "choke" to the carb. Since the fuel/air mixture has more fuel and less air in it, the idle is higher, because you don't get enough air to keep it running at lower rpm otherwise.
    Allan E.
    Curmudgeon Extraordinaire
    Charter Member, Old Hippie IH Club
    Old fashioned binder freak

  3. #3
    Join Date
    Oct 2009
    Posts
    4

    Default

    Great! Thank you for the explanation.

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