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High Idle Cold Start

Discussion in 'Intermediate CJ-5 and CJ-6 Tech' started by Kevin M, Nov 25, 2018.

  1. Nov 25, 2018
    Kevin M

    Kevin M New Member

    Denver Co
    Joined:
    Nov 24, 2018
    Messages:
    7
    CJ5 285 engine. I've noticed it idles very high after I start it and doesn't calm down for about 10-15 minutes, its cold here (snowed last night, don't think it broke 40 today) so I assume the 10-15 minutes is how long the engine takes to warm up the surrounding parts.

    After the 10-15 minute warm up period it will go back to idling at what seems pretty normal as soon as I hit the gas, ie... I'll stop at a stop light after it warms up and the next time I accelerate the idle is normal.

    I'm thinking it might be a choke problem, but this is my first experience with a carburetor so I figured I'd ask people who know more before I blindly troubleshoot.

    Otherwise it seems to run very well, so no other symptoms.
     
  2. Nov 25, 2018
    timgr

    timgr Jeepin' Nerd Sponsor

    Medford Mass USA
    Joined:
    Aug 10, 2003
    Messages:
    19,156
    Year? Can't say much without that. Presume you mean 258. Factory carburetor? Any modifications?
     
  3. Nov 25, 2018
    timgr

    timgr Jeepin' Nerd Sponsor

    Medford Mass USA
    Joined:
    Aug 10, 2003
    Messages:
    19,156
    Looking at your old posts, it appears to be a 1976 model. There is a TSM for 1976 here: JeepĀ® 1976 TSM online You want section 4, the YF carburetor. I suggest you read the section about the choke circuit first, and the electric assist. I'm not that savvy re the YF, but if you have questions, post up and I'll try to help.

    The behavior does not sound abnormal to me. Possible the fast idle is set too high. You have a dwell/tachometer? Measure the speed of the engine when cold, and compare to the TSM spec.

    This may not be exactly like the YF, but this is how it works in general. There is a fast idle cam that holds the idle speed up when the engine is cold. This is separate from the choke flap, which controls the cold-idle mixture. Both the choke flap and the fast idle cam are controlled by a bimetallic spring that coils or uncoils in response to temperature. The mixture must be rich initially since gasoline will tend to condense on the cold manifold interior and not reach the cylinders. There will be an adjustable fast idle stop that climbs the fast idle cam when you first press the accelerator and the engine is cold. This holds the throttle plate open. As the engine warms up, the fast idle cam tends to rotate and release the fast idle stop. Typically the fast idle stop will not ascend or descend the fast idle cam unless you open the throttle and release the pressure on the stop. You can also adjust the initial tension on the choke thermostat (bimetallic spring) to adjust the rate of warm-up. The TSM should have instructions for adjustment.
     
    Last edited: Nov 25, 2018
    Kevin M likes this.
  4. Nov 25, 2018
    Kevin M

    Kevin M New Member

    Denver Co
    Joined:
    Nov 24, 2018
    Messages:
    7
    No tachometer. I actually kind of like this as it makes me feel more in control, my last truck didn't have one either.

    Thanks for the link I will read it over. Maybe it isn't abnormal behavior, the engine isn't redlining or anything like that, and it is really cold out so I could see it taking a bit to warm up.
     
  5. Nov 25, 2018
    PeteL

    PeteL Member Sponsor

    Hills of NH
    Joined:
    Aug 3, 2003
    Messages:
    5,880
    Are you giving it a blip or two during warm up, to release the fast-idle cam?

    Take a look down the carb throat when fully warmed up, the choke should be completely open.

    And Timgr has the right info.
     
  6. Nov 25, 2018
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member Sponsor

    Denver, CO
    Joined:
    Jul 18, 2013
    Messages:
    1,678
    I think Tim is asking if you have a tach or dwell meter in your shop tools - not a dash tach gauge. The tool will allow you to tune the engine correctly.
     
    Kevin M likes this.

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