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Technical 51 Mercury carb choke plate adjustment?

Discussion in 'The Hokey Ass Message Board' started by solo_909, Apr 13, 2019.

  1. solo_909
    Joined: Apr 9, 2006
    Posts: 1,763



    I have a 51 Mercury and need help with the carb choke plate adjustment.

    So no matter what I cannot get the plate to open up all the way or the idle to drop
    After the flathead is warm and I press on the gas. I need to know what I’m doing wrong.

    Here’s what I was told.

    With the weather between 65-70° loosen the carb choke spring housing. Press the gas pedal in between 3/4-1/2 down and turn the choke until the plate opens then turn back in the opposite direction until the plate closes softly. Then tighten the choke spring housing and it’s set.

    Here’s the issue, doing this several times the choke plate never fully opens up when fully warmed up and the idle screw never drops onto the lower lobes dropping the idle.

    First question - What lobe should the idle screw be resting when the motor is cold? Right now it’s on this one.

    Second question - I live in California so would I still need to press in the gas pedal when making the adjustment? It seems that by doing that it would keep the carb choke plate closed longer and if the spring doesn’t unwind that much when hit it would stop the plate from opening up all the way.

    Third question - typically how many lobes will the idle screw move when it’s cold going to hot?

    Any help would be greatly appreciated.

  2. solo_909
    Joined: Apr 9, 2006
    Posts: 1,763



    So I let the car cool down, I adjusted the bar on the left so the idle screw was on the second highest lobe. I then adjusted the choke without pushing the gas pedal down and made sure it only closed ever so slightly but the choke plate still will not open up completely when hot.

    When the choke plate is fully closed the car stumbles, has a rough idle and runs super rich (smoke out the tail pipes). Once the carb warms up a little bit the plate opens up maybe 1/8” and the idle goes up and runs smoothly. As I let the motor get hot the choke plate will only open like in the photo max and the idle will only drop slightly.

    No idea what’s going on as the heat tube, manifold heat tube are all clean and clear.

  3. inthweedz
    Joined: Mar 29, 2011
    Posts: 376


    The adjustment you describe is spot on with type of choke setup..
    I am more used to Zenith and Chevy carbs, with the same setup, so I'll give my 5 cents worth..
    Is everything is working/assembled correctly inside the plastic cover??
    When the engine is at operating temp, the stepped cam should release right back to the last stage, with a small amount of clearance to the adj screw..
    To check this adj, open the throttle slightly and fully open the choke flap..
    Let the throttle go while holding the choke open, check that there is a small gap (approx 1/32'') between the end of the screw and the cams lowest point..
    If everything is ok, and it still won't release fully, you could get the engine to operating temp, and set it in reverse to the cold setting..
    As in slacken the screws holding the plastic cover, open the throttle shaft a tad, and turn the cover until the choke is (softly) open..
    This will make sure that you get full open..
    Hope this helps..
    Last edited: Apr 14, 2019
    solo_909 likes this.
  4. get the motor to operating temp, then adjust the choke till it is open all the way. it should only come off "high idle" when the choke is fully open.
    solo_909 likes this.
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  5. Petejoe
    Joined: Nov 27, 2002
    Posts: 10,406

    from Zoar, Ohio

    Not related to your question but still related.
    If running the original loadamatic distributor be sure your vacuum advance diaphragm is working and operating correctly.
    The rubber diaphragm fails over the years.
    solo_909 likes this.

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