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Thread: 2M carb linkage / pump plunger question

  1. #1
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    2M carb linkage / pump plunger question

    Hi JNC,
    Can anyone shed a bit of light on my 2M carb linkage here?

    Here's the question: the pump plunger action doesn't seem to be hooked up correctly. I'm undoing a previous owner's tinkering, so I don't really know how it was set up originally. I've got the 2M engine manual, and as far as I can tell everything is hooked up as it should be, but this means the pump plunger action on the carb is close to nil.

    example: when the throttle is open fully, the spring is compressed, and the plunger doesn't move:


    when the throttle is closed, the spring is uncompressed, and the plunger is still in the same spot.


    It's only the spring on the pump plunger linkage that moves, and not the pump. You'd think that there should be some sort of washer on the linkage to get the pump to move, but that's not how it looks in the engine manual, and there's no pinch points on the linkage that look like this existed already.

    This is a 1970 2M carb. I have a 1974 4M carb as well, and the linkage there is different: the rod is at a 90 angle at the end, and the plunger moves a lot when the throttle moves.

    Any hints on this one?

  2. #2
    Senior Member
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    Re: 2M carb linkage / pump plunger question

    Something is seized on the plunger side, or the accel pump port is plugged (causing the plunger not to move due to hydraulic lock)?

    The spring there looks ok to me.

    What happens is when you stomp on it, the plunger is supposed to go down at a controlled rate. The spring allows this to happen - the output port size controls how quickly the plunger goes down.

  3. #3
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    Re: 2M carb linkage / pump plunger question

    Quote Originally Posted by ToyGuy
    What happens is when you stomp on it, the plunger is supposed to go down at a controlled rate. The spring allows this to happen - the output port size controls how quickly the plunger goes down.
    Hmm, this makes sense. As it is now, when the throttle is fully open, the plunger has nowhere down to go, which makes me think that it doesn't have the correct plunger in there; too short maybe.

    Will investigate the possible clog / plunger question - thanks.

  4. #4
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    Re: 2M carb linkage / pump plunger question

    For future reference if anyone else has this issue - the plunger in my carb is the wrong size. It's 64mm, and it sounds like some 2M carbs had a shorter plunger, which would make the linkage / spring setup operate like it should.

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