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poor running after carb rebiuld

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    #16
    ok so your carbs have 3 circuits to them
    each one controlling a different throttle range (not to be mistaken with RPM range or speed) the range i am talking about is how far open the throttle slide is

    circuit 1
    the pilot (idle) circuit it controls from 0 to 1/4 throttle opening
    there are 3 adjustments to this circuit

    1) the pilot jet this small jet meters the fuel taken from the float bowl into the idle circuit and is marked with a number like 15 or 17.5

    2)the pilot fuel screw (the small screw on the bottom of the carb along the front edge of the float bowl (front edge being the edge closest to the cylinder bank)
    (this screw is usually 3/4 to 1 turn out from lightly seated) in your case more likely you will need 1 turn out (the more you turn this screw out the richer the fuel mixture will get)

    3) the air mixture screw (this screw is on the side of the carb body) this screw adjusts the amount of air that gets added to the idle circuit usually in the region of 1 1/2 to 2/1/2 turns out from lightly seated
    (the more you turn this screw out the leaner the mixture gets)

    that is the pilot (idle) circuit and its adjustments the pilot (idle) circuit is probably the most difficult circuit to adjust

    Circuit 2
    the needle jet and jet needle controls from 1/4 to 3/4 throttle opening
    this circuit is fairly simple to adjust
    the needle jet is the long tube with tiny holes in it that the the main jet screws into it has a letter and number designation ( O-6 )
    and the jet needle is the thin needle that comes out of the throttle slide and usually has 5 adjustment slots in it that you can put a clip in the number on it is usually similar to this 5DL31
    (the farther away the clip is from the tapered end of the needle jet the leaner the mixture will be)
    reminder( the main jet size affects this circuits operation )

    Circuit 3

    the main jet circuit controls from 3/4 to fully open throttle opening
    this circuit is fairly simple to adjust
    it is adjusted by changing the main jet it self to a different size ( it is marked with a number like 125 or 122.5
    the higher the number the more fuel it will allow into the carb )

    now also be aware that each of these circuits overlap to allow for smooth operation so getting the carbs set up right can be a painstaking experience to say the least

    i hope this helps explain how the VM carbs circuits and adjustments work and perhaps points you in the right direction that you need to go

    going by what you have posted thus far the main jets are more than likely not the cause of your problems
    i would venture a guess that you have a problem with the pilot (idle) circuit
    Last edited by Guest; 03-27-2015, 09:09 PM.

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      #17
      Brillaint to say the least...and thank you for that, I am going to print it for reference

      RB

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        #18
        you are more than welcome
        to add to this once you get it running you need to ride it because it will tell you what you need to do by doing plug chops

        a plug chop is riding at a specified throttle range for a period of time and then stopping the engine using the kill switch while pulling in the clutch to allow it to stop
        then you take out each plug look at it too see what colour it is on the center electrode your looking for a light tan to dark tan colour then you know the carbs are set up correctly
        if the electrode is white then your too lean
        if the electrode is black then your too rich

        reading the plug

        at the tip of the electrode to 1/4 down is the pilot circuit
        from 1/4 to 3/4 down is the needle jet circuit
        from 3/4 to the bottom is the main jet circuit

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