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Thread: Re-Jetting Carbs

  1. #1
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    Default Re-Jetting Carbs

    Preface by Don Nelson (Freebird)
    When Don Murray told me that he was tackling this project, I asked him if he would take some pictures so that we could add them to our tech library. He not only agreed to do so but did a fantastic job of it. Now....is this something you want to do? You'll just have to decide for yourself. The last I heard from Don, he had gone a jet size larger and though he liked the way it ran, a sparkplug inspection seemed to indicate that it was running a little rich. So..he then went a size smaller and didn't feel that it ran as well. So I think he has now gone back to the original main jets and in his case, feels that only the size larger on the pilot jet was needed. That and shimming the needles. Don did a great job so if you find any errors here they are probably my fault in editing. Let me know and I'm make any corrections needed.
    Re-Jetting the Venture Carbs
    Thanks to Don Murray for this great write-up
    After you have removed the carbs and cleaned them set them on a table facing the way you see them in the first picture...it may help you to number them as you see in the picture...you will need to refer back from time to time to make sure you're working on the right carb. The reason I say this is because only two carbs use the same size jets.(1.)

    Now you'll want to remove the bowls, it may help keep you organize to only remove the bowl of a single carb at a time so you don't install the wrong jet in the wrong carb....don't ask how I know....okay you'll need a #10 allen wrench with a ball end if you don't want to mirer up the bolts...after you get the bowl(s) off this is what you'll see....(2.)(3.)
    This jet is the main jet! From the factory (stock) it is 122.5 carb #1

    122.5 carb #2
    117.5 carb #3
    120 carb #4
    now do you see why I said only remove one bowl at a time....now this only matters if you're changing jets.



    Here is the main jet after it is removed. It is the one being pointed to with the stick.



    Okay lets say you want to increase the fuel consumption and maybe increase the HP's (maybe).....without a dyno it's anybody guess if you increased your HP. Okay here's what Joe had said about the correct jet sizes and it worked for me.... carb # 1 up 125

    carb # 2 up 125
    carb # 3 up 120
    carb # 4 up 122.5
    along with this jet size increase you should also be using K&N air filters and the "D" cutout in the air filter holders.


    Now what I found to be a big change was to change the pilot jet....the stock one is 15..replace it with a 17.5 (5.)


    Here is what the Pilot Jet looks like after you remove it. Again, it is the one pointed to with the stick.


    The next thing is shimming the needle valves. This is relatively simple and can be done without removing the carbs from the bike if this is the only thing you wish to do. What it will give you is quicker throttle response.
    Not much commentary here. The pictures tell the story pretty well. Fist thing is to remove the black cover that covers the diaphragms.
    The spring will just lift out.
    This is what you have at this point.
    This is the needle plug holder. Pull it straight out and the diaphragm, slide and needle will lift out.


    There is an O-Ring here for the vacuum to the diaphragm. Be sure not to lose it.

    Now...pull straight out on the holder to remove the needle.

    Here is what the needle assembly will look like.
    You will want to add some washers to the needle and diaphragm. Now I don't know what Don Murray did but I simply removed one of the washers from the top and and slid it up from the bottom. That shims the needs a bit and is where you get the quicker throttle response. Just disassemble the unit and move one of the washers or buy extra washers from your dealer.

    Okay...now you just need to put it all back together. After it's back togeher, check the slide.
    Move it up and let it fall.

    Use your finger to run this test.

    If they fall fast rather than just float down as they should, then you probably didn't get the diaphragms back in properly, no sealed. Remove and reinstall them.
    And that folks, is about it. Reinstall the carbs and go for a ride. If it doesn't run to suit you, just start all over. Jetting is NOT an exact science and unfortunately, it sometimes takes two or three tries to get things the way you want them. Is it worth all the trouble? I don't honestly know....if you try this, maybe you can tell us.






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    Default

    Thank you for posting this again.

    When you moved the washer, did you just slip it on the bottom side of the lock ring or did you slide it up the needle on the other side of the teflon thing?

    Also, did you have any trouble lining up the diaphragm to go back into the slots? Mine seem to be kind of stretched out and don't want to just pop back into the groove like I've seen on other bikes. I found that a previous owner or mechanic closed one back up a little too carelessly and smashed the edge of the diaphragm with the cover.

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