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  1. #1

    Main Jet issue on a 38mm sbn mikuni

    Hi all, I just picked up a 1993 sl750 the previous owner has installed new a tripple out pump, oil block off, primer, and removed the choke and oil injection. When I bought it the thing started no problem but the engine raced at idle. I remove the gas tank/lines, clean them and installed a new fuel filter. With the carbs I just pulled the bowl off and cleaned it out as well as the idle and main jets, I didn't do a full rebuild. Now the issue I am having is on the pto cylinder, the main jets just dumps fuel at idle even with the needle seated. The needles were all mushroomed where the seat hit so I sanded them all down smooth and it definitely helped but I am still getting the fuel from the pto main jet at idle with it seated. Is this issue still with the seat and needle or is the pop off pressure too high or something else needs to be cleaned? I know I need to do a full rebuild on them but I was hoping to wait until the winter.

  2. #2
    Banned User
    Join Date
    Jun 2010
    Milwaukee WI
    You definitely need to replace those needles and seats. If those don't seal your engine will never run right.

  3. #3
    johnsonmtz's Avatar
    Join Date
    Oct 2006
    Mt. Zion Illinois
    Quote Originally Posted by BryanP View Post
    You definitely need to replace those needles and seats. If those don't seal your engine will never run right.
    +1 In fact, I'd suggest you just get full rebuild kits so you know for certain that all components in the carbs are in top shape.


  4. #4
    I know I need to replace the needles/seats and I need to rebuild them I have sanded the needles on other carb and it has worked great in the past I guess I just dont understand why this carb is shooting tons of fuel out of the main jet even when I have the needle seated. Does the Main needle control air flow? Like the photo below? If so then at low engine rpm how would I flow enough to push fuel out of the main jet? Seem like I have too much pressure in the bowl and it is kicking fuel out of the jet. Also when I kill the engine it dumps the most fuel out of the main jet of that carb.

    Click image for larger version. 

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  5. #5
    Well I found the issue, the dowel that holds the float lever into position was not seated all the way down so the pump was continually pressurizing the carb. But now when I rev it there is back firing into the carb, I hope I didn't hydro lock that cylinder one time

    Edit: Well it is popping from all the carbs (running in my garage) I think it may be from it just being cold. Im going to hit up the water today and see if it runs fine on the water. It also sounds like it bogs out when I rev it up and it starts popping into the carbs

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