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Mikuni BSR 40 Accelerator Pump Screw Floating

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  • Mikuni BSR 40 Accelerator Pump Screw Floating

    I have a question about the Acceleration Duration screw.




    From this article : http://www.roadstarmagazine.com/2003...pump/#more-149




    and a virtually identical description on another form.




    My bike is a 2003 Yamaha Road Star. All stock when I got her.




    My basic problem is that I did a carburetor rebuild on my Mikuni BSR 40 and instead of improving matters, things go worse. Hitting the gas did not make the engine take off quickly as it should, and there is now a lot of backfiring through the carb.




    According to that article, the bogging problem could be alleviated by adjusting the 'Accelerator Duration Adjustment Screw' outward to increase the amount of fuel squirted. However, upon investigating this screw, it seems to be floating. Not that it's loose, it just never touches the black plastic piece underneath that is supposed to adjust the amount of gas being squirted when the throttle is hit.




    Does anyone know approximately where this end of this screw should be touching? According to the article, turning the screw (it is secured by a lock nut) clockwise should decrease the amount of fuel provided. So, just guessing . .. being retracted so that the screw never touches the lever underneath, it should be squirting the maximum amount of fuel.




    Does anyone know an approximate starting position that this adjustment screw should be set at? Is the screw too short? There is also one on the back side that sets a quantity. Yes, confusing, they both say quantity, but really one is duration and the other is quantity.




    Also, all that I have done, engine wise, besides new plugs, is to put on a high flow K&N filter. Not the small one that replaces the OEM, but the triangular large RK Series that requires a new outside plate to hold it in place. None of these changes should be causing the backfiring, sputtering, and bogging down when the throttle is applied. I've made no changes. I used the needle valve that came with the kit, the Non-adjustable one with a single notch and the main jet replacement was the same original size of 165. Testing is done with the air box off anyway, so air flow availability is at maximum.




    I would love to hear any other suggestions on solving the backfiring problem. So far, I have increased the turns outward of the Pilot Jet screw from the original 2 1/2 turns out to 3. I have a replacement Needle Jet on order that has cut slots so that it can increase richness. I don't know yet if I should be changing things into a more lean direction. Advice on that would be greatly appreciated.




    Thank you in advance for any advice.



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  • #2
    The screw should be touching the black plastic piece but it only does so while twisting the throttle. When the throttle is steady, mine doesn't touch.

    Are you sure its floating when you turn the throttle?

    You are correct turning the screw clockwise decreases the squirt. I have moved the nut on the other side so I can turn it in more. I did this because I could smell the gas each time I got on it while riding.

    Default setting of the screw is almost all the way down to the nut with a very small gap between screw head and nut. It is my understanding that this is usually too rich.

    From your pic it looks pretty far out = very rich

    Backfiring could have other causes. I would start by checking for air leaks on intake side with starter fluid and visual inspection of manifold. Then check exhaust leaks working from cyl ex manifold gaskets back the pipes.

    Once all of that checks out go back to PMS, then jets and maybe just maybe accelerator pump.

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    • #3
      Originally posted by Druegeme View Post
      Are you sure its floating when you turn the throttle?

      You are correct turning the screw clockwise decreases the squirt. I have moved the nut on the other side so I can turn it in more. I did this because I could smell the gas each time I got on it while riding.

      Default setting of the screw is almost all the way down to the nut with a very small gap between screw head and nut. It is my understanding that this is usually too rich.

      From your pic it looks pretty far out = very rich

      Backfiring could have other causes. I would start by checking for air leaks on intake side with starter fluid and visual inspection of manifold. Then check exhaust leaks working from cyl ex manifold gaskets back the pipes.

      Once all of that checks out go back to PMS, then jets and maybe just maybe accelerator pump.
      I'm pretty sure. I tested it with some quick shots, and it didn't seem to touch the plastic piece. I'm trying to find out if things are set too rich or too lean, then I'll be pulling the carb again and either changing some jets or checking for other problems. I'll try and get those screws set to the right positions when I have the carb off.

      I am beginning to suspect that it is running to rich on acceleration. I could see the fuel squirt in to and I've seen videos where it didn't appear to be as much.

      Thank you for your input, this is going to help.






      Comment


      • #4
        You have an open element air cleaner and you need to jet for that. What exhaust do you have? If you have aftermarket exhaust along with this open element air cleaner you need to do a 167.5 main jet and a 37.5 pilot jet given the changes. An adjustable bearings needle would be ideal as well.
        LET'S RIDE

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