Carb sputter after cam upgrade

Discussion in 'Engine, Fuel and Exhaust' started by glidenwide, Mar 6, 2010.

  1. glidenwide

    glidenwide Member

    I just took my first ride after doing the cam conversion I went with the 26N. Thank you to all who put up so much info. i really didnt need to ask any questions it was all right here. OK so now when im cruising in 3 gear doing 25-30 or any low RPM speed it has a hesitation and sputter coming from the carb. not constant but almost like a hiccup
  2. TQuentin1

    TQuentin1 Well-Known Member Staff Member Moderator

  3. fin_676

    fin_676 Experienced Member Staff Member Moderator Contributor

    As you have adjusted the time that the valves are open sucking in fuel/air mix you may have to adjust the fuel/air mix that the carb is producing as your problem is in the lower rev ranges then id suspect that either the slow jet needs changing or the shim height of the needle will need adjusting

  4. glidenwide

    glidenwide Member

    Sorry I forgot to put this in there it is a mikuni hsr42.
  5. fin_676

    fin_676 Experienced Member Staff Member Moderator Contributor

    It does not matter which carb is fitted the cam change has changed the suction of the engine by adjusting the time the valves are open you will need to re tune the carb to take account for this as im not aware of what this cam setup will do i dont know if you need to make it richer or leaner but if your getting more air in id suggest next size up on the slow jet

  6. Bud White

    Bud White Well-Known Member Retired Moderators

    why are you doing 25-30 in 3rd gear kinda seems like lugging to me .. you might not be spinning the motor enough with the carb you got .. do you have a adjustable accell pump?
  7. Chopper

    Chopper Senior Member

    The mikuni's are more temperamental then Keihin carb's are, here's a link to a mikuni tuning manual might help? Good luck.

    Harley Davidson Community
  8. jaxdwg

    jaxdwg Active Member

    I'm not familiar with the Mikunis on harleys but if it has a circlip adjustment on the needle like on the old ones, I would raise the needle one notch. It sounds like you may be lean on the lower end. Have you checked to see whether you are rich or lean by either checking plugs or sticking a finger in the exhaust to see if you get any soot on your finger as a basic starting point to go by??? you haven't given us much info to work with so any credible answers will be tough to give other than very general----jack
  9. glidenwide

    glidenwide Member

    It doenst happen often just so happens i went through a small town that was having election day yesterday. i didnt really want to make to much of a racket.

    there is some soot in the exhaust and the plugs are darker then they should, i had them out while doing the cams. Honestly i dont know much about carbs or bikes for that matter thats why im here. To learn
    Last edited by a moderator: Mar 7, 2010
  10. BUBBIE

    BUBBIE Well-Known Member

    On the 42 mikuni there is a BIG brass plug on bottom of float-carb. to remove..... GAS WILL COME OUT when you remove it..... the MAIN JET sits there in plain view.....
    Straight bladed screwdriver to CAREFULLY remove it and with YOUNG EYES,,,, A MUST,,,, you should be able to see the size of that jet..... LOOK at the SCREW DRIVER SLOT END on jet.. size stamped there....
    the carbs usually use come with a 155 installed, mine did.... a .150/.155/.160/ and bigger can be had...
    If your jet is a .155 or .160 you need to go to a .150 or .155 respectfully... IF IT was RUNNING RICH... usually 5 represents .005 thousands of a inch... and .005 will change the fuel mix a lot..

    There is a SLOW SPEED up and behind the MAIN JET and for now I would leave it alone... it comes in .0025 increments and is located in a tunnel and takes a very small screw driver to take it out.... I have found the one it comes with to be close enough and by setting IDLE MIXTURE as described will prove it to be the right size OR NOT.....

    the IDLE AIR ADJUSTMENT is in plain view toward rear side on mikuni... DON't TIGHTEN DOWN on it hard but .... CLOSE IT then GIVE IT 2 FULL TURNS OPEN.... START BIKE after bike is warmed WARMED means HOT from a RIDE not just sitting and idled to a warm condition..
    turn the screw in to listen if it raises the RPM a little or OUT to raise and smooth out the idle.... THAT KNOB on the flex cable is JUST TO SET engine SPEED at idle... NOTHING to do with FUEL MIXTURE just IDLE RPM adjustment. 1000 to 1200 RPM is needed by most HD's and TOO slow at 950

    IF YOU LUG THE BIKE AS SLOW AS YOU Said YOU WERE IN 3RD GEAR ..... THAT being the problem..... HARLEYS need to be RUN at NO LESS in these gears, but more if wanted.

    1st= up to 25
    2ond= 25 to 35 no less
    3rd= 30 to 45 no less
    4th= 40 to 60 no less
    5th 55 to---- no less
    6th NOT UNDER 60.....

    THIS IS A MUST lugging under these speeds in the gears LISTED will DO HARM TO ENGINE!

    the MIKUNI tuning book will show you all that I talked and then some.. good to download...