Mikuni Jetting

Discussion in 'Airheads' started by DeutschMark, Sep 16, 2012.

  1. DeutschMark

    DeutschMark Full Tilt Boogie

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    Does anyone have the jetting specs from their Mikuni carbs, specifically VM34? I'm interested in comparing what I'm using, specifically to what Rocky Point Cycle has in their kit. Here is what I'm currently running with dual plugs and the needle second from the top.

    6DH4 Needle
    2.5 Slide
    185 Main
    PO (159) Needle Jet
    35 Pilot Jet

    My main jet is probably a little rich. I think the 32mm Bings used a 145. The bike runs strong and snappy in this configuration but I think I'm losing some power on the top end.
    #1
  2. Kai Ju

    Kai Ju Long timer

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    Here is what I'm running on a 77 R100/7 in the VM 38's. Stock (Repro ) mufflers, stock airfilter and airbox.

    Description Spec
    Main Jet 210
    Pilot Jet 25
    Needle Jet 166 P-8
    Jet Needle 6DH3 middle groove
    Slide Cut Away 2
    Main Air Jet 0.5
    Needle Valve Ass'y 3.3
    Idle air screw 1 turn from seated


    I started with a 17.5 pilot which was way too lean and a 200 main, which felt too rich. Going to a 190 made it worse while a 210 made it better. So that's what I'm running now.
    I compared the stock Bing mainjet to the 200 Mikuni jet by inserting a very shallow taper reamer and the Bing mainjet, a 145 I believe, is actually larger in hole diameter than the 200 Mikuni mainjet.
    #2
  3. DeutschMark

    DeutschMark Full Tilt Boogie

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    Thanks Kai. I now vaguely recall that Mikuni jets are rated differently.

    I did some searching and found out that Mikuni jets are rated at cc/min rather than actual diameter of the hole in the jet. So a Mikuni 185 flows 185cc/min.

    Still looking for more VM34 combinations to compare.
    #3
  4. DeutschMark

    DeutschMark Full Tilt Boogie

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    I ordered up some main jets in 170, 160, and 150 sizes to give me a rough idea of what I was looking at. In the 170 package, I received a 180 along with the 170. I have since received the correct size but I haven't tried it yet.

    I immediately noted a change in the engine note and feel with the 160. Redline came easier in 3rd and 4th gears with a more eager note. When I tried the 150's, I noticed the upper rpm response was dull and and rpm increased when I backed off from WOT as the mixture richened. Definitely too lean.

    I'm going to order some 165's to see if anything changes. My riding season temperature in Michigan ranges from the low 40's to the 90's so the 165's might like the cool, oxygen dense, late fall air.

    Does anyone have any other jetting combinations they'd like to share??
    #4
  5. R100LT

    R100LT Chasing 11

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    What model are you running these on Mark ???
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  6. DeutschMark

    DeutschMark Full Tilt Boogie

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    '89 1000cc twin-plug 9.4:1
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  7. R100LT

    R100LT Chasing 11

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    Did you get the carbs from Rocky Point Cycle.

    That will my project sometime next year with my 91 RS. Thinking of the 34mm ... 336 Cam ... Dual Plugged and the 9.4:1 also.
    #7
  8. DeutschMark

    DeutschMark Full Tilt Boogie

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    I bought them second hand for a song so I'm not sure where they originally came from. They ran great right out of the box. The VM34 is probably a little small for this application.

    I'm running the Edelweiss/Scheffer asymmetrical camshaft.
    #8
  9. elmontanero

    elmontanero Practicing...

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    You decide to rid yourself of those awfully small v34's let me know.

    :lol3:lol3
    #9
  10. R100LT

    R100LT Chasing 11

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    I was thinking so getting 36 or 38 mm Dells myself ... I have a few months more of research to go before pulling the plug
    #10
  11. Uncle Pollo

    Uncle Pollo Long timer

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    You are the only one that will know, because you are in your own air density ... the bike is running ... right?

    Good ... now bend over and change jets and test.

    Unless you have access to a dyno and a lot of money ... at least to me.

    Nothing replaces a dyno, but is it going to be happy?

    I have a 140 here at 2k meters over sea level on an r80. different bike. Don't go too much by what others do. If you think you are going too rich or too lean on full throttle test accordingly.

    Go fast, pull clutch in and kill engine, coast to a stop, pull the plugs and read.
    #11
  12. Uncle Pollo

    Uncle Pollo Long timer

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    and the jump from 145 to 180 is huge ... you would be blowing smoke everywhere if it was too rich.
    #12
  13. Airhead Wrangler

    Airhead Wrangler Long timer

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    Actually it's not. Mikunis and bings use a different numbering system for jets. Bings use orifice diameter (not sure of units though), mikunis use flow (cc/min) at some arbitrary pressure difference.
    #13
  14. DeutschMark

    DeutschMark Full Tilt Boogie

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    Look at the first post, the bike runs very well

    That plug reading method hasn't worked for me in a while. The fuel is too highly refined to get much of a reading, at least for me.

    As far as others are doing, it is always a good reference point but I'm not too stuck on it. Others can benefit with a baseline to work with also. I was PM'd some specs on another 1000cc twin spark that was very close to mine. The bike was set up by a well respected tuner.

    No smoke was produced with the 210 main jets that came in the carbs. The twin plugs are very tolerant of jetting variations, that is why I'm curious about
    other combinations.
    #14
  15. Uncle Pollo

    Uncle Pollo Long timer

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    20 something percent more flow. Baby steps.
    #15
  16. Airhead Wrangler

    Airhead Wrangler Long timer

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    The 145 is a bing jet. The 180 is a mikuni jet. You can't compare those two numbers apples to apples.
    #16