Changing to Mikunis

Discussion in 'Airheads' started by Velocipede, Jun 9, 2012.

  1. Velocipede

    Velocipede Been here awhile

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    Bike Bandit has Mikuni Carbs for what seems like reasonable prices.They range from round slide 36/38mm spigot mount VM series to 38mm TM series.To my uneducated eye the 38mm VM series looks to be the easiest to fit to an airhead.My GSPD is the Canada model with 40mm carbs.

    The original carbs on my bike have over 90,000 miles on them and need a rebuild so I'm wondering how the Mikunis might work.I would be interested to hear from any inmates have done this swap and what were the results.


    John
    #1
  2. disston

    disston ShadeTreeExpert

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    Could you supply a link to where these are advertised or sold.

    Sorry can't answer your question because I haven't done the Mikunis although I have considered it. Other that the cost the main problems seem to be adapters to fit the plumbing and cables that fit. If these don't come in some kit form then they will have to be fabricated.

    In general riders report good things from Mikunis on Airheads.
    #2
  3. daveoneshot

    daveoneshot Been here awhile

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    I put Mikunis on a '71 R60/5 years ago and it was the best upgrade, ever. Had to trim the choke lever on one of them to prevent my pants from snagging it, but that's all. Rocky Point Cycle set them up, a phone call describing your altitude and whatnot was all they needed to jet them to your bike's personality. Put them on and go.
    #3
  4. supershaft

    supershaft because I can

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    90,000 Miles? Your Bing's are most likely damn near perfect. Rebuild? Chances are the only thing they need are new needle jets and jet needles, enricher gaskets and a few O-rings.

    Mikuni's? The downside is mostly the airbox snorkels. The best setups I have seen still are not that great. I have seen a lot of them letting dirt in big time. Then the flat slides have independent floats that very often do not work right just like Bing's "alcohol proof" independent floats which I believe they have since stop selling? They at least now offer an alternative regular "alcohol proof" float. Independent floats flood the carbs a LOT! I see it ALL the time. Both in Bing's and Mikuni's. Excepting the independent floats, they are better carbs. Better performance and mileage. But then chances are some real good jetting skills will come in handy to get there or at least get close to as good as they can be.

    The BMW 40mm carb spigots to the small port heads have a very abrupt taper. They look like they would cause flow issues but I know from experience that looks can be deceiving when it comes to what flows better. But still . . . .

    I run Dellorto's mainly for the better snorkel fit. With the later square airbox setup you can file about a third of the Dellorto's velocity stack threads off and the original snorkels will slide on as if they were Bing's for a super clean looking setup that works!
    #4
  5. mfp4073

    mfp4073 Long timer

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  6. mykill

    mykill odd

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    I suspect they will come jetted far from what you need. The Rocky Point kits look expensive at first, but start to make sense quickly when you consider cost of jets, cables, etc. 34mm Mik's are the biggest that come R&L.
    #6
  7. supershaft

    supershaft because I can

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    +1 Besides, you very rarely need to spend even close top $120 per carb to fix your Bing's. Plus, not only do you have to spend money on jets, you need to know which ones to use. I hate to say it but most people don't have a clue. Most people never get past the fact the jetting requires a lot of jets.

    I wonder why Rocky Point only has 34mm Mikuni's?? Too small for most models IMO. I think they would maybe be right for a R65? If RP carbs are jetted good I would bet that is a good place to start with any size round slide. The expensive part is if you need richer slides and you often do with bigger carbs.
    #7
  8. Kai Ju

    Kai Ju Long timer

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    VM 38 Mikunis on a 1977 R100/7

    I just finished my VM38 Mikuni installation and probably did it the hard way.
    I bought a pair of bodies on E-bay only to find out that one of the float pivot needle towers was broken off. I got another one at the Long Beach swap meet and then bought the jets to suit from Sudco. Total investment is less than $200.
    Modified the stock throttle cables to work with the Mikunis and used some bicycle cables for the choke using the Mikuni cable choke plungers and the stock lever set up on the stock air cleaner case.
    The stock throttle cam off an R100/7 won't open the slides all the way, I added an extension to the throttle cam to do so, made from Delrin. I didn't want to spend the money on an R90S throttle cam which I assume will work as is since it has to open straight slide carbs as well.
    The stock elbows are slightly smaller in OD than the ID of the bellmouth and I used silicon tape to make up the difference. It's two layers thick and is a squeaky tight fit.
    Since they are both right hand carbs, i.e. the idle and air screws are on the right side of the body, I added and extension to the air screw for the left carb which allows me to adjust the screw with my fingers, the idle screw is already long enough and grooved to be able to adjust it.

    The engine runs awesome, starts and idles like it should and I went from getting 34 mpg to 40mpg.

    Here are the jetting specs for anybody interested:

    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

    Here are a couple of pics of the installation:

    [​IMG]

    [​IMG]

    [​IMG]
    #8
  9. carpetburn

    carpetburn Been here awhile

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    I run the VM34s purchased new on ebay a few years ago, fitting the air cleaner pipes to the carbs is a hassle and mine is a bit crude looking[supershaft is correct there]but functional, i think they respond better than the 32mm bings they replaced with a lot lighter wrist/throttle action, also very easy to tune[i use a colortune and take it from there]
    Overall i would definately reccomend but if somebody offered me a pair of 38/40mm dellortos i would love to give that a try
    #9
  10. mfp4073

    mfp4073 Long timer

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    my 900 is getting 41 on original bings and a 100k motor. Not sure I see a bonus to going with them. Seems like if you were getting 34 something else was wrong that needed fixing with your carbs.
    #10
  11. JRP

    JRP Old guy

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    Installed these for a friend who got the carbs as part of a deal, I ordered the cables from Rocky. The bike seemed to run quite well after.

    [​IMG]

    [​IMG]
    #11
  12. oldroadie

    oldroadie Two wheel addict

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    I will gladly sell my 64/32/3&4 Bings to any Bing lover here to finance a new set of Mikunis. There, I said it! This model of Bings is lame, lame, lame...
    #12
  13. Velocipede

    Velocipede Been here awhile

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    Thanks for all the replies,I think I'm going to give the 38mm VM carbs a try.I'm going to wait until the short Canadian riding season is over as it looks like it will take a while to get them set up.

    Thanks John :clap
    #13
  14. supershaft

    supershaft because I can

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    How did you get to those jets Kai Ju?
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  15. Kai Ju

    Kai Ju Long timer

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    You're missing the point. I'm getting stronger performance and 10% better mileage with the Mikunis.
    There was nothing wrong with the Bings, I'm just a bit heavy handed with the throttle and you're also comparing an R90 with an R100.
    #15
  16. Kai Ju

    Kai Ju Long timer

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    Do you mean the specs or the jets themselves ?
    Thanks to a friend I got both from Sudco.
    The original specs were close but needed a bit of help. They called for 17.5 pilots, not even close since the engine would only idle with the airscrews seated. I then tried 22.5 which brought the airscrew setting to 1/2 a turn out and now I'm running 25s with the airscrew right at one turn out. I could probably go to 30 since the plug body is clean with no carbon whatsoever around the perimeter.
    The mainjet spec was 200 and the engine would gain power/rpm if I closed the throttle a touch. Made me think rich, so I tried 190s and it got worse. Went to 210s and I'm a happy man with a happy engine.
    The rest of the specs seem to be right on the money. I'll ride for a week and see what happens. So far the plugs are burning very clean with just a hint of color at the tip and no detonation.
    #16
  17. supershaft

    supershaft because I can

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    I meant jet numbers or specs. Who's specs? Just curious.

    I just reread your post and realized you got the specs from Sudco. They recommend jets for different model bikes? I did not know that! Is it somewhere in there manual?
    #17
  18. Airhead Wrangler

    Airhead Wrangler Long timer

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    They'll give you a rough starting point. They stay well to the rich side to be safe.
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  19. Kai Ju

    Kai Ju Long timer

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    I don't know if they have different specs for different bikes. As you can see from my previous post they were close but did need a bit of work.
    #19