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  1. #1
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    Default Mikuni Carb Size

    Exactly how do I measure a Mikuni Carb to find out what mm it is?

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    According to THIS site:

    Measuring the opening where the air horn goes is good for newer carbs.
    if you simply divide 44mm by 24.5mm in an inch and you get the english measurement equivalent.
    The 40mm plates have 165 stamped on them
    The 44mm plates have 175 stamped on them

    Which roughly equates to the english measurement, BTW....

    Early 40's had a 44mm bore to where the aux venturi mounts, so the 40 and 44 took the same parts, so measuring the air cleaner opening isn't the most reliable.
    And other sites say just to use a micrometer to measure the 'engine' side of the carb, then multiply by 25.4 to get the mm. Hope that helps.

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    Brad, just as an aside, I assume you are talking about the round slide series. Meaning you will plan on running dual carbs on a british twin bike. The carbs you want are called the VM series, and you would like to use the VM-30, VM-32, and VM-34. There will be no model number stamped on these carbs. You can make sure you have one of those carbs by measuring. You take four measurements. The total height of the carb, the total width, the width of the intake and width of the "outlet" or side to the engine. All measurements are done from the outside lip to lip. If you post those measurements in millimeters, I can tell you EXACTLY which carb you are holding in your hand. The reason you would want to go to the trouble is to get a baseline setup for your bike.

    I'm running the twin VM-32's on a 1971 T120 650 engine. I can post the "standard" setup Mikuni supplied for your carb settings, or give you mine, which are slightly different. Someone will probably post soon something like, "Just toss the Mikuni and get an Amal." Mikuni's ARE more difficult to tune, but only because they have MORE variables, and if you get them right, they run great. If you toss that make sure and send it to me... I'll pay postage.

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    Just toss the Mikuni and get a Amal.....(I had to do it, Jethro)

    Besides don't they have little tiny stampings on the back side up towards the top that have "30 or 32 or34" on the VM's
    Last edited by Torch; 02-23-2010 at 10:00 AM.

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    Quote Originally Posted by Torch View Post
    Just toss the Mikuni and get a Amal.....(I had to do it, Jethro)
    hey, torch is getting mellow! he didn't refer to the -m- carbs as "rolled up balls of dog shit."

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    LOL! Hey, Brad, I'll post the settings you PM'd me about here in case someone else has the interest...

    Main Jets 220
    Jet needle 6DH2 (the clip is in the middle)
    Slide 3.0
    Needle Jet P4
    Pilot Jet 40


    Also, I think you mentioned your airjet is 2.0 That is standard, and should be good.

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    Damn, the mikuni tuning guide for british twins suggests a 20-25 pilot. I had a 35 in before with bad results, what are your air screws at?

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    Throw me a picture of it, I will tell you in 2 seconds what it is. But like others said, you will see a stamping on the side near the throats of the carb. The only exception is if it is an older Mikuni with the thinner outlets (engine side). Some of those have stamps, some had stickers.

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    Actually that is my bust... I copied and pasted that from an earlier post and did not see the number. It should read 30. Also, I'm at 1 1/2 out on the air screw. Good catch.

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    Right now the bike has one on it. Runs good but rich. I think it needs rejetted. Gets real bad gas milage. Never fouls plugs. Might also try different plugs. Currently running NKG at .025 gap.

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    Where is it running rich in the powerband?

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    Everywhere. But i would say mostly on the decline of higher rpm.

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    I'd start with a smaller pilot and go from there. Keep in mind that the main jet is compounded by everything under it (pilot, jet needle, needle jet, etc...). So if it's pig rich up top, don't go and put a 100 main in it thinking it'll fix the issue as it will just lean out once it's past the mid-range.

    I stole this from our sister shop's eBay page:

    It's a pretty simplified thing that certainly helped me out back in the day.

  14. #14

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    Quote Originally Posted by Jethro View Post
    Actually that is my bust... I copied and pasted that from an earlier post and did not see the number. It should read 30. Also, I'm at 1 1/2 out on the air screw. Good catch.
    Hey I was trying to get this same info about my size,did you get an answer? My air hole is 1.5 as well.

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    .../// http://www.sudco.com/ \\\...

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