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How do you determine the size of a carburetor

This is a discussion on How do you determine the size of a carburetor within the CRF50 forums, part of the General Talk category; Where and how exactly do you measure on a carburetor to determine the 'size' of the carb (20mm, 22mm, 26mm, etc.)? I'm interested in both ...

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  1. #1
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    How do you determine the size of a carburetor

    Where and how exactly do you measure on a carburetor to determine the 'size' of the carb (20mm, 22mm, 26mm, etc.)? I'm interested in both Shenway and Mikuni carbs.


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    Re: How do you determine the size of a carburetor

    Measure the opening of the body on the intake manifold side in mm. That will equal the model number like Mikuni VM26 = 26mm bore.

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    Re: How do you determine the size of a carburetor

    Ya, that is what I've always known too. I've worked with PLENTY of carbs on big bikes and have never found a carb that did not follow that reasoning.



    I have purchased a Mikuni "22mm" carb, and I measure 26mm on the body at the manifold. I think they shipped me a 26mm, but they are holding to it being a 22mm.



    Here is what the dealer told me, what do you think? Am I missing something?



    "The carb measurement on the 4 stroke Mikuni is in the slide area (22mm). The carb has a stamp on the side of the float bowl that shows it is a 22mm. The 26mm 4 stroke Mikuni is 26mm by the slide and tapers to 30mm at the manifold."



    How the heck could measure the slide area and verify this?

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    Re: How do you determine the size of a carburetor

    These 4 stroke carbs are measured at the venturi, not at the mouth (filter end) or at the manifold end.

    The mikuni 22mm you have is almost certainly a 22mm (its 25mm at manifold and 38mm at the mouth)



    It is not a precise measurement on the 22mm Mikunis simply because they have an oval venturi, approximately 24mm high by 20mm wide averaging 22mm.

    Sounds like the guy that sold you the carb is correct in what he has said....

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    Re: How do you determine the size of a carburetor

    Quote Originally Posted by SDF
    (its 25mm at manifold and 38mm at the mouth)....


    Pretty close, I get 25.5mm inside the manifold end and 38.0mm outside the mouth, but 34.5mm inside the mouth.

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    Re: How do you determine the size of a carburetor

    Quote Originally Posted by Soupamoto
    Pretty close, I get 25.5mm inside the manifold end and 38.0mm outside the mouth, but 34.5mm inside the mouth.


    Yep, they can range from 25mm to 26mm, they are not OEM parts but are manufactured under license so tolerances are not exact,



    Sounds as though your not measuring the narrowest part of the venturi, but just in front of it...



    Take the needle slide out of the carb, and look through the carb as the air would flow,

    the portion where the slide was that you can now see through is the venturi,

    it will look oval'ish (more of an upright rectangle with rounded corners).



    The narrowest point (across the width) should be around 20mm and the widest point (across the height) should be around 24mm, the average gives the overall size e.g, 22mm



    If its the same as the picture then its a 22mm Mikuni
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    Re: How do you determine the size of a carburetor

    My bad. I guess the carbs I have are straight when I looked at them. Not venturied.

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    Re: How do you determine the size of a carburetor

    Here is my reply in the Roost Area Forum, same subject...



    http://www.planetminis.com/f71/name-...tml#post911344
    - dirtbkr188 -




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    Re: How do you determine the size of a carburetor

    Quote Originally Posted by project-c2
    My bad. I guess the carbs I have are straight when I looked at them. Not venturied.


    Nah not Bad

    and yes you may well have had some straight thru type carbs

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    Re: How do you determine the size of a carburetor

    Quote Originally Posted by SDF
    If its the same as the picture then its a 22mm Mikuni


    That all checks out. OK, it is a Mikuni 22mm oval slide 4-stroke carb. That is exactly what the dealer said it was. Cool on that. Good job.



    Now, why can't I get the silly low-mid RPM bog out of my XR50 with TB 108 kit?! (it was originally an 88 kit then the stroker was added).



    The dealer said my intake was 22mm on the head, so it is not the larger valves, and I will only get a 20mm carb to work.



    Think that is right? The 20mm that came with the 88 kit had a bad bog at low rpm; enough to kill the engine.

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    Re: How do you determine the size of a carburetor

    Quote Originally Posted by Soupamoto
    That all checks out. OK, it is a Mikuni 22mm oval slide 4-stroke carb. That is exactly what the dealer said it was. Cool on that. Good job.



    Now, why can't I get the silly low-mid RPM bog out of my XR50 with TB 108 kit?! (it was originally an 88 kit then the stroker was added).



    The dealer said my intake was 22mm on the head, so it is not the larger valves, and I will only get a 20mm carb to work.



    Think that is right? The 20mm that came with the 88 kit had a bad bog at low rpm; enough to kill the engine.




    The 22mm should still work fine, oil up a good filter and try needle settings, may even need to go up or down on the main jet.



    Sorry I cant help with what settings (slightly different half way around the world)



    Perhaps someone local can give a good starting point..

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    Re: How do you determine the size of a carburetor

    LOL, ya, I hear ya SDF. You've been great though! Thanks.



    I've got a 95 main, 15 pilot. I've tried the clip in all 5 positions. Best position is clip 4th from top, but that isn't close to good enough). About 1 turn out on the air screw. 2 turns was pretty bad.

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    Re: How do you determine the size of a carburetor

    Quote Originally Posted by Soupamoto
    LOL, ya, I hear ya SDF. You've been great though! Thanks.



    I've got a 95 main, 15 pilot. I've tried the clip in all 5 positions. Best position is clip 4th from top, but that isn't close to good enough). About 1 turn out on the air screw. 2 turns was pretty bad.






    Ok, just so your not dialing in the wrong direction its actually a (Fuel Screw) out richer/in leaner, and I would say the 95 main is to lean even at a guess we'd run about a 105 main jet, needle in middle or second clip from bottom with 95-98ron



    Guessing you want to be over a 100 main for a starter and about 1.5-1.75 turns out on the pilot screw....... then dial from there.

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    Re: How do you determine the size of a carburetor

    Quote Originally Posted by Soupamoto
    That all checks out. OK, it is a Mikuni 22mm oval slide 4-stroke carb. That is exactly what the dealer said it was. Cool on that. Good job.



    Now, why can't I get the silly low-mid RPM bog out of my XR50 with TB 108 kit?! (it was originally an 88 kit then the stroker was added).



    The dealer said my intake was 22mm on the head, so it is not the larger valves, and I will only get a 20mm carb to work.



    Think that is right? The 20mm that came with the 88 kit had a bad bog at low rpm; enough to kill the engine.


    If you are running the racehead, INTAKE=25mm and EXHAUSt=22mm
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    Re: How do you determine the size of a carburetor

    Quote Originally Posted by cmktech
    If you are running the racehead, INTAKE=25mm and EXHAUSt=22mm


    Does/did TB sell a head that is not a "race" head? The intake on my TB head is 22mm.

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    Re: How do you determine the size of a carburetor

    The earlier TB race heads had a 22 or 23mm intake, and a 20mm exhaust, I believe. I just browsed through the TB website, and did not see any "older" race head rebuild kits, just the kits for the newer head.
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