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Thread: New Carburetor Jet size?

  1. #1
    Senior Member cowboyjeff's Avatar
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    New Carburetor Jet size?

    I just was wondering is anyone knows what Main Jet Yamaha sends there replacement Carbs out with or what altitude they are set for.
    I have mine all installed and I have to have the Idle mix screw turned out 4.50 turns to even get it to start.
    littletommy likes this.

  2. #2
    Senior Member Tweaker's Avatar
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    You need to tell us the year of your bike as there are 2 carbs and if it is a CA model as these have different jets. My 2014 CA carb has a 126 mainjet as stock. I am not sure what older carbs have. Here is a list of jets.

    TW carb jet part#s

    If you have particles in your gas tank you can plug up a new carb especially the pilot jet as it is so small. Or if the gas is bad. The gasahol here in CA goes bad incredibly fast and seems to affect the pilot jets.
    littletommy likes this.
    Twin 2014 TW200's made side by side on the assembly line, Moose rear racks, Protaper Bars one with risers, DG oval pipes. rejetted carbs, 130 main jets, 2 -.020 shims on the needles and @ 2 1/2 turns on the pilot screw, #34 pilot jets, Aserbis hand guards, Shinko 241 front tires, modified Krator foot pegs, 14-55t sprockets, Ricochet skid plates and 90 degree fuel filters.

  3. #3
    Senior Member cowboyjeff's Avatar
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    Hi Tweaker, It is a 91 no Cal. Bike. My tank got a little rusty then plunged my Carb up. I could never get it to run right again.
    So I bought a brand new Yamaha Carb, removed the tank and cleand all the rust out of it, replaced the Petcock, and put a new Generact fuel cap on it.
    Im running fresh Ethanol free Gas. It now only runs if i have the Screw out 4 or 5 turns.
    Im wondering if that new Carb might have been shipped with Sea level Jets installed. Im at 4500 right now but spend most of time with this bike 6500-10,000
    Now my Rubber Boot that connects the Carb to the air box is about 1/2' short.
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  5. #4
    Super Moderator littletommy's Avatar
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    Replace the boot and yes, the bike most likely is jetted for sea level and lean to pass emissions tests.
    Quote Originally Posted by cowboyjeff View Post
    Hi Tweaker, It is a 91 no Cal. Bike. My tank got a little rusty then plunged my Carb up. I could never get it to run right again.
    So I bought a brand new Yamaha Carb, removed the tank and cleand all the rust out of it, replaced the Petcock, and put a new Generact fuel cap on it.
    Im running fresh Ethanol free Gas. It now only runs if i have the Screw out 4 or 5 turns.
    Im wondering if that new Carb might have been shipped with Sea level Jets installed. Im at 4500 right now but spend most of time with this bike 6500-10,000
    Now my Rubber Boot that connects the Carb to the air box is about 1/2' short.
    Ken likes this.
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  6. #5
    Senior Member Fred's Avatar
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    Cowboyjeff, 4.5 turns out on the mixture screw is indicative of a very lean fuel mixture for some reason. Increasing the altitude you ride at should require a leaner fuel mix than sea level but your evidence indicates an excessively lean situation so richer "sea level jets" ( if there were such a thing) would lessen your problem. However factoryYamaha jetting only varies by California vs. 49 state bikes.
    What what year did you get the new carburetor for? The 1/2" shortfall of carb to air box length after installing new carb sure indicates something is not right.
    Have you checked other assumptions? I would first open petcock and carb's float bowl drain and confirm that indeed there is a good consistent supply of fuel to and through the carburetor's float bowl. It is entirely possible that new carb has been fouled by rust or debris from the aforementioned gas tank issues.
    Ken likes this.
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  7. #6
    Senior Member cowboyjeff's Avatar
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    Hi Fred, I ordered the Carb for the 91, It looks the same as the one I took off. I think that Dab Gon Boot shrank.
    I have the Tank so clean Im sure were good there, there is a steady stream of fuel to the carb & to or out the Carb Drain Screw.
    One thing I did not mention is I do have a bad batterie. But thats never done anything but make it so I have to kick start it in the past.
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  8. #7
    Senior Member Fred's Avatar
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    OK, good feedback. So we know a good supply of ethanol free fuel is getting to the correct carb and into to float bowl to supply the various fuel metering circuits. The factory jets included should be correct.
    Now to figure out why the metering seems so lean.
    While fuel gets to the float bowl are the floats set at correct height to then permit enough to feed fuel to the various fuel metering circuits?
    I would check the float height as well as confirm carb vents unobstructed as well as possible air leak at cylinder to intake manifold, and intake manifold to carb. Testing for leak typically done while bike running; spray a bit of WD-40 or other flammable aerosol at connections and listen for a RPM surge.
    I would also check that all fasters on new carb are tight, nothing is missing or causing a possible air leak. Maybe compare side-by-side with original carb?

    p.s. - the new Generac fuel cap is vented, correct?
    Last edited by Fred; 03-19-2017 at 04:43 PM.
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  9. #8
    Senior Member Tweaker's Avatar
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    What Fred said. If your 1/2" short boot is leaking on the intake side that would make it lean so LT's suggestion to replace is good. If you have a propane torch you could use that without lighting to test for leaks too, that would be less messy than WD40. Almost everyone has WD40. The propellant in WD 40 is propane that's why it is good to use to check for air leaks. Plus putting a 90 degree fuel filter on the fuel line is a good idea.
    Ken likes this.
    Twin 2014 TW200's made side by side on the assembly line, Moose rear racks, Protaper Bars one with risers, DG oval pipes. rejetted carbs, 130 main jets, 2 -.020 shims on the needles and @ 2 1/2 turns on the pilot screw, #34 pilot jets, Aserbis hand guards, Shinko 241 front tires, modified Krator foot pegs, 14-55t sprockets, Ricochet skid plates and 90 degree fuel filters.

  10. #9
    Senior Member cowboyjeff's Avatar
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    Fred, Tweeker you both are awesome, This sure will give me a few more things to check to day, thank you very much.
    Ken, littletommy and Tweaker like this.

  11. #10
    Member boylstonrider's Avatar
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    I've seen old carb boots shrink like that.. I have a friend with a 87 and his shrunk a 1/2" compared to the new one..
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