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  1. #1
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    Default Mikuni TM 38 TM38 Needle jet jet Needle HELP when on a 250cc

    So I have an issue with needle range jetting being too lean.

    I have to pull the carb apart yet again but as I search I find many different needles numbers.

    There is a needle jet and jet needle? WTF?

    I came up with this:

    6fj40 for the needle ???
    389 Q-6 for the needle jet (or should I use Q-2 ?)
    35 pilot ????


    But here is what I found for the LT500R:
    6DK3-3
    R-2
    22.5 pilot


    Now does anyone know what needle and needle jet I should use?

    The carb appeared to be quite new and when you buy them pre jetted they could be setup for a 125cc even and the needle jetting and everything is way off. I find new carbs with different jet setups listed. And every parts diagram I look at has a different needle number.

  2. #2
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    I searched all over and came up with this but hit a wall looking for the supposed rf40 needle

    http://www.3wheelerworld.com/showthr...s-carb-on-250r

  3. #3
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    Dec 2007
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    Quote Originally Posted by nd4speed View Post
    So I have an issue with needle range jetting being too lean.

    I have to pull the carb apart yet again but as I search I find many different needles numbers.

    There is a needle jet and jet needle? WTF?

    I came up with this:

    6fj40 for the needle ???
    389 Q-6 for the needle jet (or should I use Q-2 ?)
    35 pilot ????


    But here is what I found for the LT500R:
    6DK3-3
    R-2
    22.5 pilot


    Now does anyone know what needle and needle jet I should use?

    The carb appeared to be quite new and when you buy them pre jetted they could be setup for a 125cc even and the needle jetting and everything is way off. I find new carbs with different jet setups listed. And every parts diagram I look at has a different needle number.
    Ok, just to be clear here, you currently have the 6fj40 for the needle, the 389 Q-6 for the needle jet and a 35 pilot? I have the 38mmTM Mikuni both the 85 and 86 series as being listed as a 22.5 pilot, 389 Q2, and the 6fj41. If you purchased the carb from some one running 125cc stuff, it may have different stuff in there. I have run the TM successfully on 250's with the above combination (as listed) at sea level with no lean condition. Have you checked the bell of the carb to see if there are any air bleeds and what they are? For clarification the jet needle is the actual needle that you adjust the clip on to set your fuel rate in the 1/4 to 3/4 throttle range. If you raise the needle (lower the clip) the engine will become richer because your are exposing more of the needle jet (The hole that the needle slides in and out of. It also houses the main jet on the bottom side of the carb) allowing more fuel to flow out of it. Conversely if we lower the needle (raising the clip), we will be leaning the engine out the same amount through out that range. If you have the needle in its highest position and you still have a lean problem and there are NO air bleeds in the air bell, then we have to take a look at the either REDUCING the size of the jet needle or INCREASING the size of the needle jet, or both. Different jet needles also have different tapers on them to allow different fuel rates at different RPM's. A jet needle can have up to 6 different tapers on it. Now we also have to keep in mind that a certain size engine requires a certain amount of fuel at wide open throttle for a given horsepower. As you remember I mentioned that the needle jet also houses the main jet, so it is important if we have a larger engine that we have a larger needle jet (with a corresponding needle of course to maintain proper A/F ratio through out the throttle range). We do this because we want the main jet to be the restriction in the system and not the needle jet. If the needle jet is too small for the given engine size and it does become the restriction, you will install a larger main jet and the engine will still be lean.


    Here are the jet needle part numbers starting with the richest and going to the leanest.

    6DP04
    6FJ40
    6FM46
    6FJ41

    Here is a link to the needle jet numbers and what they mean.
    http://www.motorcyclecarbs.com/Mikun...rt_W319C36.cfm

    Hope this helps a bit. Happy tuning!

  4. #4
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    Not sure what is in it yet. Trying to go by memory. It probably has the 22.5 pilot.

    The issue is the needle clip is one from the bottom and now it is winter. It ran fine this summer but I just want to fatten up the mid range for the winter cause I think I fried a crank or main bearing with that setup last winter.

  5. #5
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    Found my needle to be 6FJ47

    Not sure about the needle jet.

  6. #6
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    Quote Originally Posted by nd4speed View Post
    Found my needle to be 6FJ47

    Not sure about the needle jet.
    With the slide out you need to take the bowl off, remove the main jet and tap it out gently from the bowl side. There are numbers on the side of it that will tell you what it is. I didn't find that needle on my jet sheet, but going by the numbers would be leaner than the 6FJ41. You may just want to swap the jet needle to one of the ones that I listed in my earlier post. I was wondering about this thread yesterday and was going to dig it up and see how you made out. Thanks for the update.

  7. #7
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    In all these years I've never had to tap out this needle jet...... or was it jet needle? WTF. This just makes me dislike Mikuni even more.

    I swapped a PWK on and rode a couple times including a trip to some snowmobile races nearby.
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    Last edited by nd4speed; 01-16-2011 at 02:00 PM.

  8. #8
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    OH and yes I believe the 6fj47 is leaner , and thus my complaint and fried motor.

  9. #9
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    OK i'll update my my comical results.

    The whole time I had a 6fj41 needle and ordered another one (my dumbass thought the one was a seven).

    So the new needle fixed it, the old one was bent or somethin.

    Now onto main jets. I run 280-300 but I see people saying they run 360-410 on this carb. I've ran this a couple years with no meltdown.