Setting Idle Mixture Screw
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Thread: Setting Idle Mixture Screw

  1. #1
    Senior Member Welder's Avatar
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    Setting Idle Mixture Screw

    IBack when cars had carbs I remember setting mixture screws while using a clamp on tachometer and a vacuum gauge. I still have both of these tools I bought back in 80s.

    Reading all the jetting threads, I haven't seen mention of this. I am curious if guys are using any instruments, or are just adjusting the screw by seat of the pants feel?

    My 2015s have an unused manifold vacuum line that runs from the top of intake manifold, up under the tank, to the left side of the steering head. It is plugged with an aluminum plug. So this would be an easy place to tap manifold vacuum.

    With the coil where it is located under the tank, hooking a tach to it would be a little tricky unless a short lead was installed first that would hang down for accessibility.

    Anyone know which lead on the coil is negative?

    Is anyone using a tach for mixture screw adjustments? If so, how did you find easiest to hook it up?
    Last edited by Welder; 07-17-2015 at 12:11 PM.

  2. #2
    Senior Member RockyTFS's Avatar
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    It's a TW, use the seat of your pants.

    Turn it all the way in (gently) and back out 2.5 turns if you live at sea level, 2 turns if you live at 5,000 feet. If the bike comes off idle properly with a rapid throttle movement, call it good and forget about it for the next 10 years. If not, there is most likely some other issue than number of turns. I adjusted mine once five years ago and haven't touched it since.
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    Rocky
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  3. #3
    Senior Member skase98's Avatar
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    Semi-related... I have been preparing myself to rejet my bike and have seen mention of drilling out the idle mixture screw cap. I just haven't found a 'why' to do that but I sure would like to know!
    '03 TW200 - QuasiMoto, the king of wheeee!

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  5. #4
    Super Moderator littletommy's Avatar
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    You have to be able to get to the jet. It's a cover.
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  6. #5
    Senior Member Welder's Avatar
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    Yes, the screw is under a plug that must be removed for acces to turn the screw.

    BTW, I tried hooking a vacuum gauge to the intake manifold and it was unusable as there was massive needle fluctuation on the little thumper. I also tried hooking my tuning tach to the coil and all it would do is provide an erroneous reading with RPM registering in the 70 range..... So back to hearing and seat of the pants.

    (128 main jet in use for info below)

    Now I found that idle speed kept increasing up to 3.5 turns out where I stopped turning any further. Following conventional wisdom that indicates a bigger pilot jet is in order. However the bike runs pretty good at half that setting so I figured I would leave well enough alone for now. I put it back to 1.75 - 2 turns where I am now still testing. May end up with more but I recently took out the needle shim I had tried (.032") due to a hesitation on easy throttle opening and found throttle response meaningfully improved without the shim.

    So, so far I have found a slight increase in pilot screw opening (about 1.5 - 1.75 turns out from bottomed) has solved almost all of the lean drivability issues by itself. I will incrementally be trying more pilot screw opening in the future, watching the plug for signs of excessive richness.
    Last edited by Welder; 07-31-2015 at 03:40 PM.
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  7. #6
    Super Moderator littletommy's Avatar
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    Too bad about the vacuum gauge experiment, I was interested in the progress on that one. I used to tune a lot of carbs that way and was interested in fine tuning my TW. Would any of the carb fittings work, I wonder? If you find anything, let me know please. Thanks bro.
    1st John 1:9
    If we confess our sins, He is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.

    John 3:16
    For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.

    John 3:17
    For God sent not his Son into the world to condemn the world; but that the world through him might be saved.

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