Idles strong at first, then not so much

Discussion in 'Motorized Bicycle General Discussion' started by fischer550, Oct 6, 2015.

  1. fischer550

    fischer550 Member

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    Have a little issue and I can't seem to figure it out. My bike, (2 stroke 66cc), has a strong idle when I first start it up. Along the ride, the idle slowly weakens, and will eventually not idle when I pull the clutch lever. This issue has been on both the NT and my new Mikuni vm18. thoughts?
     
  2. crassius

    crassius Well-Known Member

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    first thing to consider is whether engine has run long enough to get the rings seated well - idle & overall power will be iffy till then

    if it is run in already, then I'd guess that your carb is a bit too rich at idle (a cold motor likes a richer mixture than a warm motor) - I have had motors that needed the idle adjusted when warm, and needed the throttle feathered a bit to keep it running for the first few minutes when cold because there was no happy medium setting for idle
     
  3. Davezilla

    Davezilla New Member

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    I agree with crassius to set the idle mix and speed with the engine fully warmed up and just blip the throttle to keep it idling when cold and as the engine warms up.
    the best way to set the idle is by adjusting the idle mix screw in until the engine starts to stumble on the lean side, then slowly adjust out to the rich side counting the turns until it stumbles on the rich side, then go back half way to the lean stumble point, this will put the mixture where the engine wants it and not where the EPA wants it (lean stumble, then back off 1/2 turn).
    the idle mix on the mikuni carbs also plays part in the cruise mixture so getting this adjustment as accurate as possible can really help the bike with its low speed manners. Mikuni also makes 3 different throttle slides for these carbs but they can be hard to find to buy individually, but the slides can be trimmed with a dremel or with the sandpaper on glass trick to help richen or leaning the off idle mix, I wouldn't sand or file on the slide bottom tho unless it simply won't tune without doing so, but it is an option.
    Also, if your engine isn't fully broken in, you may need to readjust your idle mixture a lot as the engine loosens up and the rings seat better.
     
  4. fischer550

    fischer550 Member

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    Ya I am looking for a mikuni 3.0 slide, but cannot find one.
     
  5. fischer550

    fischer550 Member

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    I don't think you understand, it idles strong when cold, then as it warms up as I ride the idle weakens, and eventually when i pull in the clutch it just dies.
     
  6. crassius

    crassius Well-Known Member

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    since I started selling a diff model of kit, I've not had to adjust any of the carbs

    I did notice right away, tho, that for first 30 miles or so idle was actually faster when cold than when warm - once the motor ran in that 30 miles, they went back to having a slightly lower idle when cold than when warm.
     
  7. Goergedave

    Goergedave New Member

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    Have you tried a new fuel filter? According to your thread it would seem the carb isn't the issue, but still possible. Have you tried a different gas cap? If it isn't venting correctly that could cause a stall.
     
  8. fischer550

    fischer550 Member

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    I have an in tank filter, I can check it. This is happening with bot the nt speed i have, and my new mikuni vm-18.

    It is VERY strong at idle when cold, and then maybe about 10 mnutes of riding it just DIES or idles VERY weak.
     
  9. crassius

    crassius Well-Known Member

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    still thinking that setting for a good idle when warm and running it till the motor runs in more will solve it - I suspect that it is a bit rich at idle which is what a cold motor likes
     

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