Jetting headaches? CV or slide carb? Emulsion tube...

Discussion in '2 smokers' started by dhallilama, Oct 11, 2013.

  1. dhallilama

    dhallilama Long timer

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    Apr 3, 2012
    Oddometer:
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    Location:
    Portland, OR, USA
    Go check it.

    i know carbs, i've probably rebuilt 100 motorcycle carbs and countless car/truck/tractor/equipment carbs over the years. and i still can be a complete dumbass...

    my GSXR wears the emulsion tubes out in ~5k miles... while ordering new ones again today, it occurred to me: my damned yamaha IT250 that seems to be as temperamental as could be... never dialed in on the jetting, what works one day completely doesn't the next.
    i never checked the emulsion tube / needle jet for wear. i checked the needle... even ran 3 different ones. hell, i ran three different entire carbs! and as it turns out, all 3 had worn emulsion tubes. they LOOK good, until running a drill rod down them when you can tell they're slightly off. (or put the needle fully in the emulsion tube and blow on it, shouldn't stream air)

    popped the tube out... happened to have a new one in my drawer of Mikuni parts. total transformation. the old one is out by less than 0.2mm. enough to eff the jetting up all to hell.

    old:
    pilot - 35 and it'd go from rich to lean
    needle - they all sucked
    main - 275

    now:
    pilot - 50 (stock spec) and the low end is smooth as butter
    needle - 6f15 (stock spec)
    main - 360

    0.2mm made the thing run like crap at any given time, as the needle would bounce around.

    so... no matter how much you're sure, go check if'n ya have problems :D

    (even my daily driver harley has a slightly worn 'tube... explains the mid-rpm sputter)
    #1
  2. Twin-shocker

    Twin-shocker Long timer

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    Jan 28, 2012
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    A carb like any other part is a wear item. Bikes will run with a badly worn carb, but replacing with a new one will make a very big difference. In terms of cost and time, in most cases its more effective to replace worn out OE parts with a new OKO carb, which in most cases will be cheaper than trying to repair OE and will work far better.

    GSXR needle jet wear is a very well known problem, and can be greatly reduced by fitting a Dynojet kit, which uses better materials.
    #2
  3. dhallilama

    dhallilama Long timer

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    Location:
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    yes, carbs contain wear items... i was posting this as a friendly reminder that the needle jet can have MINIMAL wear and still cause significant issues.

    GSXR needle jet wear is generally due to the slide guides wearing. mine are pretty worn, but they're $$$$$$ new (literally, $200 each last i saw) and the used ones are seemingly all as bad as they wear very quickly themselves. that's a different discussion for another place though...

    the dynojet needles may as well be rat tail files... they chew needle jets to hell... and the early kits were effing aluminum needles. screw dynojet. if you want a better product, go Factory Pro.
    #3