WOT Jetting Issue

Discussion in 'General' started by whereshaldo, Apr 19, 2019.

  1. whereshaldo

    whereshaldo Member

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    Ive got a curious jetting issue that, as I write this, I suspect may be a needle height issue. I've got an 88cc with a slightly hotter cam in it I've been riding all winter with it just fine, but I've started to notice that going from 95% open throttle to 100% WOT, the bike actually loses a little bit of power. It "feels" like its running lean, but maybe its over rich. When I'm riding, i feel like the engine runs smoother and goes faster at the 95% WOT, and then opening the throttle all the way makes the bike slow down. Any ideas? Raise or lower the needle a notch? I'm not sure which way makes more sense...

    Hal
     
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  3. wanrep

    wanrep Active Member

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    Needle height won't have any effect at wide open throttle. Needle height will typically effect 1/4 - 3/4 throttle performance.
    It's all on the main jet at WOT.
    What you're describing sounds like the main jet is a size too big. Especially since it didn't do this during the winter months.
    I'd try going down one size on the main jet.
     
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  4. spencer2986

    spencer2986 Member

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    ^^^exactly what I had to do to mine the other day. I was running a stock engine with a cone filter and #62 main all winter and it ran AWESOME! Once the weather got warmer it didn’t rev out as clean, checked the plug and was fat black. Dropped down to a #60 main and runs better but can certainly tell it lost power compared to the cooler months. Actually, there is a night and day difference and feels like a snail now but what can you do.
     
  5. racerx

    racerx Administrator
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    Contrary to popular belief, needle height can affect a:f ratio at WOT. This has been verified using an EGA, fitted w/wideband O2. It's a matter of whether, or not, your carb works this way and, if it does, to what degree. Stock 16mm Keihins do work this way, it's easy enough to change needle height.

    I'd try going 2 grooves in both directions (separate tests) to make the effects more obvious.

    Here's where a lot of shadetree tuners can draw the wrong conclusions. It is possible to get a richer WOT mixture from a smaller main jet, leaner from one that is larger. The lion's share of the impact will be a part-throttle. In some instances, it comes down to choosing between 2 mains, with very different needle height settings. Short of realtime exhaust testing, trial and error testing is the only option. With a bit of patience, it can deliver equal results.
     
  6. whereshaldo

    whereshaldo Member

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    Oh man it's amazing what a simple jetting change, some adjustments to the A/F mix, and resetting the points gap will do. It's a whole new ride. I'd love someone who knows these bikes better than me to take it for a spin, but I think I'm nearing optimal tuning, as least based on how fun it is to ride.

    Thanks all

    Hal
     
  7. racerx

    racerx Administrator
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    Glad it's working for you. As for "someone who knows these bikes...", you're joining the ranks and what you've gone through is how it's done.
     

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