The Crying Shame of Jetting

Discussion in 'Trials' started by motobene, Apr 22, 2017.

  1. lineaway

    lineaway Long timer

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    For a reference, the newer Gasser 125`s run a larger pilot than the big bikes. That just blew me away with old style jetting knowledge or lack of.
    #61
  2. motobene

    motobene Motoing for 46 years

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    I appreciate your confidence, Norman, to dare significant changes.

    I recently did another Dell'Orto small bore and ended up with joy at a smallish 98 main jet and biggish low-speed jet of 44 (1,600 feet). Malasec told me of 125s with Keihin carb getting joy with a 60 low-speed jet (my Sherco like a 55).

    One purpose behind this thread was to promote confidence regarding the changes bikes are 'asking for.' with 'asking for' being like a saucer-eyed kid who tugging at your pant leg and saying, "My bottom is starving! Please feed me! My head is drowning, please stop waterboarding me!"

    If something doesn't work one can always go back.
    #62
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  3. laser17

    laser17 Long timer

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    +1: You never know if you're jetting is optimized until you test it.
    #63
  4. jonnyc21

    jonnyc21 Trials Ninja

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    Well worth the experiment, and as you stated easy to go back.

    So, I picked up a 40, 42, and 44 low and as they didn't have anything smaller I got a 102 and 104 main. (Wanted a 100).

    After pulling the 38 low I put in the the 42 feeling it was the best starting point. And not having a 100 I went from a 105 to the 102. (Will see if I can get a 100 and a 98 later)

    For now a marked improvement! Much crisper response on the low and seems to be ok at the top at just over 3000 feet elevation. I suspect it will be a bit rich at higher altitudes though, hence my desire for the smaller mains I plan to order.

    More to come after my wife and I have a bit more time on the bike.
    #64
  5. jonnyc21

    jonnyc21 Trials Ninja

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    quick update...

    Current verdict... it dose get a little to much fuel over time if I don't get on the throttle a bit from time to time... So, I think its almost there, I am going to drop the needle down one more notch to help with the 102 jet and believe that should do it unless we go way up in the hills.

    Over all my wife loves the lower end power it has and how much more responsive it is. I can't say I blame her, it rides so much better.

    Now if I could justify updating the suspension.... LOL
    #65
  6. motobene

    motobene Motoing for 46 years

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    jonnyc21, your experience is following a pattern. When I my enrich the bottom and lean the main there can be some richness bleed-over into the mid, making low-slide mid-riding a touch too 4-strokie, as in rich.

    I compensate by dropping the needle one or at most two clip notches. Then low, mid, and high work exceptionally well!

    I should mention this when recommending jets.
    #66
  7. jonnyc21

    jonnyc21 Trials Ninja

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    I have reviewed lots of your posts on the jetting and did know needle position can make a positive change and that often when making the low richer needs to be dropped (hence why I indicated I think I need to do it). :thumb

    I do admit that it isn't as clear on that with the way the posts have been ordered, however it is in there. However I admit I don't remember witch post or thread it was that I got it from. So yes making sure its clear in your jetting recommendations is good!

    By the way, thank you for sharing your advice and experience it really has benefited myself and I believe many others here in the group!
    #67
  8. motobene

    motobene Motoing for 46 years

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    Welcome! Often the threads evolve or get hijacked, so their volume can clutter things up. Or there is learning experience later in a thread that may contradict what was written earlier.

    It's good to read a whole thread, but that gets tough with LONG ones.

    A rewrite summary us sometimes good, but as s said Ben Franklin said, "I would have written you a shorter letter, but I didn't have the time."
    #68
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  9. Takataka

    Takataka Been here awhile

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    I posted the following in the Gasgas starting thread about my 300 pro with Keihin carb.

    "When cold, my bike likes first kick with choke on and no throttle. Second kick choke still on and full throttle. Fires right up."

    Lineaway suggested the following "Sounds like a bad needle and seat or a float issue."

    I have cleaned the carb and everything looks ok. Forgot to note jet size.

    The mixture screw is 1 turn out and the bike runs really well when putting around but does seem a bit lean when thrashing up a big hill and knocks a bit. If I wind the screw in about 1/4 turn it runs better but is then a bitch to start cold. Even 1/8 turn makes it hard to start cold. Hot starting is not a problem.

    What do you think these symptoms suggest?

    Thanks
    #69
  10. motobene

    motobene Motoing for 46 years

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    Your symptoms suggest the main emphasis of the thread above your post!

    You are in 'crying shame' territory with -at least - the classic undersized low-speed jet. Next time you get into your carb, document what's in there.
    #70
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  11. mung

    mung Been here awhile

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    My 300 pro had 45 pilot stock. Went to a 55 and life is very good.Starts easy, runs smooth and strong.
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  12. lineaway

    lineaway Long timer

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    Full throttle starting suggests you have too much fuel, not a lean condition.
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  13. laser17

    laser17 Long timer

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    Does it start the same way hot?
    If only cold, do you trailer your bike to go riding? Bouncy trailers can cause the needle valve to bounce as well (and let too much fuel into the carb) and is one of the ways to create the condition Lineaway is pointing out. I left my gas on one time before trailering and the whole bottom end filled up with fuel - had to turn the bike upside down and kick it with the plug out to "dry" her out. Still needed wide open throttle to start after that.....Never did that again.
    #73
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  14. Takataka

    Takataka Been here awhile

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    Thanks for the answers :)

    Ok I'll open it up again and see what jets I have.

    I was thinking the same but don't have a full understanding of the effects of jetting. I thought maybe a lean jet forces you to tune the screw and needle in a way that creates rich starting or something. Just guessing...

    Doesn't require choke when hot but still likes a good twist of throttle. Almost always starts first kick then.
    Generally I start my bike without trailering so it has just been sitting with the fuel off.

    The bike warms up quickly from a cold start and only needs the choke for 10 seconds or so before it will idle with no choke.
    #74
  15. jonnyc21

    jonnyc21 Trials Ninja

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    screw direction making it rich or lean depends on if its an air screw or fuel screw. I believe that Keihin's are an air screw so would make it richer screwing it in and lean going out. Dell'Orto I seem to recall is a fuel screw so its reversed so going in will lean it out and out will make it richer. (someone will correct me if I remember wrong, I hope.)

    This to me sounds running like your rich someplace in the jetting... however I am not as experienced as bene or lineaway.
    #75
  16. jonnyc21

    jonnyc21 Trials Ninja

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    Oh, almost forgot to update status on the needle position change.

    It seems to have done the trick. After dropping the needle one position I turned up the idle just a little and then set the fuel screw. After readjusting the idle back down a bit the bike seems to run just spot on. So much more low end then I would expect from a 125 and will rev out strait away without needed anything but a single twist of the throttle.

    :wings
    #76
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  17. laser17

    laser17 Long timer

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    You have it right. It was always told to me that:

    If the adjusting screw is on the airbox side of the carb slide = air screw.

    If on cylinder side of the slide = fuel screw.
    #77
  18. motobene

    motobene Motoing for 46 years

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    Takataka, go here:

    http://advrider.com/index.php?threads/2017-sherco-250-factory.1225988/page-3

    ...and look at post #56 for a summary of jetting on my Sherco Factory 250. That specific jetting is way better than stock for any brand 2 stroke with PWK 28 Keihin, 125, 200, 250, and 300. It follows the general patter I have scattered all over in this long thread.

    Compare my stock settings to modified. Lots of good info there in one place.
    #78
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  19. motobene

    motobene Motoing for 46 years

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    BTW, if atmospheric conditions change and my idle drops a touch and the bike starts better with the throttle a little more open, I can fix the slight richness by opening the air screw around 1/3rd of a turn. The opposite holds true when the idle rises and the motor start sounding and feeling a wee tiny soft.

    Once you have better jetting, it's minor tweaks in the adjustments. The main jet becomes the only jet to change when you jump up a bunch in altitude.
    #79
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  20. motobene

    motobene Motoing for 46 years

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    Heard Dale Malasec say recently he puts a 60 low speed jet in some Gasser 125s. Got a 55 in my Sherco 125, same as I use in a 300.

    Curious how wide the application is for the exact same Keihin or Del'Orto, with the factories learning to make making minor jetting changes. Same needle, nozzle, float setting, etc. 125, 250, or 300... Then when jetted very well, one carb set up one way will out perform stock jetting no matter what engine size you put it on. Mass flow drops with displacement, creating a need for larger jets, so it balances out.

    Fit well with the direction designs have gone. Unlike the very old(er) days, today's 125s are almost entirely the same bike as a 300. Only the stroke, top end and ignition are different.
    #80
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