The Crying Shame of Jetting

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

  1. PSchrauber

    PSchrauber Long timer

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    So much about jets but I haven't found anything about the needles beside dropping or rising them. There are so many different needle and needle jets available and the needle steers about three quarter of the throttle range.

    Also different slides are not discussed?

    Both have much basic impact to the performance of the carb!
    #81
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  2. UstaKood

    UstaKood Adventurer

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    Yeah , for sure . I can relate as to my little Triumph Spitfire with twin SU carby's . Needle change made a significant result . One would hope though that the MFG's got something right . But with at least getting the basic jetting corrected and realizing improvement is a best step forward .

    One thing not mentioned is basic Ignition timing . It should be set / confirmed before chasing after jetting . This might could explain differences between individual bikes .
    #82
  3. motobene

    motobene Motoing for 46 years

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    The reason is you can use the stock needle/nozzle/slide cut angles on the modern bikes is they are right enough to get excellent results simply with jetting and settings.

    That is one reason for 'crying shame.' The factory gets bikes almost there, but unfortunately the mass of carburetors are sub optimal and some are problematic.

    When I was fitting the excellent Mikuni VM26 round slide to '80s/'90s to get rid of the then-troubled big-body Dell'Ortos, I had to develop nozzle and needle combinations through many jetting iterations.

    Interesting about the Keihin is the concept of fixed nozzle size, and tweaking by needles. But again, the JJH is fine, that no one is pushing JJK or whatever.

    The Dell'Ortos have had the 60 slide, D36 needle, and I can't remember what nozzle, for a long time.
    #83
  4. motobene

    motobene Motoing for 46 years

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    Here we hit another simplification in the moderns. One will see a slotted stator plate just like previous bikes, but the hall-effect sensor is no longer on the stator, but rather is mounted on the outside of the flywheel in the bulge seen at about 0200 on the cover. The Hall-effect sensor is mounted on posts that have no intended adjustment (though you can adjust a little by opening up the holes).

    The trigger point for firing is by a milled step on the outside diameter if the flywheel. This is very accurate to reproduce in modern production, as is posts location. Whatever variation there is will fall into a tight bandwidth. I think they don't time bikes today, just assemble them. One could say that justifies checking timing, but it's a pretty fool-proof system.
    #84
  5. PSchrauber

    PSchrauber Long timer

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    Especially the different cone shapes of the needle at with which diameter the tip starts has such a big effect it unbelievable if you haven't tried that out.

    Also the cut away of the slide offers more or less under pressure which results in more fat or more lean adjustments.

    I recently replaced the Amal carb of my 242 against a modern PHBH type C which is more a Mikuni clone coming with an air screw too.

    Now I'am very pleased with the outcome, sadly I have now some slides that I think I will never use also needles bit these are luckily cheap.
    #85
  6. gu9cci

    gu9cci Adventurer

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    Anyone try smart carb or similar here for trial bike?
    I did run some in enduro bike and it was very good
    #86
  7. motobene

    motobene Motoing for 46 years

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    Did lots of that kind of stuff in the distant past. These days I'm happy if I can get great results with just jets. I guess I'm getting lazier as I age :-)
    #87
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  8. Norman Foley

    Norman Foley Devotee of the Husqvarna

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    Be glad you don't have a new KTM 2T Enduro bike.... With the switch to Mikuni for '17, guys are chasing slide cut outs, needle jets and all the rest too!
    #88
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  9. 2whlrcr

    2whlrcr gooligan

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    I'm happy with my new KTM 2T enduro bike...Lectron.
    #89
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  10. Norman Foley

    Norman Foley Devotee of the Husqvarna

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    Buying a new Husky 2T... Lectron is already on the shelf!
    #90
  11. motobene

    motobene Motoing for 46 years

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    I'm afraid I'm making haste, slowly, too! A Lectron or Smart Carb seems just so exotic... and expensive :-)

    I remember Lectrons from the `70s! Here's how they work:



    The guy talks about eddy behind the needle. The flat back side of the Lectron needle probably does have more eddy effect turbulence down stream, but round tapered needles have eddy effect too.

    What I see with the design is stark simplicity arising from the pre-CNC era. Much easier to manufacture and control variables on designs when surfaces are flat, parallel, etc. The slide and needle are good examples of lack of complex contouring. The needle I just a rod that can be fixture, then then it's set angle and depth of cut. Keihin parts, as an example, have very sophisticated complex contours. That's made possible by 900-pound gorilla design capabilities and very high production volume tooling such as pressure die casting.

    The power jet is a cool idea. One can be added to any carb. There was, or may still be, a company that sells power jet kits for any carb. You run one, leaner main jet then compensate for altitude by varying the amount of supplemental fuel added when the slide is high via tweaking the metered orifice.

    Sounds like the details, being long iterated over, are well worked out.
    #91
  12. PSchrauber

    PSchrauber Long timer

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    Electron and Smart carb are nice items for mid to high power, I have heard it is possible to archive even a good result for 1/4 to half throttle.

    But they where NOT invented and designed for trials bikes, not at all.

    Show me a trials bike with a Lectron or Smart Carb that is running better as with a standard carb. I think there is a reason why these carbs where not mounted to or gems.
    #92
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  13. jonnyc21

    jonnyc21 Trials Ninja

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    I remember hearing they, not sure if it was Lectron or Smart Carb, looking into trials bikes at one point, however I don't remember where or who.

    If they do it and get it correct at some point we will see them available maybe...
    #93
  14. Takataka

    Takataka Been here awhile

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    Following on from the above, I have now pulled the carb and noted a few things.

    Slow jet is a 45
    Main jet is a 122
    Needle is in mid position
    Float was slightly rich

    I adjusted the float to slightly lean. Not sure how to judge the degrees but my feeling is that it's 1 degree as anything less would be barely noticeable.

    After re assembly I kicked the bike with choke on/ no throttle. It fired on the second and third kicks but didn't catch. On the fourth kick I gave it some throttle and it started and ran well. Seems like the float level may have been too rich which was effecting starting and could still be a bit rich.

    I think I'll order some jets and have a play with that now..
    #94
  15. Buschog

    Buschog Been here awhile

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    A Dial-a-Jet will be getting added to my dual sport before too long to hopefully deal with the massive elevation changes I encounter on most any ride. I may look into whether or not they have anything for my trials machine. I haven't ridden a competition yet where I see 4000' elevation changes, but I certainly see them when I leave my house at 4800' ASL and get to an event at 200' ASL.
    #95
  16. motobene

    motobene Motoing for 46 years

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    Yeah, that's it, Dial-A-Jet.

    I pissed them off about 9 years ago. They sent me several examples for a test magazine write-up, then I got busy with other things and didn't get to it. Sent their stuff back with apologies.

    BTW, I found this pic today of the setup on the `16 Factory 250 June of last year. That `16 was an incredible-running scoot! Note the main jet, low-speed jet, and needle clip position for 8,200 to 10,000 feet written on the float bowl:



    The only change after my return to 1,600 feet was a 120 main jet.
    #96
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  17. motobene

    motobene Motoing for 46 years

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    New info on the variable of bike individuality in jetting.

    At an altitude event now. The 2017 Sherco 125 runs gloriously perfect at 7k plus altitude with 115 main and 58 low-speed jets, needle clip in mid slot.

    My 2017 250 is breaking a pattern and not following my 2016 specs (same engine and ignition as '16). This particular bike cannot handle a 55 low-speed jet. Even a 52 resulted in a touch blubbery on bottom. This condition coincided with the opposite in the mid when loaded with fast throttle openings. It was leanish with needle clip in the middle slot.

    In '16 I fit Boyesen CarbonTech reeds to deal positively with some contradictions in jetting. In 2017 I fit them before riding the bike, so I'm missing what jets the bike would 'asking for' with the stock reeds. My bad.

    Anyway, several iterations here and what's working for me at 7k plus is 112 and 50, with air screw 2-1/2 out. This bike might be 'asking for' a 48, though once loaded the motor is very tractor like and clean running.

    The needle clip got moved down one slot to 2nd from bottom to enrich the mid, and that fixed the bog.

    I haven't fit an under-50 low-speed jet to any Keihin bike in long time, and I didn't bring one. That experiment will have to wait until I return home.

    Following 'recipes' in jetting like, 'X needs such and such exact jets' just does not work to capture all bikes.
    #97
  18. gu9cci

    gu9cci Adventurer

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    Can you elaborate please what change/benefit you get from the carbon reeds?
    I'm doing some research on them now and like to get some more input

    Thanks in advance
    #98
  19. motobene

    motobene Motoing for 46 years

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    Most OEM reeds work great. My '16 250 Sherco had a different small conundrum than the small conundrum I have going with the '17 and CarbonTech reeds solved a problem for me. On the '16, the CarbonTech reeds just happened to push one or more mystery variables in a beneficial direction.

    On the '17, I don't know if the thinner carbon fiber weave versus thicker glass weave OEM reeds are helping or hurting. Insufficient data. My experience has been that CarbonTech reeds are not hurting but helping, slightly, by enhancing the smooth gruntiness of my 250s.

    Then again, some skepticism regarding confirmation bias is warranted. I paid money for it so it's better fuels many a bling market.

    Caveat emptor!
    #99
  20. motobene

    motobene Motoing for 46 years

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    My '17 250 is not liking my usual largish low-speed jet. A 48 is about as rich as I can go without too much 4 stroking low load and slide barely cracked open. Scratching my head on this bike. I suppose another crying shame is bikes are like people. They don't all ask for the same things.