The Crying Shame of Jetting

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

  1. lineaway

    lineaway Long timer

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    You have not opened up the tower yet? It`s not like you to be shy of the unknown.
  2. motobene

    motobene Motoing for 46 years

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    I dare not do that before knowing for sure it's not jetting. Rode two hours to test a modified shock and it ran consistently the whole time. I'll know more with more ride time.
  3. UstaKood

    UstaKood Adventurer

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    Don't S'pose it's the rings setting' in ?
  4. Yard Sale

    Yard Sale Long timer

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    I have a new 2017 GG 250 with a Dell'Orto PHBL 26. It had a 120 main, 35 pilot, 60 start, D36 needle with clip in the middle. I lowered the needle one clip and adjusted the fuel-air screw (which is is on the Dell'Orto? I read it was fuel not air like a Keihin) to 3.5 turns out. I'm at 5,000 ft, burning a mix of pump and race fuel. I have new jets on order: leaner mains and pilots bracketing the stock 35 with leaner and richer.

    It starts easily on the choke when cold. When the engine has been dead for several minutes it requires choke to re-start. Power is smooth and soft but not crisp like a properly jetted 2 stroke should be. It just doesn't have that bark to it.
  5. lineaway

    lineaway Long timer

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    You need a 118 main (115 maybe) and a 38 - 40 pilot jet.
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  6. motobene

    motobene Motoing for 46 years

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    No. That idea is from the old days when manufacturing practices weren't so precise.

    With modern Nikasil cylinders, if they breathe clean air in their lifetimes (real foam filter liquid oil, sticky to the touch), there will be no rider-discernable difference in compression between first startup and 3 decades later. There isn't on my Fantic with Gilardoni Nikasil cylinder.
  7. motobene

    motobene Motoing for 46 years

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    Lineaway is saving you, Yard Sale, from a classic mistake, the crying shame of a leaner low-speed at altitude.
  8. UstaKood

    UstaKood Adventurer

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    LOL .

    That sentence pretty much sums me up .
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  9. gu9cci

    gu9cci Adventurer

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    [​IMG]
    I resolved my jetting with elevated technology smart carb and bike is running so crisp and linear
    That stupid keihin drive me crazy!
    [​IMG] IMG_3932.JPG
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  10. motobene

    motobene Motoing for 46 years

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    I rode a 300 GasGas with Smartcarb in '16. It worked well. I'll check it out some more, thanks.
  11. VxZeroKnots

    VxZeroKnots Long timer

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    Hmmm after lineaway's post in the sparky thread I cracked open the float bowl on the new to me 16 ST300. 48 low speed low jet, which I believe is stock.

    At 8000ft it feels like it should have better pull off bottom, I need to slip the clutch to keep the front wheel up on really grippy steep ledges. I definitely prefer underpowered (adequately powered) and it seems like the 300 should have no problems hefting my 200lbs.

    I'll pop in a 55 and see where it gets me. The mid and top on it is pretty spunky so no worries there, throttle response across the range is good.
  12. jonnyc21

    jonnyc21 Trials Ninja

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    If you bump the low speed from 48 to between 52 and 55 without adjusting the main jet I suspect you will find it runs a bit ritch at 8000... just a suspicion though.

    Unless you already have...
  13. VxZeroKnots

    VxZeroKnots Long timer

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    hmm might just go to a 50 then. So much for one thing at a time eh?
  14. jonnyc21

    jonnyc21 Trials Ninja

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    Ya... the comment was because I experimented when I did my wife's bike. When I increased the low jet without dropping the main it was rich and I didn't even go trail ride yet so my altitude hadn't changed. So had to drop the main jet and the needle to get it running correctly before it even hit the trail.
  15. VxZeroKnots

    VxZeroKnots Long timer

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    Rode for an hour or two and it seemed pretty good, better than before for sure. Maybe a plug chop would show different but ive never done one of those in my life, the concept seems ridiculous and wasteful so i just tune for throttle response.

    Thanks for the tips and this thread motobene, very helpful!
  16. 2whlrcr

    2whlrcr gooligan

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    I'm wondering what the Honda and Vertigo owners do with all their extra time...?:1drink
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  17. lineaway

    lineaway Long timer

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    They work on clogged fuel filters and broken frames.:lol3
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  18. Champe

    Champe Been here awhile

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    I don't know about the others, but I am riding when I could be jetting. Life is too short for wrestling with stinky carbs.
    The never-ending patience and skill you guys apply to carbs is admirable though. :D
    Viva Montesa (and Vertigo)

    Back when the technology of ski boots was changing rapidly, the saying went "Are you still lacing while others are racing ?"
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  19. motobene

    motobene Motoing for 46 years

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    I have some time on my hands so I can reply in some detail. I'm sitting in an orthopedic hospital waiting room while my spunky wife gets hip replacement #2. After healing, no more excuses for not riding events :-)

    Yes, I'm picky. Being SO sensitive to machines, fortunate for me also comes with a joy for tweaking. I'd rather be riding and perhaps perversely, wrenching:tb

    It's SO easy to work on many trials bikes that I can tolerate multiple iterations without frustration. Dual sports and especially road bikes... yeech!

    New Sherco, GasGas, etc take me about 10 minutes at a relaxed pace to pull, mod, and replace the carb to start a test ride.

    On 80% of all Keihin 2 strokes I go to up 4 to 8 on the low-speed circuit to typically 52 to 55.

    The 52 or 55 decision is based on where the bike 'asks for' the air screw to be during the first test ride. If the bike runs best with the air screw half a turn out or less, I go right to a 55. And yes, I drop the main to 120 so as not to mess up the overall results.

    I ride it as is and check where the air screw is. How many turns to touching at full in? I count by half turns, three being 1-1/2 turns out, for example.

    I play with extremes. The bike should die when I close the air screw. If not, I know the low-speed mixture is very lean.

    I turn the air screw 1-1/2 turns out and double check the idle. Sometimes riders lower the idle on a lean or lean-and-pinging bike, or raise the idle to compensate for a bike that is too rich on bottom.

    I screw the air screw in to where the idle drops. That right where the low-speed mixture goes too rich of ideal. I open back up a quarter turn, then count the turns in and record that.

    I check loaded performance by riding with both brakes on in the lower third of rpm with more throttle. A lean-running bike will sound and feel 'soft' and may even bog or stall easily. A too-rich bike will be OK on power but sound 'poppy' and will start to four stroke fairly quickly when the throttle backed off.

    I check needle position. If it is in the middle three slots I pull the carb off to re jet it. If the clip is in an extreme position, I put the clip in the middle slot then re test the bike, noting the loaded response.

    To remove and replace needle clips And lay the needle on a flat surface with a cloth or towel or firm rubber pad on the surface. Cover the clip with thumb and index finger so it doesn't 'dink' into another universe when you push down on the needle to pop the clip off or push it on. Make sure the clip pops fully home when you push it back on.

    I'll note the needle clip position. I prefer
    to re jet bikes not yet messed with because inexperienced wrenchers can do silly things to carburetors!

    test rides are critical to knowing what direction and how much in a direction to go and if there will be departures from normal patterns. Jetting is as much a from-to process as it is following recipes and patterns. I care less about where the numbers are or end up than I do about results.

    There are definitely patterns, but not all bikes follow a tight pattern.

    Sometimes carburetors are just messed up and the inexperienced tuner can get in a tail-chasing swirl of re jetting. Just yesterday...

    I was texted by a desperate person preparing two bikes for a National. Subject bike was a NEW Scorpa 125 Factory. From the multiple jetting iterations described I ended up prescribing a carb swap off the second bike, a Sherco 300, as I suspected a bad Keihin. Some time elapsed then came this text: Swapped carb. It works (scream)!

    I the advised getting a warranty carb from RYP at the National to avoid more tail chasing.

    The lesson is to know when to stop trying to jet your way around non-jetting-related problem. The fastest way to do that is the carb swap as 125 through 300, the jetting is almost the same.


    Back to test ride results and re jetting direction and magnitude. If a bike asks for half a turn or less out on the air screw, I'll go right to a 55 low-speed jet and 120 main. If it's 1 to 1-1/2 turns out I'll fit a 52.

    And what is this 'asking for' business regarding the air screw? The right turns out is somewhere between where idle drops when you turn the screw in, and where the bike will bog slightly when the throttle is snapped full open very fast.

    I do the fast throttle-opening test while stopped.
    I use both hands flat on the throttle grip, then I pull both arms apart fast, opening the throttle full very fast. I let the throttle flick shut on its own fast right after the rpm zooms up. I look for quick response with no bogging and especially no pinging. Just a clean rev-up without bogging or blubbering (accumulated oil down in the crank should already have been slung out and burned off.

    I re test and iterate as needed.

    These bikes are very sensitive to needle-clip position. You can feel, hopefully, a move of one slot. With heavy loading from low-to-mid rpms while riding, with any bogging and soft response, lower the clip one position (richer mid). With some 'dirtiness' and excess 4 stroking when backing the throttle off from a load, move the needle clip up one position (leaner mid).

    Re test and iterate as necessary.


    Then I get bikes like my 250 that only let me go up 4 on the low-speed jet, so I can't get as much loaded power as I like while avoiding 4 stroking at low load and when backing off from a moderate load.

    A touch of four stroking in putt putt mode is fine, but more than a bit is a no-go for me. With my '17 I can't go richer than 48 from the stock 42, so I don't have quite as much grunt as I had with my '16 250 with 55 low-speed jet. Yet the 48 is what the '17 bike asks for, so that's what it gets.

    Some bikes would be better off with a needle profile other than the JJH, the standard needle most bikes comes with.

    I've needed with most outlier biked a larger diameter big end at the start of the needle taper to have the larger low-speed jet to increase power, while reducing four stroking when backing off from loadings. I've got the Keihin chart at home. Will study later....
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  20. Eric B

    Eric B Adventurer

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    Very informative post, Motobene. Thank you.
    Is it possible to explain "4 stroking"? I've heard the term on the KDX forum.
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