Jetting help

Discussion in 'Trials' started by heffergm, Nov 18, 2018.

  1. heffergm

    heffergm Long timer

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    Keep it simple, I can't get through a tenth of that other jetting thread that's gone awry.

    Current setup:
    Keihin pwk28 on an '18 GG 280
    48 pilot, 122 main, needle (stock) one clip richer from the middle (needle raised one clip, was like this stock). Air screw 1.5 turns or so out around freezing, 2.5 turns out in the summer. Stock slide.

    The symptom I have is that after some idling around as you sometimes do in a section, if I go to pick the throttle up quickly, the bike will hesitate just a bit. If I give it a few revs, it won't do it again for a bit. It just has that bit of a miss when quickly opening the throttle from nearly closed (not going wide open, just to 1/4 or so throttle), but only if it isn't cleaned out a little first (and I don't mean WFO kind of cleaned out, just a few revs).

    Everything is cleaned regularly. The float height looks fine (parallel using the hold at an angle method, I haven't had to mess with it).

    Otherwise the jetting feels pretty good...
    #1
  2. lineaway

    lineaway Long timer

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    For something simple I would drop the needle (raise the clip) and go from there.
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  3. heffergm

    heffergm Long timer

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    Was thinking similar, I'll give it a try
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  4. motomofo

    motomofo Been here awhile

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    Don’t be afraid to try things... it’s part of the learning process of figuring out how to properly jet a bike.

    I’m agreeing with lineaway, try #3 needle position. Or a 45 pilot.
    #4
  5. heffergm

    heffergm Long timer

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    I dropped the needle, and I'm kind of shocked at how lively the bike is right off idle.

    Weirdly, the more I mess with it the more it feels like the stock 45 is probably the way to go, at least in the summer, if I can keep it from knocking.
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  6. lineaway

    lineaway Long timer

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    So where is the air screw at?
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  7. heffergm

    heffergm Long timer

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    Right now? 1.5 out, but it's around freezing. I'm basically just opening it up until it knocks then going in a half turn.
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  8. lineaway

    lineaway Long timer

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    If you have a 50, I would give it a try. But you are correct that temp and humitity has alot to do with jetting. I used to see guys with big bore 2 strokes sieze the motors in the Winter after jetting in the summer.
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  9. heffergm

    heffergm Long timer

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    Feels pretty good with the needle clip change, got rid of the fluffyness off idle.
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  10. alpineboard

    alpineboard Been here awhile

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    Sounds like you were too rich earlier, should it be turn air screw out until rpm increases then stops to increases as you unscrew, then go back in 1/2 turn. I am assuming when you say knock you also mean ping...? and if so, you should not have to turn it out there that far. Just go until it stops increasing in rpm. IMO...

    Either way 2 1/2 sounds like too much turned out, concluding too rich. It should be approx. 1 1/2 out summer, a bit less for winter air. So going from 48 to 45 , correct direction. Keep in mind I'm on a beta evo .

    The turning screw method gets you close, which is ok fine, but the real proof is to check your spark plug appearance , and fine tune from there, looking at color and carbon, etc. and also feel of motor gets thrown in to the recipe. But if your plug looks good, chances are the motor will feel good.
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  11. motobene

    motobene Motoing for 46 years

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    Back to the crying shame, eh?

    55 pilot and 120 main. Back down to 52 if a bit rich.
    #11
  12. PMK

    PMK Been here awhile

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    From the sounds of it, you are content with the setup with dropping the needle one clip.

    Based on your description of idling around in a section, there is a good likelyhood the needle being one clip rich had you running on the needle, which was rich and may have mildly loaded up the engine.

    Keeping it simple, try and optimize the pilot air screw setting between 1 1/4 to 1 3/4 turns open while idling after being run and cleared out. If the air screw is beyond these settings, adjust your pilot jet size as needed. The screw setting is not carved in stone, but this range optimizes the taper on the screw for pedictable changes when adjusted.

    If the bike runs well, above 1/4 throttle, but below wide open the needle clip position really works well there.

    You can easily tune the transition from pilot to the needle by swapping needles. The swap involves a needle with a change in the needle code last letter. Assuming you currently run the typical JJH needle, going to the next letter up in the alphabet after H, but they do skip letters, so the next letter up in the code would be JJJ, then JJK. Most times Keihin avoids the letter I, maybe not to be confused by 1, but there are some needle codes that do use I.

    Needle straight adjust mixture upto 1/4 throttle. It does require a needle swap. Often riders and tuners avoid the needle swap and jet around it. How well you want the engine to operate and accellerate off idle this is a setting to optimize.

    Hypothetically, if your needle clip change had made the mid range slighly lean. Maybe you went from no knocks or pings to now feeling / hearing them, at above 1/4 throttle settings. You would want to raise the needle via the clip. However this would again create richness off the bottom. With the pilot jet and air screw optimized, you would swap to a different needle straight diameter. Since the first two needle code letters remained the same, you would retain the same clip slot setting. Since the pilot circuit is indepentdent of the needle, you should not need to make adjustments to the pilot jet or air screw, but experience indicates, on occasion very slight air screw changes then compliment the new needle.

    I am linking a PWK needle info chart. On the second chart, note the upper horizontal title bar. It indicates rich vs lean based on the needle straight diameter.

    If the engine runs well, no doubt enjoy, if you care to tinker a bit and fairly inexpensively, test a different needle.

    https://jetsrus.com/a_jets_by_carburetor_type/needle_keihin_N427-46.html
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  13. heffergm

    heffergm Long timer

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    This method has never, ever worked for me. I could turn the air screw out until the cows come home and the idle would just keep going up, regardless of what pilot I've got in the bike. It'll do the same right now with the 48 in it, and there's no way it's too rich, at least for the way I want the bike tuned (which goes to the below).

    I start at 1.5 turns out, bike warm, quickly open the throttle, look for a lean bog (you'd have to be really lean to get one) and listen for any knocking. If it knocks, turn air screw in a half turn and repeat until it doesn't anymore. If you end up around 1/2 turn out from closed (or you can't get the knock to go away), go up a pilot size. I'm perfectly fine with anything from 0.75 to 2.5 turns out as an adjustment range... it's a screw for a reason.
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  14. Twin-shocker

    Twin-shocker Long timer

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    Firstly the best carb settings will vary dependent on the fuel oil mixture which is being used. Overly oily mixtures will need bigger jets as the viscosity of the fuel is higher.

    If there is a stumble off idle, the pilot jet size may not be correct. The way to check this is to warm bike up to operating temperature and then turn the air screw till best possible running is obtained. Check this by opening the throttle very slightly from slowest possible uniform tickover speed until there is hesitation whatsoever.

    When that is achieved turn air screw in about 1/16 of a turn. Check how many turns air screw is out from fully in position. If its from 1 1/2 to 2 1/2 turns the pilot jet is very close and will not need changing. If its under 1 1/2 turns try the next size up, if its over 2 1/2 the next size down

    Its worth bearing in mind that for best possible performance from PWK carbs that the air screw should be adjusted on the day you are riding, as changing atmospheric conditions and oxygenated fuels means the best air screw setting does not remain the same.

    Finally bikes which are gently ridden by entry level riders who are using very oily mixtures, may well have exhaust systems which are choked with unburnt oil residues and need a good hard ride to clear them out properly.
    #14
  15. UstaKood

    UstaKood Been here awhile

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    When you do this , are you able to adjust the idle down and readjust the air screw ?

    Something seems odd ...
    #15
  16. Twin-shocker

    Twin-shocker Long timer

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    If turning the air screw either right in or right out makes no difference the bike has other problem issues.
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  17. UstaKood

    UstaKood Been here awhile

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    Forgive if I've missed something in your statement .

    In my experience , I have always adjusted for desired idle speed . If the speed picks up with mixture adjustment , adjust the idle accordingly . Eventually resulting in a "range" for the mixture adjustment that should be evident . Go from there .

    Again , just going by your statement that as you open the air mixture it starts revving .

    Or ...

    Is my thinking flawed ?:dunno
    #17
  18. Twin-shocker

    Twin-shocker Long timer

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    I wonder if many are aware of the fact than on PWK carbs the air screw needs to be adjusted properly on the day you are riding for best possible performance?
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  19. Brewtus

    Brewtus Buffoonery, Inc.

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    Christ, here we go again.

    Out.
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  20. UstaKood

    UstaKood Been here awhile

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    Yep , me too

    I should have known ...

    :fpalm

    92 days til spring !
    #20
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