The Crying Shame of Jetting

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

  1. Buschog

    Buschog Been here awhile

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    I've heard this term here a ton and have no idea what it means either. Off to the tube but I'd love to read a definition from our resident experts
  2. alpineboard

    alpineboard Been here awhile

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    4 stroking, too rich, so that it is only firing every other, like with the choke on.
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  3. LemmeTry

    LemmeTry Over-thinker...

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    Motobene, I copied that post and saved it on my computer for future reference. Thanks for that write up!!
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  4. ridenfly

    ridenfly Been here awhile

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    Thanks M. for the instructions that are now posted on wall in shop.
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  5. motobene

    motobene Motoing for 46 years

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    Four stroking. What a great question! It makes one think about what is really happening when an engine is skipping firing, jumping over power strokes. Here is what I think is happening, but I bet I don't have the full story below.

    As I've explained before, jetting must be understood as more than supply of fuel. There is also demand for fuel (load), engine temperature, and rpm to consider. Load is a Newtonian action-reaction thing. Fuel supply, as in the ratio of fuel to air, must be matched to the demand created by a load. RPM minced up the load into bigger or smaller pieces. For a given load, less RPM means more load per firing cycle and thus more demand for fuel.

    Most folks only think of RPM and throttle opening in jetting, as in low, high, bottom, mid, top, degree of throttle opening... but load at a given RPM and temperature are also factors.

    There is a band of air-fuel ratio relative to load and temperature wherein an engine can fire and produce a strong power impulse. Outside of that bandwidth the engine can't and won't fire. No bang for the fuel buck!

    The ratio bandwidth is called 'stoichiometric ratio or ratios.' I don't understand the concept fully (it's heavy into thermodynamics), but you all can look it up and get the empirical concept down.

    Two strokes behave a little differently than four strokes when the air-fuel ratio for some RPM and load and combustion chamber temperature falls outside of, or at the edge of, more ideal mixture to have the engine fire strongly on every power stroke. When lean for a give condition, the power impulses soften. You can hear and feel that, but the engine will fire every turn of the cranks. At idle it sounds like 'mwum mwum mwum. If way too lean to fire happens you get a bog or the engine just dies. When too rich?

    When the air-fuel ratio seen at the combustion chamber under a given set of conditions is on the rich side of stoichiometry, a kind of choking action happens and the air-fuel mixture is too rich to fire on every stroke. First will happen every other stroke or a minority of strokes. Why that is I don't know, but it's like trying to force a kid to eat something disgusting to him. That mouth isn't going to open every time the clip is pressed up against lips!

    Four strokes will blubber and may put out black smoke when overly rich. Two strokes are particularly sensitive to being on the rich side of stoichiometry at low loads, and given they CAN fire every compression stroke, every cycle, skipping a cycle or two is easily felt and heard.
  6. motobene

    motobene Motoing for 46 years

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    Your comments on how to jet remind me of where I need to get to: a well written summary of how to jet a 2 stroke trials bike. That seems impossible, but given enough time (work), one can shorten almost anything. What you all printed out I'd read again and probably do heavy editing.

    Forum postings can be like thinking with the mouth open. Things can evolve to where at the end of the post more ideal may contradict findings earlier on. Threads get long, and not everyone will study a whole thread to get the info in balance, which is why heavily edited summary documents are important.

    There are SO many variables in jetting a bike to conditions and riders. Maybe my understanding is getting there where I can do a good job with a how-to document?
  7. bfbike

    bfbike n00b

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    This is my first post here, so hello from Austria (not Australia) and please forgive my bad english.

    To lead to my question, I have to give some background information:

    I started with trials in last Oktober, when I bought two used Montesa 4RTs for me and my wife. Before that we did only ride on road and a little gravel with our BMWs GS, she on F800, me on R1200. The reason for the Montesa was the legendary build quality and that I heard, they are easier to handle with softer power, which is a nice thing when you start a so difficult sport with 41y (me) or 35y.

    Other reasons were no experience with 2T engines (besides trying to do motocross with ATVs on a Yamaha Banshe 350 (2T, 2 cylinders, motor from the RD350) 20 years ago, which ended after a year with many crashes and injuries) and we thought to do more trails riding.

    We trained a lot in the winter, learned that we do love riding sections and not trails, planned for the summer and saw, that we want to go to trials events almost every weekend from May to Oktober, so we sold the BMWs.

    We got in touch with the Slovenian trials champion (we are both born in Slovenia), had some lessons with him and became friends to him.

    As I mentioned to him one time, I am wondering how it is to ride a 2T bike, he offered me a new Beta Evo (he is the importer for Slovenia) for a very good price and let me test his Beta. The Beta did not impress me, but he has contacts to Sherco and TRS, so I tried both and bought a beautiful TRS One 250 Raga Racing.

    As I got it, I was astonished, how controllable the throttle was, which was my biggest concern in changing to a 2T. It has a Keihin PWK 28 with Main 125, Needle JJH 3rd clip, pilot 45. With airscrew between 1,5 and 1,75 turns out it ran great between sea level and 5000ft, I did not even touch the airscrew, just adjusted the idle.

    So, where is the problem? It is in my mind! I am always in search for the optimum of things, even when there is none!
    So I read everything about jetting in the "Crying shame" thread and elsewhere on Advrider, especially the posts of motobene.

    After that, I was sure, the jetting can not be the optimum. After 20 hours of riding the TRS with the original jetting I wanted to try an other jetting.

    I had half a week off, so me and my wife (she is still happy with the Montesa) did a trip to Italy to a big trials area, where we have not been before. I had only got a 50 pilot and a 118 main, which I fitted into the Keihin. I don't know exactly, how I set the airscrew, I think about 1,5 out, because there were the highest idle and a crisp throttle response (without load in neutral gear).

    At the same time (which maybe was not a smart idea), I adjusted the suspension front and rear, because after 20h of riding it should have been broken in. I followed the instructions of the manufacturers and had a good feeling about it. When standing on the bike and compressing the suspension, it seemed very balanced front and rear (before adjusting, the rear appeared softer than the front). I set the rebound on the faster side, because I read, with that you will have more traction on the rear and easier hopping front and rear (I am far away of learning to hop!).

    With this changes we started to the training area, which appeared beautiful in the woods, but quite difficult to ride with a lot of stones, tree roots and always a little wet and slippery.

    The TRS behaved like a completely new bike, but not for the better!

    The throttle was very nervous, it was like I totally lost the fine feeling and control, I had before. Instead I whiskey throttled, like in the first days of the beginning of our trials riding. On the climbs, where I normally had a lot of control and traction, I was most of the time riding with the front wheel up in the air and out of control.

    Also the suspension, which felt very smooth after the adjustments, turned to be very nervous with all the stones and roots. The nice planted feeling of the bike was gone.

    I was frustrated.

    The next day, when I was getting a little bit used to the "new" bike and training area, it was better. I learned, that with this faster suspension, I have to do a lot more work with my legs and whole body. It is maybe a good thing to be forced to do so, because one of the biggest problems for me as a beginner is being to stiff on the bike.

    I also got more used to the more nervous throttle response. The biggest problem was now (which also appeared the day before), that I had a lot of stalls, when idling around and over stones and the front or rear wheel got stuck in front of a stone. I had the impression, the tendency to stall is noticeably bigger than before the rejetting. First I tried to raise the idle, no big effect (that normally helped a lot before the rejetting, when I got to higher altitude). Than I opened the airscrew one quarter turn, than an other. That seemed to help a little, but still stalling easier than I would like!

    Sorry for the very long explanation, I hope the one or another has read until this point:

    What shall I do?

    1) About the suspension: Maybe I leave it as it is and get used to it and will be getting a better trials rider (that is the ultimate goal, to get a better rider and still having that much fun as up to now)?

    2) About the jetting: That is much more complicated, as I don't feel, hear and see all this signs, that are described in the jetting instructions.
    I can get used to the more aggressive throttle as it is now, but I don't like the stalling.
    If someone can analyze from my long explanation, what the problem could be and would give me easy to realize instructions (the changing of the jets was a very difficult and new experience for me, first time to open a carburator, it took one hour to put it together!) to reduce the tendency of stalling, I would stay with the new jets.

    Otherwise I think about just installing the original jets.

    Greetings,
    Bostjan
  8. Yard Sale

    Yard Sale Long timer

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    I put it in a 115 main and 36 pilot. It started on the second push, not kick, of the starter. (I push through the starter a few times slowly before kicking to start a cold engine.) The main feels lean, even with warm temps at medium-high altitude. I'll go to the 118.
  9. jonnyc21

    jonnyc21 Trials Ninja

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    Hello, welcome to... the best place ever!!!

    I have rather pore English and its my first, and only, language so hard to forgive what was never an issue. :thumb
  10. motobene

    motobene Motoing for 46 years

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    Bostjan, welcome! For English as third language you write very well.

    Reading your reply I got the impression you are trying to work through many variables. Some might say you are over thinking optimizations when you don't yet have the experience to know when something is already working well. That is OK because trying many things without fear is a good way to learn.

    With jetting pay less attention to numbers and more attention from-to. I am here now, what happens if I move over there? And in some cases a smaller low-speed jet is no crying shame but right for a particular Moto, situation, and rider skill level and personality.

    If your Moto runs very well at 1-1/2 turns out on air screw, you probably have the right (or right enough) low-speed jet. If you fit a larger low-speed jet for more loaded power, you'd have to open the air screw more and increase idle rpm to prevent stalling. But once compensated for you can increase loaded performance. Heavier loading in busts, once one becomes more skilled, is no longer 'whiskey throttle', it is skill.

    When you are learning in the early phase, the challenge to to maintain a steady pace and gentle throttle hand. After many basics are mastered, and terrain becomes more difficult, the next breakthrough is learning to rapidly change the pace of body and bike in bursts, while maintaining balance and traction. Terrain becomes too hard unless one learns to rush and brake at critical times. The steady pace then becomes a 'glass ceiling.' Whiskey throttle is bursts of the throttle at the wrong time, rider position, or wrong weighting. At your learning level now, you will not be taking advantage of heavy loading at low RPM, so the jetting as delivered will be better.

    The crying shame of jetting happens when motos are definitely jetted sub optimally from the factory. That can depend on individual bikes. The factories jet one size fits all, yet bike are individuals. It sounds as though your individual TRS was well jetted and running well as delivered to you.

    Adding fuel down low resulted in more power, but more power was something not needed at this stage of your trials career. Again, the air screw being 1-1/2 turn out from the factory would need to be farther out, and the idle increased, to reduce stalling upon enriching.

    During off days when the world comes at you faster than you can process, or when terrain difficulty goes up, the bike can seem as though it changes more than a modification actually changed it. Sometimes one must walk away from a situation without making a further modification, returning later when the machine our consciousness rides - our body, is working more optimally.

    Performance of body and mind vary, and the Moto can get the blame.
    Takataka and Buschog like this.
  11. bfbike

    bfbike n00b

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    Hello Jonny,

    thank you for the warm welcome.

    Hi Motobene,

    thank you very much for the explanations.

    Today I tried again at home, but at a location, were I rode the last time more than a half year ago at the total beginning and it is also not an easy terrain.
    I opened the airscrew a bit more (now it is 2,5 turns out) and raised the idle a little bit.
    It was better, less stalling, it is difficult for me to tell if it is still a little bit more prone to stalling than the old jetting.

    I am getting more used to the suspension, too, so it was a good day. I think I will try some more days and than decide, if I rejet to the factory setting.

    Allow me two more questions,please:

    1) If I go higher in altitude, I have to raise the idle. If I want to adjust the airscrew also to the higher altitude, in which direction do I turn?

    2) If I would decide to put back in the original 45 pilot instead of the 50, should I stay with the 118 main instead of the original 125 or is it better to rejet both to the factory ones (I think I almost never use full open throttle...)?

    Have all of you a great time riding trials,
    Bostjan
    Norman Foley likes this.
  12. motobene

    motobene Motoing for 46 years

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    Going up in altitude by how much? I don't typically change jets +/- 1.000m

    If you are well jetted, and the air screw opening is right at low altitude, the changes in air screw will be minor, +/-1/4 turn.

    There should be no need to return to the original jets, especially not the 125 main jet. If you are unsure about the 50, try a 48.
  13. Yard Sale

    Yard Sale Long timer

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    My bike stalled and would not restart today. Felt like it ran out of gas. Plug was white as a ghost on one side, brown on the other.

    [​IMG] [​IMG]

    It only restarted with the choke. Earlier, after a short break, it wouldn't restart and I had to bump it. I went up from the stock 35 to a 36 pilot. I guess I need to go up again.

    Can anybody help a brother out with some Dell'Orto pilots?
  14. motobene

    motobene Motoing for 46 years

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    Yes, definitely. 38 or 40 (depending on the individual bike) as written about extensively above. And depending on altitude, the main jet should span somewhere between 110 and 118.

    Plug color like that with a 5 heat-conduction NGK plug is typical in a heavily loaded (mostly on the main jet), well-jetted bike, or a bike that is WAY lean in the bottom-mid circuits ridden in putt-putt mode. Or, hard loaded run with an altitude jet in place at lower altitude.

    Float 'level' (angle), tilted to the float tang loading the needle spring, should be parallel to float bowl surface.
  15. PSchrauber

    PSchrauber Long timer

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    Nach Österreich:

    Grüzie bfbike,
    ich wechsel jetzt wieder in's Englische damit es die anderen auch verstehen.

    I think the primary jetting was OK and wouldn't change anything in first step.

    I don't know where you have been, I did too some riding in the Alps and in heights between 1500 up to 3000m, self coming from 50m above sea level. First time I went so Italy and in such heights I took a bunch of jets and even needles and needle jets with me. I haven't used one of them!

    And this with modern and classic bikes from Aprilia, Beta, Montesa to SWM.

    The only adjustment in need was a bit fine tuning of the air- or fuel screw depending what is mounted. That was sufficient for riding between 1000 to 2250m above 2250 the engines ran rich but still OK, first when we rode above 2750m and higher the engines lost power and a carb readjustment would have been in need, we simply didn't do it because we did not stay long enough in that hight.

    My personal tipp is to keep the standard jetting, furthermore I believe that you as a novice rider haven't used the complete potential of the bike thus just "puttered" around trying this and that.

    When riding in the Alps with long climbs even on mountain tracks you used more of the power and and usual and thus had a different fee. Also the engine might too be freed up again.

    I would do ar home some road work with the bike using too more then half of the throttle which too should reduce any carbon build up then start to probably rejet your bike at home id you don't like the behavior anymore.

    just keep in mind 0 - 1/4 throttle is controlled by the air- or fuel- screw in combination with the idle jet or nozzle.
    1/4 to 3/4 is controlled by the needle,
    3/4 to 1 is controlled by the main jet,
    the slide can be also changed to Kremer or richer with the cut from 0 (rich) to 60deg (lean).

    To the suspension what are your static and dynamic sag numbers front and rear?
  16. motobene

    motobene Motoing for 46 years

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    As one of 'die anderen', thank you for changing back to English.

    There are quite a few riders who go to high altitude events and don't bother to re jet since they are there only a day or two.

    At my last high altitude event, 2.500 to 3.000m, a rider broke a part which I fixed after day one. Test riding his bike, it ran like crap. He informed me he did not re jet. His air screw was way off and he hadn't even bothered to raise the idle!

    I fixed that as best I could, but he
    left the event and did not ride on day two. Perhaps if he had prepared more....
  17. motobene

    motobene Motoing for 46 years

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    PShrauber you were right to say one can get by fine not rejetting for altitude. It just takes more concentration.

    I provided an example of someone who did not know how to set the air screw and idle for new conditions and thus had more 'calculation burden' when riding.
  18. Yard Sale

    Yard Sale Long timer

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    Went to 38 and 116 and was generally happy. However, after stopping the bike for a few minutes, sometimes it requires choke to restart. Pilot still lean? Also, it still doesn't have the crisp snap I've gotten from my 2 stroke dirt bikes; maybe the flywheel prevents it? I've read the Dell'Orto is a smoother carb than the Keihin; maybe it will be never be crisp and snappy?
  19. jonnyc21

    jonnyc21 Trials Ninja

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    Sounds a little lean on the pilot with the choke to restart to me. my wife's Dell'Orto isn't as snappy as a Keihin but much better after a good re-jet.
  20. motobene

    motobene Motoing for 46 years

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    My 2017 Sherco 250 with Keihin has broken a long-running pattern. My '16 loved a 55 low-speed jet, but not my '17. I've come down in steps from 55 to 45 (stock is 42) to get it to clean up right off bottom. So now I'm at 120 (stock is 125)/45 with air screw 1-1/2 turns out.

    That did the trick to clean things up (though some loaded-power loss resulted). To make this smaller low-speed jet work, however, I had to raise the needle to maximum, clip in lowest slot. In the next slot up I get some bogging. Anything leaner, like clip in middle position or higher is just, well, pathetic.

    This bike is a lesson in bike individuality and the need to iterate problem bikes sometimes over many months because they refuse to follow 'cooking (jetting) by recipe.' This bike was 'asking for' less fuel in a narrow spot from idle to the mid transition and I was hard of hearing because I expected a different result from much past experience.

    Contrast the '17 250 with my '17 125. Only a few iterations to optimize for cleaness and power. The little guy runs awesomely with a 58 low-speed jet (stock is 50), the same 120 main jet (stock is 125), and the needle clip in the middle slot.
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