The Crying Shame of Jetting

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

  1. motobene

    motobene Motoing for 46 years

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    I've gone through such a long string of re jettings that I've built up a bunch of useless jets. Dell'Orto... Keihin. Fed up with buying the same jets and Gavin all these leftovers made me try drilling jets after a long hiatus. So far, so good!

    But this isn't about drilling but about the crying shame of factory jetting being suboptimal in most cases and problematically wrong with some bikes.

    I've guessed the lean problems on the low-speed end are for homologation reasons, and over rich on top is for minor added insurance when throttles stick wide open. Factories, after all, have no control over variables like machine awareness or what oil is used in the premix. The cost of the full ester-based synthetics like Fuchs Silkolene Pro 2 SX and Maxima Formula K2 may put some folks off....

    But another reason may simply be carburetors being available in volume a long time with the same old jets that work OK for most bikes.

    Some of the worst jetting cases I've run across are 125s and 200s fit with the long running Dell'Orto PHBL 26 BS fit with the standard-issue 60 start circuit jet a 36 low-speed jet, the D36 needle, and main jet leaner than the more common 118, but often still too rich.

    The net effect is hard cold starting, a revvy and soft running long warmup, and blubber running on top with cleanouts required even after warmup.

    About the smaller-displacement bikes, it is easy to assume - but wrong - that smaller engines require a smaller low-speed jet. The reduced mass flow of smaller displacement requires compensation with jet size closer to, say, and 300, and it is curious how similar jetting can be from 125 to 300.

    Another unhelpful myth is that higher altitude requires a leaner low-speed jet. This thinking us not prevalent in trials, thankfully.

    Altitude does require main jet reduction, but not low-speed jet reduction. The point is, if you are already factory lean on bottom at a lower elevation, with a further drop in mass flow with altitude, it may run real poorly, requiring excess clutch work to stay alive.

    My rule of thumb for jetting bikes on bottom is to fit whatever jet the bike 'asks for' that results in 1-1/2 turns out on fuel screw or air screw in average atmospheric conditions. Then it is typical to need to tweak for atmospheric extremes like storms and altitude changes any more than a quarter turn either way.

    Often I find Keihin bikes that will just run OK with the air screw almost shut, and some will run lean and erratic because they are starving at and above idle.

    Most Dell'Orto bikes with stock jets require fuel screws turned way out... to 3 or 4 turns to run OK after a delayed warm up.

    I recently rejetted a 2004 GasGas 200 that gained a third more power going from a 36 low-speed jet to a 44. The typical go-to jet for 250s, 280s, 290s, and 300s is 38, but 40 will work great too. The smaller-displacement bikes often need something bigger. If you have a bigger bike and the common 36 jet that is running really lean, go with a 40.

    Also your Dell'Orto bike will cold start much easier with the choke-circuit jet (the long one with the small o-ring) drilled from .60mm to .80mm (.031")! The start jets have forever been 60... and under sized.

    As for the main jet, most of the time the bikes are too rich on top, especially if a spark arrestor is fit (these affect exhaust flow and standing waves at certain rpms and thus jetting). The 200 yesterday 'asked for' a 97 versus stock 105 main jet to run clean and strong on top. It has a spark arrestor. Without spark arrestor maybe a 100 would have been better in this bike.

    My 2017 Sherco Factory 125 is a classic case of too lean on bottom. Factory air screw setting was 1/16-turn out. Clue! I went from the stock 50 low-speed jet to a 55 to achieve 1-1/2 turns out on the air screw. It might even handle a 60.

    As a general rule you can fit a 55 low-speed jet and a 120 main jet in any size Keihin bike and be better off than stock, at any altitude.

    Refining things a bit, with 250s and 300s with Keihins, 122 is a good jet right at sea level. At 1,200 to 3,500 feet a 120 is fine. At 8,000 to 11,000 feet a 112 is peachy. You can proportion for the altitudes in between.

    For Dell'Orto On 250s to 300s I like a 118 main right on sea level and a 110 main at 8,000 to 11,000 feet. Proportion in between.

    Caveat:. Every bike is an individual and 'asks' for some minor differences. The above assumes 2 strokes and the stock D36 Dell'Orto and stick JJH Keihin jet needles, a needle clip close to the center slots, floats set to level or slightly lean of level, and no other gorilla mechanic monkey business.

    My jetting is also paired with soaked and one squeeze out of Maxima FFT liquid filter oil.

    I did a mid 2000s GasGas 125 recently wherein the owner thought premix squeezed out or WD40 spray was good enough and would not 'restrict' air flow. I told him that was a seriously bad policy, and he was stunned to see the entire intake tract coated with brown grit.

    I told him air filters have over capacity and that the air restriction myth is a disaster for top-end life. Teal foam filter oil off gasses evaporants until they turn sticky. Don't use the choke to start the bike within a few hours if applying the oil.

    That bike, BTW, had a clutch pack get destroyed from slipping the clutch insanely to keep the motor from bogging out on slopes. It was that fuel starved. After re jetting it gained a third more power despite loss of some compression from a grit diet.

    Hope this helps your bike run better....
    #1
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  2. Takataka

    Takataka Been here awhile

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    Great info thanks. Good to hear your take on oiling filters. I think I may have been a bit lean on oil when I gave mine it's first clean a few days ago.
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  3. motobene

    motobene Motoing for 46 years

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    Soak 'em and squeeze 'em out once. That's been my method for decades.

    So far, nothing nasty has made it into any of my motors, even when water splashes on the filter.
    #3
  4. motobene

    motobene Motoing for 46 years

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    There are a lot of ideas running around that aren't right, but to too many are apparently true. Air filtration is a less serious issue for society than the loss of intellectual freedom and free speech on the modern university campus, but pondering top-end immortality is an important nuance for technical nerds :-)

    The idea of more air flow and the horrors of imagined air restriction is firmly marketed into people's brains. It's the K&N filter syndrome. Because it's hard to directly measure, there is lots of room for religious thinking.

    Those little filters in our bikes actually have over capacity, even when very amply oiled with that sticky blue Maxima FFT that will pull off in little spider-silk strings if you touch the filter.

    I have had these get pretty caked over with dirt after a serial run of dusty trials with no noticeable change in performance. When I finally did check, it was like, 'oops!', but there was zero dirt on the inside!

    I don't want grit to be rolling around in the tight piston-cylinder interface. You can't stop all of that, but an amply-oiled filter (with the sticky, water-repellent stuff), will capture the vast majority, and gain is top ends we can consider immortal, at least in out lifetimes.
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  5. motobene

    motobene Motoing for 46 years

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    I missed a detail above. What does lean and rich sound and feel like? That's hard to teach, but here's a go at it:

    Lean: Soft sounding exhaust pulses. A 'mwum mwum mwum' sounding idle, bogging with quick throttle openings, and soft power on top if very lean. A lean mid range may produce pinging on sudden load demands.

    Rich: Raspy and dirty sounding idle and mid range. Lowered idle speed or dip in idle speed when the throttle is suddenly chopped. Too much 4 stroking when load is low and the throttle slide is opening. If you need to clean out a bike after warm-up.

    I NEVER clean a bike out after warmup.

    BTW, there is a self-reinforcing mythology about the need for clean outs on warmed up and already cleaned out bikes.

    First, what is a clean out? Oil coats 2-stroke innards. Over storage time the oil is pulled by gravity to pool at the bottom of the cavity at the crank. This excess of oil is flung up by the crank and pumped through during heavy loading at warmup. Thereafter, during a typical riding day, there isn't sufficient time for the oil to re pool, so no clean out is needed.

    It's easy to get fooled into thinking you need a cleanout, especially when respected riders tell you to, "Clean out your bike!", and you do, and it 4-strokes with a puff of smoke.

    A properly-jetted bike using good oil will provide adequate fuel when LOADED. If you rev a properly-jetted bike way up while unloaded (disconnected from the ground), it will 4 stroke as there is insufficient load to use up the supplied fuel. This effect fool a rider into thinking he or she needed another clean out.

    Think Newton's law regarding action-reaction as it applies to load with respect to the available fuel. Then what I'm saying will make sense.

    Understanding action-reaction with respect to available fuel is the key to becoming proficient at discerning carburetion and being great at jetting.
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  6. Dorian

    Dorian huge carbon footprint

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    Great info motobene. I agree and have jetted low speed (aka pilot) to achieve ~1 1/2 turns out on air-screw. Maybe you cold speak to this.. how decreasing the premix ratio (less oil in gas) effectively richens the mixture?
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  7. motobene

    motobene Motoing for 46 years

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    Yes, about premix. The simple idea is that if your volume proportion of oil is less, there is more fuel available for that volume.

    But maybe it's not that simple because oil is fuel too, especially when it is thinned way out buy being diluted in other fuels. So it is possible that 2% (50:1) versus 1.25% (80:1) would not make much difference to jetting.

    I did run across these two interesting bits in a paragliding website:

    Top 80 engine -- 2% (50:1) semi-synthetic oil OR 1.5% (67:1) 100% synthetic oil with AVGAS or ethanol-free gasoline
    Polini Thor engines -- 2% (50:1) semi-synthetic oil OR 1.5% (67:1) 100% synthetic oil with AVGAS or ethanol-free gasoline

    Hmm. Less oil when using a full synthetic. also the usual ethanol as boogieman:

    Semi-synthetic oils ONLY 100% synthetic oils do not mix well with gasoline that contains ethanol. When the mixture sits in the fuel tank, the oil tends to separate out. Simonini notes this problem in their manual for the Mini 2 Plus. Some 100% synthetic oils e.g. Castrol Power RS TTS 2T mix so poorly with ethanol fuels that Simonini will void the engine warranty if those oils are used.

    I've long been using a full ester-based synthetic, Fuchs Silkolene Pro 2 SX in my witches brew of fuel which may contain up to 5% ethanol. Can't say oil separation doesn't happen, but never noticed an issue with oil separation in my fuel.
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  8. motobene

    motobene Motoing for 46 years

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  9. SportSawyer

    SportSawyer 4RT Herder

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    Every now and then, I'm reminded why I like EFI so much.
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  10. 2whlrcr

    2whlrcr gooligan

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    :rofl
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  11. Dorian

    Dorian huge carbon footprint

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  12. alpineboard

    alpineboard Been here awhile

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    Very lucky to have a 91 eth free designated pump near me. Am told it is coming into northern NH from a Canadian supplier.

    With my 300 evo, need to run a 125 main jet, tried the 122 and is too lean, the 122 is fine/no pre detonate, out on the road, but when climbing a steep mt trail at 1000' to 2000' it pre detonates at the start of mid range, put the 125 main in the very next day w/same weather conditions and same ride, the 125 = no pre detonate. Air filter clean and oiled. So what you speak of , having a load applied to your motor to find out what perfect jetting is makes sense. And the road I have to test on has some hill climbs, just not a mt climb. So there is a difference in , how much load. And differing applications.
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  13. motobene

    motobene Motoing for 46 years

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    Yes! Jetting must be tested loaded.

    Interesting on going up with the main. I wonder if you are compensating for being lean in the mid range with bleed-back from the larger main?

    What happens if you put the smaller main back in but raise the jet needle (lower the clip position)?

    As an experiment, put the clip in the bottom needle slot with the smaller main. If the problem is over compensated for the more precise answer was in the needle, and the clip should end up in one of the lower slots.
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  14. alpineboard

    alpineboard Been here awhile

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    Understood, On the 2 stroke carb tuning articles that I have read, the information said, change your jets first, and if that a good tune is not obtainable, then try changing your needle position , and if still not good, try a new needle size. Just trying to be careful, and make the proper judgment. The difference between a JJH and a JJE needle for a Keihen pwk28 is only 0.0015" on the diameter. So a little makes a big difference. But guess that is why there are needle positions?

    Also, on that mt trail climb, in order to make it up that grade/hill , a 1/2 throttle or more throttle must be held at a constant. Not many places have that application.
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  15. motobene

    motobene Motoing for 46 years

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    In the 2017 Factory Sherco 125 thread, Norman reports, "I just installed MJ 120 and PJ 55, with AS 1 1/2 turns out. 1100 ASF and 67F at home. I rode it in a couple of my sections and a run in the field through the gears. What I thought was excellent with the stock jetting.... was just very good. With your jetting, the bike is now excellent... cleaner off idle and can rev out further and smoother in each gear.

    Thank you!"

    This is the milder end of 'crying shame': bikes that run apparently fine but could better by taking the advice of nuts like me.

    Norman's experience gets repeated on every bike that comes my way. There are exceptions of course, but the general pattern predominates of too lean on bottom and too rich on top. What varies is the degree. At the bad end of 'crying shame' are those bikes that take a bunch of kicks to cold start, a while to warm, then are missing a good portion of potential power.
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  16. motobene

    motobene Motoing for 46 years

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    Back to the needle issue, the JJH works great across all the engine sizes. Another needle profile might be more optimal, but I don't know as I haven't found where to buy the damned Keihin needles to experiment with! But my motivation to try other needles was reduced by the following incident.

    My 2016 Sherco Factory 250 drove me nuts in jetting, as it wasn't following the pattern. And talk about nuts lean! The factory fit a 42 low-speed jet.

    I was all set to try two alternate needles that were leaner than JJH to try to reduce some dirtiness in the mid when not fully loaded, but then I fit Boyesen CarbonTech reeds and the dirtiness went poof! The reeds solved the problem with overkill and the power became wonderful.

    The reeds changed what jets the bike was 'asking for' and lucky for me they happened to be according to pattern, with the leaner 120 and much richer 50.

    Going to try the CTT132 CarbonTech reeds in the Sherco Factory 125 as well, even though it has no dirtiness tendency. If there is no improvement I'll just save the reeds for the coming 250.
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  17. alpineboard

    alpineboard Been here awhile

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    JetsRus.com
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  18. motobene

    motobene Motoing for 46 years

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    I thought I looked there....
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  19. alpineboard

    alpineboard Been here awhile

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    You need to be a little clever to navigate/find stuff on that site, you can always call them.
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  20. heffergm

    heffergm Been here awhile

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    Hey look ma! No jetting!

    c4.png vertigo.jpg


    Not kidding aside, every time I've gone up in the pilot on my GG, it runs horribly. The stock 45 is right on the money with the air screw 1.5 turns out, at least as far as the plug reads go.
    #20
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