The Crying Shame of Jetting

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

  1. Norman Foley

    Norman Foley Devotee of the Husqvarna Supporter

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    Montesa... play with plug heat range to make it run right?
    #21
  2. motobene

    motobene Motoing for 49 years Supporter

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    Interesting case, yours.

    My Sherco 250 would run with the stock (ridiculous) 42, but not very well. Then putting in a 50 fixed the power issue, but caused some partial-throttle 4 stroking until I put in the CarbonTech reeds. Never had that happen on a Gasser.

    Every bike is an individual, and use of throttle (degree of loading relative to rpm) must be factored in. Do you push a lot of throttle, or rather ride more putt putt and revvy?

    What is 'runs horribly?' Which size GG? What is the float setting?

    Regarding the FI bikes, you are right in your humor... no carburetor to jet is nice. I did tech inspection at a recent national and found it interesting that almost all of the balky starters were FI bikes.
    #22
  3. My GG is a 280 with a 45 pilot. No idea what the main is, whatever is in there stock (same with all the jets in it). The float is set like this: http://www.splatshop.co.uk/blog/2012/12/setting-the-float-height-on-the-keihin-pwk-28/. No idea what that equates to in a measurement, I've never bothered to check. Runs horribly is an exaggeration, but a bigger pilot just makes it feel way too fat coming off the bottom.

    As to how I ride, I'm generally either at idle-ish with a lot of clutch or grabbing a handful for something big, although that's rare.

    Even with the 45, when the bike came delivered with the pilot at 3/4 turn out the plugs would tend towards the black end of the spectrum. At 1.5 turns out they generally come out more of a nice brown :)
    #23
  4. There are half a dozen Montesa's floating around here, including a few new ones. I've never seen or heard of any of them having to muck with plugs to make them run right. Plus they all run E10 pump gas, which is quite the time saver.

    There are plenty of other things I don't like about them, but how they run isn't one of them.
    #24
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  5. Norman Foley

    Norman Foley Devotee of the Husqvarna Supporter

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    Well Champe raised the question in the Montesa 4RT thread and he has '06 and '17 models....
    #25
  6. And he's been riding trials for how long?
    #26
  7. Norman Foley

    Norman Foley Devotee of the Husqvarna Supporter

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    I think about 4 years.... he did tell me when he bought my Fantic 300, but I'm not sure. He isn't a dirt bike novice though.
    #27
  8. jonnyc21

    jonnyc21 Trials Ninja

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    Ok been trying to figure out jetting better and love this thread... however I am still a little confused on comparison Dell'Orto jetting to Keihin comparison...

    So would the Dell'Orto equevelent to the Keihin be say - Dell'orto: 44, Keihin: 55 and Dell'orto: 105, Keihin: 120?

    just ball parking here, want to get a size or two up and down from a good starting point and play with it a bit on my wife's 08 Gas Gas 125.
    #28
  9. 2whlrcr

    2whlrcr gooligan

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    I've never changed the jetting in any of my trials bikes. I couldn't even guess what's in them for jets, I just ride them, with an occasional turn of the air/fuel screw. My latest is a 250 GG with a Keihin. If I was going to altitude, yeah I'd have to fiddle with it. I do change jetting in my 2t race bikes, but now I have a Lectron on my latest and haven't had to bother with it. Maybe I'm just lazy...
    #29
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  10. jonnyc21

    jonnyc21 Trials Ninja

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    I ride an EFI Montesa 4RT so dose that make me lazy?

    Then again, I am jetting my wife's bike so maybe half lazy. :dunno
    #30
  11. Champe

    Champe Been here awhile

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    Jetting is an important aspect of competing on a 2 stroke. I like reading about solutions here - it's alot easier than running a bike hard. chopping the throttle and reading plugs.

    My first trials bike was a Greeves with an Earles fork, bought in the early 70's. It was seized when I bought it and never ran for me. In the early 90s I had a Can-Am, but was not impressed with it since I was still into MX then. Have owned 2 Fantics and 2 Montesa/Hondas in the past year having changed my focus in my old age.

    Generally dislike the "stinky finger" syndrome from carburetor work. In my younger years I kind of liked stinky fingers, but not from gasoline.

    My preference is for riding, over cleaning or wrenching. That explains why I like (and own) BMWs and Hondas, especially the fuel injected variety. I used to race Maicos (MX) and Buell (road racing) and still ride older KTMs. All of the fast types were carbureted, high performance, and high maintenance.

    Add me to the lazy list. The 4RT requires only the most minimal of fuel handling. It likes ethanol diluted pump gas just fine. In an emergency I can even throw in pre-mix if I want. No pulling the carburetor - ever.

    To be honest, I have had non-starts with both my 06 and 17 4RTs. It was during very cold weather and was temporarily fixed by wirebrushing the plug. Since changing plugs, it has not happened again. Luckily, when it did happen, the other bike would start, so no riding time was lost. Now that we are into the normal (warmer) weather, I expect no more issues - but you can bet there are spare plugs in my toolbox now.
    #31
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  12. motobene

    motobene Motoing for 49 years Supporter

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    One problem with writing about jetting is that unless you have jetting experience already, it's so easy to get lost in the details. I can't explain everything, though I make feeble attempts to. Also, I'm more interested in general principles or trends than recipes for some exact jetting.

    If I do suggest jetting numbers there are resultant caveats from a wide array of variables, not the least of which is the loose nut behind the handlebars :-)

    The number systems between brands are often not the same. Which carb you have is the one to focus on.

    I can discern a lot about how bike and rider are working together in 5 or 10 seconds while checking a section by watching and listening. I often just hold my tongue because what rider in the stress of competition wants to here some engineer say, "your jetting is sub optimal."
    #32
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  13. lineaway

    lineaway Long timer

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    What rider wants to hear some smart ass say your foots on backwards while riding. :lol3 Sorry Chris that the first thing that popped in my head.
    #33
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  14. motobene

    motobene Motoing for 49 years Supporter

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    I should try that sometime and see if anybody notices.
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  15. motobene

    motobene Motoing for 49 years Supporter

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    The new owner of my project 2005 GasGas 225 is coming to pick it up tomorrow.

    One last go-through on jetting has made this Dell'Orto bike run spot on! ...And instilling sellers regret!

    The re jet followed the typical pattern: too rich on top and a little too lean on bottom.

    From: 108 (4 stroking under all but heaviest loads) and 35 (soft running than I like). Needle clip 4th slot down.

    To: 104 and the classic go-to 38. Needle clip 3rd slot down... a fine guess.

    Big positive difference! Why did I wait a year to do this final step? Another 225 I did a while back for some reason was 'asking for' an atypically lean low-speed jet in the 30 to 34 range during setup (it may have a 36 now). That threw me off on my 225, so I kept the 35 all this time. I suspected it was a bit lean, and I definitely knew it was rich on top, but didn't order in some smaller mains until more recently.

    I also thought the way it ran that the reeds were old, but it was just the need for one more jetting iteration.

    #35
  16. Thesolidman1

    Thesolidman1 Adventurer

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    I am puzzled why trials bikes, at least in my experience, do not follow the rules with regards to jetting and elevation. Every 2 stroke jetting chart I have seen indicates leaner jets as the elevation goes up. I practice near sea level. Go to events some in the 6000' range, some even higher. I have tried slightly leaner jets for those locations, and the bike just runs lean. So now I leave it alone. Usually just turn the idle speed up, sometimes fine tune the air mixture screw. I have heard others indicate to actually go richer for high elevations. Seems weird.
    #36
  17. motobene

    motobene Motoing for 49 years Supporter

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    Maybe the jetting charts are wrong :-)

    But bear in mind the trials bikes tend to be jetted too lean down low and too rich on top to begin with - at low altitude.

    The contra-intuitive part comes when experience has found that maintaining the same corrected low-speed jetting for higher altitude is better than stock, but this has been proven, over and over.

    Much that is apparently right in the world isn't, from the flat world to the idea that a feather and cannonball should, in a vacuum, drop at a different rate.

    My explanation for the need for larger jets is that air density affects the venturi effect and pressure differential that pushes fuel up from the float bowl against gravity. A non linear effect.
    #37
  18. lineaway

    lineaway Long timer

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    Atmospheric pressure and the fact that trials bikes are finely tuned down low. Every thing else you twist the throttle.
    #38
  19. motobene

    motobene Motoing for 49 years Supporter

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    Just jetted a Sherco X-Ride today. From 126 main to 120. From 36 pilot to 40. Start jet from 60 to 80. Needle in middle position.

    Even with the 40, the fuel screw 'asked' to be 2-1/2 to 3 turns out.

    Sure runs much better though. Snappy power and no more blubbering at top.
    #39
  20. kenny robert

    kenny robert Long timer

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    most excellent truths bravo can you answer some vintage keehin carb "?
    4 stroke shit
    #40