The Crying Shame of Jetting

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

  1. motobene

    motobene Motoing for 49 years Supporter

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    Ask and (maybe) you shall receive!
    #41
  2. motobene

    motobene Motoing for 49 years Supporter

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    Just a reminder on how other variables figure in. A putt putt rider is a significant variable. Floating around with the throttle slide low creates little fuel demand, and will make a bike jetted for who pushes the throttle seem over rich.

    Altitude, float level, spark arrestors (make bikes run richer), traction (richer when there is little), clogged nozzles from long-term storage with fuel in the bowl, even this weirdness:

    http://advrider.com/index.php?threads/s3-bl-700-r-carburetor-screws-warning.1223037/

    ...can factor in.
    #42
  3. kenny robert

    kenny robert Long timer

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    ok so i got cb750ko keihins on my african twin cb350 based heater http://advrider.com/index.php?threa...onda-never-built.1181059/page-4#post-32053345
    when i rode 15,000 milkes on the bike as the sl350 the 24mm keihins smaller version of same oval venturi crab,were outstanding easy to work on clean crisp running etc
    but it has different spec now 24 would not do,so i got some 28's\
    anyway but then i got a parts lot of nos brass as well
    i really dont expect the carbs to be way off but the geedam nozzle that presses in and is from what i can tell the flow control for needle when on taper
    lol i got a hundred of them but they got no marks ass to size
    i have checked a bunch with a tapered hole gauge deal and they are not all same
    issat only way measure the pricks?and does each needle have only 1 correct nozzle ? or is it a way to tune ? going up or down nozzle size with same needle ?
    #43
  4. motobene

    motobene Motoing for 49 years Supporter

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    Woa! That was like a plate of spaghetti in the face!

    Hey why not just fit the old SL350 oval bores back on? One of the assumptions of the non-trials world is bigger-bore carbs are better, and there is an over emphasis on WFO power.

    I don't have direct experience in the CB750KO, so I can't really solve your particular problem, but if it was working excellently before why muck with it?
    #44
  5. kenny robert

    kenny robert Long timer

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    well the kick start only sl350 is very slow, honda made it that way compared to the cb/cl 350,super mild cam, less lift ,less duration, no overlap !and the intake ports are small as well
    so while it puts really well and will plonk really low rpm it does indeed simply not have the ability to use anything bigger you can feel it run out of breath but i did modify fy mine just smoothed up the oval opened up throats a tiny but probably gained 1mm actual flow size
    lol it ran a touch better up at 9000+ rpm no bullshit i ran it wfo a lot
    the engine i'm using now has a nos early 350 high rev cam correct long pipes,tuned intake cam dialed in a kilo i \turned off the crankshaft no sparkler arrest a bit os on bore but that dont mean shit but yeah i am with you lots of fools try to put 32mm or even 30mm "prejetted" mikuni vm's on the 350 twins poor dumb bastards
    28mm is all you need and vm mikuni is a goddam 2 stroke carb unless it has correct needle jet etc
    that said my nos head is a sl head same valves and exhaust port sizes all cb/cl 350 twins ,but had the smaller intake ports that i have matched and opened up to the 28 keihin
    my pipes are not much bigger than stock pipes inner diameter is about same maybe .010 larger at begiuning but very slightly tapers in steps starting midway finally about 1/4'' total at silencers
    another falacy fools figger small header pipes create back pressure that an engine is tuned for and it needs that back pressure to run right stupid f$%*&ers
    but its just the opposite bigger pipes can easily ruin power because flow is slowed down so much it creates "back pressure" a dam of slow cooling gases
    im not going bigger but am optimizing it will be 25-30% up on hp to an sl motor
    it is astounding how =many think that rejetting a set of carbs on a 4 stroke is just something anybody can do in a couple afternoons
    they dont realize that only 1 out of 50 really good mc mechs can properly jet a 4 stroke with a non stock carb rack and i am not one of them
    i kept my sherpa t 350 stock when i was competing observed trials sheeit it never fouled a plug
    its the biggest scams in motorcycle history "prejeted" vm mikunis for your cafe racer lol
    take voff the airbox and use pods for more air ,another huge myth wrong again
    the stock pipe has a lot of backpressure lol z
    #45
  6. motobene

    motobene Motoing for 49 years Supporter

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    Wow. Barkbuster things too.

    That bike is out of my core competency, and this venue is trials, so this is probably not the best place to get answers. Been fun thinking about it though!
    #46
  7. kenny robert

    kenny robert Long timer

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    bah trials schmiales it will never be very popular at all in the usa ,even more so since it was radicallized 30 years ago,just as i lost interest, by the bicycle trials boys,bernie and others that started using the clutch to wind up, launch and hop
    i really loved trials ,early mid 80's nothing poppy yet,very traditional but too totsally weird for 99.99% of dirt bikers, to serious,lol took skills to do it but i knew better i used it. trials competition. (man donner ski ranch a whole mountain of granite traction) to go fasster in my quest to make top 20 enduro expert and to win a national enduro overall
    im not sayin because of what i learned in trials that allowed me to achieve those goals but it was a huge help learning things like rear brake and more throttle to gain traction again
    and the focus and weight transfer all of it i even moved the pegs back 2"and up 1-2" on all my dirt going machines after doing some trials and analysing the deal
    my buddies laughing and shaking a finger man you gotta stopp doing that
    doing what ? moe-toe-crossing the sections,fool, you are terrifying the checkers,you cant do that
    i think trials is still bitchen but in usa its leper-like in its unpopularity a shame indeed
    bitchen information you have a fantastic great thread it should be applauded
    as to my problem well it wasnt a problem,well problem is we still dont knowif nozzle sizes alone are a tune or nit
    we think alike i am still using same
    oval bore keihin carbs,the 24 and 28 are exactly the same design
    my heater now pumps more air thats all,24's cannot pass the air needed for the hp numbers ,i have not tested yet, soon will and saw this thread keihins,
    thanks a ton,you are a highly knowledgable fellow ,didnt ythink i was high \jaking,your "trials" thread ,see,i saw the thread as more a learning ,tuning,carb jetting thread than trials one....dont you see?see you trials nuts are seperatists. ya weird, nerdy pricks:lol3
    hang up the chain mail,put down the drawbridge and empty the moat ,ffs :photog
    maybe someday 4 strokes will be used in trials:-)
    i don't expect to be doing any major carb work at all the 350 twin is almost exactly the same engine as a cb750 sohc per jug
    the carbs are 100% new and ill try the stock cb750 jets
    i was just wondering about the unmarked keihin nozzles
    and i am still learning and asking thats all
    #47
  8. motobene

    motobene Motoing for 49 years Supporter

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    OK, wow.

    Maybe we'll get you back on the dark side some day. At least the carburetion is much simpler:-)

    BTW, modern or not, 90% of trials riders are just ordinary Joes who don't use advanced clutch techniques, so not much has changed since the '80s. My experience spans from then since. The bikes are much better now, but the fundamental techniques still rule the majority.
    #48
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  9. The clutch is the most fun part of riding a modern trials bike...
    #49
  10. motobene

    motobene Motoing for 49 years Supporter

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    One finger without Popeye forearms!
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  11. motobene

    motobene Motoing for 49 years Supporter

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    Just jetted a year 2000 GasGas 321 with spark arrestor. Ran way rich on with a 112 main. Bike has a spark arrestor. Reduced the main to 108. Low speed jet was increased from from 36 to 38.

    Needle is D35 instead of the more common D36.

    Will test ride tomorrow.
    #51
  12. kenny robert

    kenny robert Long timer

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    yeah i know i was trying to be funny something i usually fail at horribly
    but eye sure like motorbikes any and all that are ridable anyway lol
    lets just hope the latest cafe racer fad dies off soon ,before they come up with a magic fix ,ie an engine tune de-molester,so the shit actually can get past 20mph,and the morons start hitting public roads
    #52
  13. Norman Foley

    Norman Foley Devotee of the Husqvarna Supporter

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    Mid 80's National Enduros... You must of ridden my club's National.... Ithaca Dirt Riders/Speedsville Enduro

    Start Control 1987. Dave Bertram and Kevin Hines in the foreground...
    [​IMG]
    #53
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  14. kenny robert

    kenny robert Long timer

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    AWESOME well i did compete in a few west coast nationals and was in district 36 dave bertrams district, where i got my number 16 trophy plate 16th overall expert for a season the coveted top 20
    sheeit we had i think 8 enduros maybe 10 or 12 per year in district 36 a huge district with 1000's of enduro riders i rode about half -3/4 of them for 6 or so seasons
    never rode on same minute as dave but for sure on the minute before a few times
    kinda humbling when he would come freight training through in the special test sections
    bertram ,so fast ,you cannot comprehend it till you ride with em
    so smooth on those husky autos,casual observer , he never looked that fast but just try to stick with him a few times in a special test everybody at risk of or indeed running late,you get your mind right,luke, yes you do
    for those that might ask believe me you enduros are ONLY won by the fastest mother ^&*() on the day racers with top pro mc racing talent
    who keeps there wits and keeps from burning the check into a special test
    usually one before one after gas
    see you have a secret check and its an easy one to zero,to be on time, infact they are trying to burn,catch you coming in early ,see because if you study the route chart you pretty much are knowing the special test is here or there ,and for sure you want to zero it
    thing is, instead of one point per minute late ,burning ,being early is like 5x the penalty points
    and the special test is special cause it is knarly ass trail w3ith a speed average so high, it is difficult to maintain foe even grissled experts
    it jersy shredding time
    of course dave being double A expert(top 20 overall in the nation to get that double A we were never close minute wise in a national
    notice he is on minute 24 AA group
    whearas us district A's would start early at the nats first 25 minutes
    i may not be exact in my recollections about those details
    i think it was 1984 the RATTLESNAKE national enduro the west coast rattle snake put on by the redwood scramblers out of crescent city ca,disrtict 36
    lol i was on minute 2 only 4 riders as minute ahead
    the old caldor mill site was base camp for the run right on the mighty klammath river in klammath ca
    funny thing when you turned off the coast hwy to go up river to the start there were concrete bears on either end of the coast highway spanning the mouth of the klanmmath river
    so minute 2 as you know you can just haul ass first i think 5 miles were "free" miles no nchecks to burn gave you a chance to warm up and stop and rest a few minutes ,maybe adjust yer rolloffs or front brake,then get to racing when yer rollchart tellsd you its time
    that first five miles was a lot of fresh "trail" lol sheeit you rarely saw dirt it was willow saplings or whatever soaking wet and just barely layed over as even the club saved it for the hapless soles on early minutes to get it cleared out
    it was not easy at times to keep feet up on pegs and the green wet wood whips would flail at you like they pissed off
    it was a mild climb up a crik dranaige beautiful blue sky sun flittering off the dew and a goddam had to be 400 lb bear obviously scared shitless from the first minute
    she wasd in a dead panic came busting up from the crik i had to lift fer a moment or we would have had a closer encounter
    then she proceeded to show me the way for a good 2-300 yards up the trail
    right in front of me i nearly tagged her a few times but it was astounding how fast she ran fur ripling in big waves of red brown and black colors enhanced by the crystal clear morning sun
    steam coming out of her so dense i had to turn on my smith turbo fan goggles lol
    i mean it was a once in a life beauitiful thing ,man,you might call bullshit but i was there and so were da bears not just the ones in concrete
    she finally lunged off trail ,exisiting stage left
    back down into the crik drainage direction into the thick greannery vasnishing in a flash
    it was a brutal run, one in my favor because i was almost always too late to even worry about burning checks
    just pin the throttle and stir the box t
    i finished with my new jersey hanging off shredded like it was wet newsprint
    i got 10th overall 1rst overall a only the aa pros beat me and i beat quite a few
    i was so awesome lol all puffed up and proud as a newly hired playboy magazine titty fluffer
    wall of words
    #54
  15. Norman Foley

    Norman Foley Devotee of the Husqvarna Supporter

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    National AA guys started on 21, with National Champion on that row and everyone else by National Championship status the year before, so Dave was 4th place and Kevin 5th. All different now, as there is a Pro Class and AA/Expert Class. This year Pro riders moved from starting on 21 to 31 swapping with the Pro/Experts who are now on 21. The Pro's lobbied for this, to get another 50 riders to burn the lines in ahead of them. Also start rows are set by last finish position now, not last years Championship position. Last event winner goes to 31A and so on. Of course time keeping is history and fastest guy between checks wins...
    #55
  16. motobene

    motobene Motoing for 49 years Supporter

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    When threads go entirely OT... :-)
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  17. kenny robert

    kenny robert Long timer

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    huh aka hare scrambles
    you could have posted that start and asked who is here?
    bertam i recognized him right off , he was the man in dis 36 for a long time
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  18. kenny robert

    kenny robert Long timer

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    bertam shir had that huskautotrials bike jetted crisp
    it barked like a =noir-wigin sex slave
    #58
  19. Norman Foley

    Norman Foley Devotee of the Husqvarna Supporter

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    Sorry Chris!
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  20. jonnyc21

    jonnyc21 Trials Ninja

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    Time to bring this one back up to the top...

    Finally felt like I got enough information to properly re-jet my wife's 125. Where I had it better than stock it wasn't perfect. Still lean as a result of most jetting information I could find at the time was MX bike based.

    It has a dellorto carb for reference:
    I now have a 42 pilot and a 102 main (was going to go 100 but My source didn't have smaller). I haven't had a chance to do much on it yet but right away it now starts first kick. I can then take the choke off and it will stay running with only little blips on the throttle or a momentary flip of the choke lever. The short test was promising and I should be able to give a full post jetting ride report tomorrow.
    #60
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