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    CARB Tuning / Jet Sizes / Needle Height data etc.

    Heres an update on my, carb tuning. Mainly for people searching for carb settings to reference.

    Moriwaki wide open exhaust
    Uni-Pod filters
    Factory stage 3 jet kit.
    Racetech springs and emulators (an absolute must)
    Ohlins shock on it's way.
    Close enough to sea level but I have the japanese alps on the otherside.

    Jet Needle Main Jet Pilot Jet fuel screw Spring float
    position size size turns mm mm

    Original
    Front n/a 142 42 1 and 3/8 168 9.1
    Rear n/a 138 42 2 and 7/8 168 9.4

    40hp Smooth linear Hp curve from red baron dyno
    Any amount of chop and WOT will not budge the front wheel off ground

    1st
    Front 3rd clip 168 42 2 and 1/2 194 9.1 1/4
    Rear 3rd clip 162 42 2 and 1/2 195 9.4 1/2-3/4

    Throttle sloppy but ok until 4K, bogs down after 5 k badly. Pretty disappointing really and made me feel like I had made a mistake in deciding to Jet. When I cooked the spring it shrunk a bit but I thought it would be o.k. On the next rebuild I re cooked the spring to the right length and properly set the floats with a decent vernier (Very very important)

    2nd

    Front 3rd clip 168 42 2 and 1/2 203 9 1/4
    Rear 3rd clip 162 42 2 and 1/2 203 9 1/2-3/4

    Deffinitely better than the last try but alot more work to do.
    Good power right through range with WOT
    throttle sloppy but ok until 4K bogs down after
    bogs down at 5 k badly

    3nd
    Front 2nd clip 165 42 2 and 1/2 203 9 1/4
    Rear 2nd clip 160 42 2 and 1/2 203 9 1/2-3/4

    Throttle smooth as original much better power right through range with WOT Still not achieving full potential
    Still flat spot from 4.5k but not severe. Definitely wants to be tuned out but is livable. 2nd clip position made a big difference right through the range. Much much much smoother.

    4th
    Front 2nd clip 158 42 2 and 1/2 203 9 1/4
    Rear 2nd clip 155 42 2 and 1/2 203 9 1/2-3/4

    Power wheelied (albeit small) at mid revs WOT, throttle a little less smoother than original but an improvement on the last try.
    Further reduced flat spot from 4.5k.
    I reckon I'm gonna try 155/152 next and then 152/150. I think one of them should hit the sweet spot and finally let me move on to the rest of the tuning.

    Best advice i've got o far is, if you've got an M4 exhaust then you should start at 168/162 however if like me you haven't then your jetting will likely be down towards the low 150's. This is based on all the jetting settings that i've found from other peoples post here and elsewhere on the internet. I don't think it necessarily means less power if your using a smaller jet sizes, I think it's alot more complicated than that, bigger is not necessarily better. Saying that at the moment i'm all about trial and error and don't really no shit about the finer points of the carb specific fluid dynamics involved.

    As the bike is right now, It's a massive improvement on the original. I'm holding off going for a proper ride until I get the suspension balanced out with the ohlins hopefully before this weekend.

    After that, it's heating up here and I find a nice sticky 40 mile piece of tar macadam up in thems Japanese alps about 30minutes ride from my door.

    Let the summer time roll on.

    I'll update with any more changes I make for the carbs.

    I'd suggest that anyone else with this kind of data should post it in this thread and i'll format some sort of carb tuning database for others venturing into the black art.

    I've also got pictures of the carbs internals if anyone would like to host them.

    Good luck with your tuning,

    Happy Hawking.
    '88 BROS 650
    Race Tech Springs + Emulators
    Factory Jet Kit + UniPods
    Moriwaki exhaust
    Ohlins Shock
    Want : corbin gunfighter, black pearl wheels, 916 chain guard and lot's lot's more.

    http://www.bikepics.com/pictures/1410547/

    #2
    Sound good rorycooney I am guessing that is for the 650 engine

    If you got pics you can host them on http://photobucket.com/ for free. and link to them using the img button on this reply screen.

    Comment


      #3
      650 it is. Sorry about that. I've gotten so use to everyone on the forum having a 650 that I completely forget that only a year and a half ago I was in Ireland on my 400 looking at the forum with jealousy. Saying that, my BROS 400 was completely stock and I did up to Donegal and down to Cork on a few occasions and have never had so much fun in my life.

      Cheers for the pointer to photobucket, that was pure unadulterated lazyness on my part.

      How's the insurance situation back in ireland, I heard quinn direct had moved in but I might be mistaken. I was paying 2,700 euros for one year on my BROS 400 which is more than I paid for the bike. I was getting royally screwed.

      now I'm paying 600 euros for 2 years insurance on my 650 in japan (better coverage than 3rd party in ireland with insurance for me up to alot of money and theft insurance. Saying that though, where you save on one area you gain pay in another. The NCT equivalent over here is very expensive, and road tolls are ridiculous and cover almost all intercity roads of any use.

      Happy hawking.



      Inside the float bowl. Jets and floats
      Bottom of the cluster of three thingies - main jet held by the needle jet
      Left of the cluster of three is the slow jet/ pilot jet
      the other thingie is not changeable so I don't car what it is, as long as it's clean i'm happy.


      Setting the Float height with a vernier (Someone please correct me if i'm doing this wrong.


      The big round black thing is the carb slide it goes up and down as the air velocity changes. In turn it lifts the needle in and out of the main jet.
      '88 BROS 650
      Race Tech Springs + Emulators
      Factory Jet Kit + UniPods
      Moriwaki exhaust
      Ohlins Shock
      Want : corbin gunfighter, black pearl wheels, 916 chain guard and lot's lot's more.

      http://www.bikepics.com/pictures/1410547/

      Comment


        #4
        I just did a trip around Ireland with much bigger bikes and the 400 kept up with them and when we swapped bikes they all agreed mine was the most fun. But like you I crave the 650..

        Insurance is still pretty bad here, I pay 800 euro for third party (no fire or theft) with fully licence and 5 years no clam bonus. I do here there is more companies coming into the market.

        I too am messing round with carbs, I have a two spare sitting here and trying to get a good match for the 400 with with the Fuel exhaust no baffle with stock air box( K+N filter) --reading everything I can related to them.

        oh and nice pics.

        Comment


          #5
          You have to make sure when you check float heights that the float is not depressing the little spring plunger on the float valve. If you set the carbs upside down and measure, you will get the wrong reading because the weight of the float pushes the plunger down. It's better to hold the carbs at a sideways angle so that the floats just baaaaarely touch the float valve and take a reading. Could explain why you are needing leaner mains that normal, if the float is too high it will run quite rich.

          jH
          J.D. Hord
          Keeper of Engine Nomenclature, 9th Order

          Comment


            #6
            Re: CARB Tuning / Jet Sizes / Needle Height data etc.

            Originally Posted by rorycooney
            1st
            Front 3rd clip 168 42 2 and 1/2 194 9.1 1/4 Rear 3rd clip 162 42 2 and 1/2 195 9.4 1/2-3/4
            What's the figures in red s'posed to represent?
            '95 ShaftHawk 650P>
            Front: Bros Mk2 front wheel; Mk1 forks; MetalGear disc; NC30 caliper; cb1 yoke.
            Breathing: Hacked up and shortened ART tri-can on collector and link pipe from DemonTweeks.
            Other: RGV bars; DefT dash; modded subframe; modded rearsets; relocated ignition.

            '91 Bros 650 Mk1>
            Currently off the road.

            Comment


              #7
              I'm guessing 9.1 and 9.4 are the float height, and needle groove postions are counted from the top 194 and 195 are the spring lengths
              "Life may begin at 40, but it doesn't get real interesting until about 150."

              • '88 in Candy Flair Blue + '90 in Italian Red
              • Ohlins Rear Shock
              • F2 front wheel
              • VFR750 rear wheel
              • Hiperform seat&headers
              • MSMotorsport Seat Cowl
              • Steve Lenac Tokico six pot caliper

              Comment


                #8
                Hi deftrap, JR is right, here's the format I was using. It didn't transfer well from excel.


                Jet Needle Main Jet Pilot Jet fuel screw Spring float
                position size size turns mm mm

                JD, thanks for the advice on the floats. Your procedure of tilting the carbs so that the floats are not touching the valve and then tilting the carbs back just enough so that they are touching is exactly the procedure I was using. It was just a bit difficult to get a picture as it was tricky enoug by itself.

                I figured that the difference between the moriwaki and M4 was the biggest reason for the difference in jet sizes. I will be however be re-checking the float height next time I rebuild just to be sure. I did notice that one side of the float is deffinitely lower. I don't think it would effect the float level though. Should they be exactly the same or what is the tolerance.

                Thanks for the input again.
                '88 BROS 650
                Race Tech Springs + Emulators
                Factory Jet Kit + UniPods
                Moriwaki exhaust
                Ohlins Shock
                Want : corbin gunfighter, black pearl wheels, 916 chain guard and lot's lot's more.

                http://www.bikepics.com/pictures/1410547/

                Comment


                  #9
                  update

                  I just installed a 152 front / 150 rear and the results are excellent. seemingly more power again, these mini power wheelies are quite a lot of fun. The flat spot from from 5 k up with mid throttle is almost gone. I think I'll probably try 1 size down again just to see if I deffiniteley have found the best main jet setting then i'm going to put the needle position back to 3rd from the top and see how the mid range feels then.

                  I re-checked float heights and they are perfect at 9.0mm when just slightly resting but not compressing the valve. So after finding the best clip it'll be on to the fuel screws and then the mountains. Gotta get some riding in before rainy season.
                  '88 BROS 650
                  Race Tech Springs + Emulators
                  Factory Jet Kit + UniPods
                  Moriwaki exhaust
                  Ohlins Shock
                  Want : corbin gunfighter, black pearl wheels, 916 chain guard and lot's lot's more.

                  http://www.bikepics.com/pictures/1410547/

                  Comment


                    #10
                    Re: update

                    Originally Posted by rorycooney
                    ..... these mini power wheelies are quite a lot of fun. ......
                    I wish my bike would do those.....

                    -Jeremy
                    Jeremy N.
                    1988 Hawk GT sorta custom-ish?

                    Comment


                      #11
                      Re: CARB Tuning / Jet Sizes / Needle Height data etc.

                      Well done for this thread, I think it's a great idea to stick it in one place. MODS can we make this a sticky?

                      Originally Posted by DefTrap
                      Originally Posted by rorycooney
                      1st
                      Front 3rd clip 168 42 2 and 1/2 194 9.1 1/4 Rear 3rd clip 162 42 2 and 1/2 195 9.4 1/2-3/4
                      What's the figures in red s'posed to represent?
                      So I'm reading in this example (just doing the front cylinder for simplicity) Needle clip in third slot from top, Main Jet 168, Pilot Jet 42, Air/Fuel screw 2.5 turns out, Spring length 194mm, Float height 9.1mm and 1/4 of???

                      I'm curious what the last fractions are, after the float heights (1/4 and 1/2-3/4 in the above quote)

                      E

                      Comment


                        #12
                        The fractions are actually throttle openings. In my spreadsheet I had written down how the bike felt at 1/4, 1/2 - 3/4, and WOT. It was late and spaces and tabs wouldn`t stay in when I submitted the post.

                        As for the mini-wheelies, don`t be too jealous, they really are very miny wheelies although I haven`t been trying very hard. I`m going out tonigh to see what it can really do now that the paranoia of what will happen when the front wheel touches back down again is gone.
                        '88 BROS 650
                        Race Tech Springs + Emulators
                        Factory Jet Kit + UniPods
                        Moriwaki exhaust
                        Ohlins Shock
                        Want : corbin gunfighter, black pearl wheels, 916 chain guard and lot's lot's more.

                        http://www.bikepics.com/pictures/1410547/

                        Comment


                          #13
                          Quick question from newbie on Carbs

                          I have a quick question. I decided to clean out my carbs and now I'm not sure where one hose connects. I was careful to label things when I took it apart, but I have this one long hose that runs on the top of the carb. towards the front of the bike. It appears to connect near the vacuum chambers. I've looked in the manuals and a number of places but cannot find an appropriate connector nearby.

                          Where does it go??????? what does it do?

                          88 Hawk NT650

                          Comment


                            #14
                            Re: Quick question from newbie on Carbs

                            Originally Posted by zmanbill
                            I have a quick question. I decided to clean out my carbs and now I'm not sure where one hose connects.

                            Where does it go??????? what does it do?

                            88 Hawk NT650
                            Brewsy posted a pic or two a while ago for just this purpose. Search for his posts and it ought to come up there. Can't remember if it was in pics or in Mech and Tech...

                            E

                            Comment


                              #15
                              Originally Posted by zmanbill
                              I have a quick question. I decided to clean out my carbs and now I'm not sure where one hose connects. I was careful to label things when I took it apart, but I have this one long hose that runs on the top of the carb. towards the front of the bike. It appears to connect near the vacuum chambers. I've looked in the manuals and a number of places but cannot find an appropriate connector nearby.

                              Where does it go??????? what does it do?

                              88 Hawk NT650
                              Do you mean the hose that connects to the top of of both carbs near the intake with a little "y" connector and then dumps over the side? Those vent the carb bowls as far as I can tell. I think on CA models they go to Air Vent Control Valve, but on 49 states it just hangs loose.

                              Comment

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