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Help me ID these? and can they be tuned? Mikunis

Discussion in 'The Garage' started by gggGary, Mar 4, 2011.

  1. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    1980 and 82 mikuni BS34 carbs BOTH need the rubber plugs to seal the pilot jets. otherwise it basically won't run because it won't pick up any fuel for the midrange.

    The 80 carbs came with brass floats the 82 with foam floats. The 82 carbs were painted black and use a handle bar choke otherwise they are the same as the 80 carbs.
     
  2. wrmdovr

    wrmdovr If it were easy.....everyone would do it!

    i have a 1979 motor with BS34's and am completely lost on tuning them. the carbs have velocity stacks....currently have 27.5 PJ and a 130 MJ and running straight back 1 3/4 pipes with no baffles. i also have a PAMCO installed.

    left side idles o.k. however right side seems not to be firing...exhaust is cool not warm as other side.

    Any help would be greatly appreciated.

    WRMDOVR
     
  3. twindallas

    twindallas XS650 Addict

    Are all these Mikuni carbs stamped with the model I/D's? I haven't found it yet. I'll look again.
     
  4. Mattk11

    Mattk11 XS650 New Member

    3
    0
    1
    Boston
    I got a TM34 flatslide with 210 main Jet, needle set to lean, pilot jet seems good. Throttle feels Asymmetrical, as in 0-20 is rough. WOT is like a race bike. Is this jet low or High?
     
  5. jabcb

    jabcb XS650 Enthusiast

    Look at the flat surface facing the motor where the choke cables are attached.
     
  6. 5twins

    5twins XS650 Guru Top Contributor

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    The VM slide carbs have I.D. numbers stamped there by the chokes. As far as I know, there are no numbers stamped on the BS vacuum carbs. We can I.D. them because we've studied them and know how they look. We know the little changes made through the years.

    You may find one letter stamped on the bottom of the float bowl but that has nothing to do with the carb or carb set I.D. I'm really not sure what that letter means.
     
    gggGary likes this.
  7. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    An excellent jetting compilation from threads here and on XS650 rider, by 2many xs1bs

    MJ = Main Jet
    PJ = Pilot Jet
    NJ = Needle Jet (emulsion tube)
    JN = Jet Needle
    Slide = Cutaway number
    _________________

    Roy Fisk
    Carb: VM34
    MJ: 190-200
    PJ: 22.5
    NJ: P6
    JN: 6DHS, 3rd slot
    Slide: 2.0
    Air jet: 2.0?
    Air screw: 1.25- 1.75
    Float height: 25mm
    _________________

    Richard, from the Aussie site
    Carb: VM34
    MJ: 180-190
    PJ: 20-25
    NJ: P5, P6
    JN: 6F9, 3rd slot
    Slide: 2.5
    Air jet: Removed

    Dwayne, from the Aussie site
    Carb: VM34
    MJ: 180
    PJ: 25
    NJ: P6
    JN: 6F9
    Slide: 2.5
    Air jet: 2.0?

    Bob Hart, from the Aussie site
    Carb: VM34
    MJ: 180
    PJ: 27.5 - 32.5
    NJ: P5, P6
    JN: 6F9
    Slide: 2.5
    Air jet: Removed
    _________________

    HALCO (UK)
    Carb: VM36
    MJ: 220
    PJ: 25
    NJ: 159-Q2
    JN: 6DH3
    Slide: 2.5
    Air jet: 2.0

    BOB BERTAUT (VM36 CARB KIT)
    Carb: VM36
    MJ: 190
    PJ: 22.5
    NJ: 159-P6
    JN: 6DP1
    Slide: 2.5
    Air jet: Removed
    _________________

    Yamaman
    Carb: VM34 with Ramair pods, Triumph silencers ,Halco 1 1/2" pipes.
    MJ: 185 - 190
    PJ: 25
    NJ: P6
    JN: 6DH4, 3rd slot
    Slide: 2.0
    Air jet:
    _________________

    gggGary's Madness
    Carb: VM38
    MJ: 260 (220?)
    PJ: 25
    NJ: 166-P8
    JN: 6DP1, 3rd slot
    Slide: 1.5
    Air jet: Removed.
    Float valve: 3.3

    gggGary's Pristine, never had gas in them yet.
    Carb: VM34
    MJ: 180
    PJ: 25
    NJ: 159-P6
    JN: 6F9, 3rd slot
    Slide: 2.5
    Air jet: ?
    Float valve 3.3

    gggGary's Unknown
    Carb: VM34
    MJ: 260
    PJ: 40
    NJ: 247-Q2
    JN: 6DH4, Top slot
    Slide: 2.5
    Air jet: ?
    float valve 1.5
    _________________

    Grizld1's 700cc
    Carb: VM36
    MJ: 190
    PJ: 22.5
    NJ: 159-P5
    JN: 6F9, 3rd slot
    Slide: 2.5
    Air jet: Removed
    _________________

    TwoManyXS1Bs 750cc Dragon
    Carb: VM36 (Vintage 1975, opened to 37.5mm)
    MJ: 270-280
    PJ: 35
    NJ: 159-Q5
    JN: 6FJ6, 3rd slot
    Slide: 3.8
    Air jet: 2.0
    _________________

    Barncat Ported head, performance exhaust
    Carb: VM36
    MJ: 190
    PJ: 25
    NJ: 159-P6
    JN: 6F9, 2nd slot
    Slide: 2.5
    Air jet: none
    air/idle screws out 1-1/2 turns
    Very smooth power delivery and pulls hard through the entire range.
     
    Last edited: Jul 20, 2019
    TwoManyXS1Bs likes this.
  8. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Weaselbeak posted up this jet size comparison

    Keihin, Dynojet, Mikuni Jet Size Cross Reference

    All Hole Diameters Are in Inches

    Hole Diaâ€---Keihin # -- DynoJet # -- Mikuni #
    0.0350---- 92.5--------- 92----------- 86.3
    0.0360---- 95----------- 94----------- 88.1
    0.0370---- 97.5--------- 96----------- 90
    0.0380---- 100---------- 98----------- 91.9
    0.0390---- 102.5------- 100---------- 93.8
    0.0400---- 105--------- 102---------- 95.6
    0.0410---- 107.5------- 104---------- 97.5
    0.0420---- 110--------- 106---------- 99.4
    0.0430---- 112.5------- 108--------- 101.3
    0.0440---- 115--------- 110--------- 103.1
    0.0450---- 117.5------- 112--------- 105
    0.0460---- 120--------- 114--------- 106.9
    0.0470---- 122.5------- 116--------- 108.8
    0.0480---- 125--------- 118--------- 110.6
    0.0490---- 127.5------- 120--------- 112.5
    0.0500---- 130--------- 122--------- 114.4
    0.0510---- 132.5------- 124--------- 116.3
    0.0520---- 135--------- 126--------- 118.1
    0.0530---- 137.5------- 128--------- 120
    0.0540---- 140--------- 130--------- 121.9
    0.0550---- 142.5------- 132--------- 123.8
    0.0560---- 145--------- 134--------- 125.6
    0.0570---- 147.5------- 136--------- 127.5
    0.0580---- 150--------- 138--------- 129.4
    0.0590---- 152.5------- 140--------- 131.3
    0.0600---- 155--------- 142--------- 133.1
    0.0610---- 157.5------- 144--------- 135
    0.0620---- 160--------- 146--------- 136.9
    0.0630---- 162.5------- 148--------- 138.8
    0.0640---- 165--------- 150--------- 140.6
    0.0650---- 167.5------- 152--------- 142.5
    0.0660---- 170--------- 154--------- 144.4
    0.0670---- 172.5------- 156--------- 146.3
    0.0680---- 175--------- 158--------- 148.1
    0.0690---- 177.5------- 160--------- 150
    0.0700---- 180--------- 162--------- 151.9
    0.0710---- 182.5------- 164--------- 153.8
    0.0720---- 185--------- 166--------- 155.6
    0.0730---- 187.5------- 168--------- 1575
    0.0740---- 190--------- 170--------- 159.4
    0.0750---- 192.5------- 172--------- 161.3
    0.0760---- 195--------- 174--------- 163.1
    0.0770---- 197.5------- 176--------- 165
    0.0780---- 200--------- 178--------- 166.9
    0.0790---- 202.5------- 180--------- 168.8
    0.0800---- 205--------- 182--------- 170.6
    0.0810---- 207.5------- 184--------- 172.5
    0.0820---- 210--------- 186--------- 174.4
    0.0830---- 212.5------- 188--------- 176.3
    0.0840---- 215--------- 190--------- 178.1
    0.0850---- 217.5------- 192--------- 180
    0.0860---- 220--------- 194--------- 181.9
    0.0870---- 222.5------- 196--------- 183.7
    0.0880---- 225--------- 198--------- 185.6
    0.0890---- 227.5------- 200--------- 187.5
     
    TwoManyXS1Bs likes this.
  9. 650Skull

    650Skull Dinosaur Lives Top Contributor

    Credit where credit's Due...................Those Aussie site setups do not come from the Aussie site..........Read the thread i linked to where this info come from
     
  10. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Sure, I did it all myself! :redface:
    It's a compendium from a 650 archive thread and at least one thread on here. Many authors, all sharing in the spirit of the 650 brotherhood. Skull brought the thread with most of the set ups to our attention. 2many did the clean up and organization. Kudo's to all mentioned or not.
     
    TwoManyXS1Bs likes this.
  11. 650Skull

    650Skull Dinosaur Lives Top Contributor

    Point i'm making is critical information has been left out and location is wrong on some..............Elevation and climate makes a difference to the set up so if that is left out.....................
     
  12. X77S

    X77S Unmotivated..... HELP!

    Gary, so in your opinion is it worth it to move from the bs38 to the vm34? My bs38 carbs will need a total rebuild. If there are better characteristics in the VM carbs where throttle response is concerned it may be worth the couple hundred dollars for the change.
    My experience with bikes so far are limited to 883 Sportsters. I had the stock CV on one bike and a Mikuni HSR42 on another. The Mikuni had way better throttle response and was easier to tune.
    I appreciate any advice one way or another.
     
  13. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    I'm a big fan of stock carbs, unless you have built a performance motor it's hard to beat a well fettled set of 78-79 BS38s. Heck I like BS34s too. Just sold a really nice set of BS34s maybe I'll try the set ninja carbs I got out in the shed next. But I'm also very happy with how the VM34s have run on madness this year put 3-4K miles on it. Want to try a set of Keihin pumpers someday also.
    Guess the point is, if set up properly and tuned for your engine, altitude they all work pretty darned good.
    PS if you are using CV carbs you need to keep big airboxes in front of them. CV's just don't like tiny air filter volume. There's a reason manufacturers go to such lengths making huge airboxes that squeeze every speck of room available into the box. A Ducati forum did some testing on one bike, a larger airbox from a previous model was worth 10 horsepower, Yamaha got similar results increasinging air box volume on the V4 Ventures.
     
    Last edited: Dec 9, 2018
    grizld1 and TwoManyXS1Bs like this.
  14. X77S

    X77S Unmotivated..... HELP!

    I've modded my seat frame so the factory airboxes are out of the question. I wonder if I could fabricate my own to fit in the remaining space. Or perhaps it's a good excuse to go ahead an get the new carbs.
     
  15. grizld1

    grizld1 Grumpy old man Top Contributor

    Guys, those jet lists are nice, but they're not likely to be very helpful unless the user keeps a few things in mind. First: note the inverse relationship between NJ size and MJ size. Second: note the inverse relationship between slide cutaway and PJ size (smaller number=richer slide, richer slide forces leaner PJ, etc). And finally, understand that needle position directly impacts pilot circuit tuning: A change of 1 step will always force mix screw adjustment and will often force a PJ change. The pilot circuit never turns off, and the main circuit impacts the mix even at idle, through the slide cutaway.

    The variation in those lists demonstrates something to keep firmly in mind. Carburetors have to maintain a usable fuel/air mix across a wide and shifting range of demands, and they have to do it with a few simple components. The most accurate jetting involves compromise. Bottom line: there are many combinations that work, there are some that work better, and there are some that don't work at all.

    As far as jet lists go, treat them as possible starting points. Some guys take to tuning aftermarket carbs like fish to water, some guys get lucky, and some guys wind up wishing they'd never been tempted to fool with old motorcycles at all, let alone modify one. The best advice I can offer the beginner who wants to hang aftermarket carbies on his bike is first: Read some threads by others who've done the same, asking yourself if you have enough patience and frustration tolerance to enjoy the tuning job as a project. And second: Buy a responsibly baselined set of carbs from a reputable vendor who owns and knows the XS650 and offers tech support after the sale. In the USA, I can recommend Michael Morse of 650 Central and Gary Hoos of Hoos Racing.
     
    gggGary and TwoManyXS1Bs like this.
  16. grizld1

    grizld1 Grumpy old man Top Contributor

    X77S, Mikuni flat slide pumpers like the HSR42 are indeed a joy to tune; component effects are much better isolated than they are in early designs like the VM, which are touchier (in spite of being simpler). You should see an improvement in throttle response with well tuned VMs, but not as much of a difference as you've seen between a vacuum diaphragm carb and a pumper with flat slide and direct connection between twistgrip and slide.
     
  17. X77S

    X77S Unmotivated..... HELP!

    Thanks Griz. Perhaps it's best if I just rebuild this set I have. I will look into what kind of air cleaner/ box I can make work if the Uni filters work decent I will probably go with that and be done with things.
     
  18. 5twins

    5twins XS650 Guru Top Contributor

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    As far as pod filters go, the long, straight foam UNIs work best on the stock CV carbs. I know from experience that the K&N pleated style pods don't work as well. You won't be able to tune the carbs perfectly with them. The bike will run, just not as good as it could. I finally have an airbox equipped 650 and truth be told, I don't see much difference compared to my other pod equipped one. Both run well and I'd be hard pressed to pick one over the other as far as performance goes. But, I haven't had my way yet with the carbs on the airbox bike, lol. They're still jetted all stock and being BS34s, I know they're a tad lean. Maybe once I richen them up a little bit, my airbox opinion will change. At the moment, the only airbox "plus" I'll concede to is rain riding, of course they're better for that.
     
  19. grizld1

    grizld1 Grumpy old man Top Contributor

    I've used Outerwears rain covers with K&N filters on Mikuni VM round slide and TM flat slide 4-stroke pumper carbs, and they do a great job of keeping the filters dry and breathing freely. The covers create very little restriction. Outerwears will make covers for filters of any dimensions, and I've often wondered how they'd work with Unis.
     
    gggGary likes this.
  20. 5twins

    5twins XS650 Guru Top Contributor

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    The only issue I've experienced with my pods in the rain is when I come to a stop. The bike can run rough and even stall if they're really wet or sucking in water because it's down pouring. At speed, running down the road, they seem OK in the rain. I don't do much rain riding so it's not an issue I have to deal with very often. What I enjoy and take advantage of much more often is the ease of access the pods provide.
     
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