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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

    Here's what I know; the engine was on a go cart with these on it. Motor ran but never "well" The bores measure 38mm they are on new carb holders. Engine looks to be a 72 or 73 it has a decompressor valve.
    Haven't looked inside to see how they are jetted. So can a set of 38 mikunis be tuned to run "well" on an XS650? any ideas of where to start? Assume pods and a Kerker 2 into 1 with "some" baffling to start.

    grnbyxs650 008 (1024x768).jpg grnbyxs650 004 (1024x807).jpg grnbyxs650 005 (1024x768).jpg
     
  2. 5twins

    5twins XS650 Guru Top Contributor

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    There should be an I.D. # stamped on the carb body. This is a VM32 .....

    [​IMG]

    Here's a chart with the I.D. #s and off-the-shelf jetting specs for the generic VMs. The # in the 1st column (without the "VM") is what you'll be looking for stamped on the body .....

    http://www.carbparts.com/mikuni/vm__round_slide_standard_specifi.htm

    I'm sure they can be tuned but are a bit larger (if they're 38s) than what most like to use (the 34s). I would follow the jetting specs for the 34s. You probably will need slightly different mains and pilots compared to the 34s, maybe needle jets as well.
     
  3. Dogger

    Dogger XS650 Addict

    I know very little about this kind of thing, but I have 36vm roundslides on my bike and they raise rpm at which you get optimal torque, the problem is they raise it up into the less usable range for the street. Where as the 34s keep it down in the rpm where we run every day.
    Deduction would conclude that the 38s would raise the torque range even higher. From what I understand, the bigger carbs are great for race bikes with 750 kits and 1 3/4" pipes that like to scream.
    I haven't got my bike to run they way I want since I installed those carbs but I've been told that they will work with some patience. My plan this spring is to work at them till they're perfect.
     
  4. grizld1

    grizld1 Grumpy old man Top Contributor

    That chart's a bit odd; generic NJ and needle in the VM36, for example, is Q2 (not Q5) and 6DP1 (not 6FJ6).

    Anyway, gggGary, you'll give up performance down low and likely won't gain a thing up high with the 38's. You'll need to jet a tad richer on mains and pilots than you would on VM34's, but I'd guess that the combination of P-6 NJ and 6F9 needle would be what you'd want. With some carb/motor combinations the full-throttle mixture leans out at high rpm, and that condition is corrected with the main air correction jet; if you roll to WOT at, say, 4K rpm and get clean response, then try the same at 6.5K and get lean signals (motor falls on its face or misfires), you go to smaller MAJ's until it clears up. The 0.5 in the chart might be fine for some 2-stroke applications, but on our twins you should start with no correction, and if you get down to 1.9, look elsewhere for the misfire. As a baseline, I'd start with PJ: 25; MJ: 200; NJ: P-6; N: 6F9-3; PAJ: open.

    Can those carbs be made to run well? I guess if "runs well" means they'll get you down the street without embarrassing misbehavior, yeah. But even with a big bore motor you can expect idle and off-idle operation to be just a little ragged with the 38's due to the weaker vacuum signal; you might try a pair of #2 slides if that's a problem and you want to throw a bunch of money at it. If you want better performance on a stocker than a good pair of BS38's will deliver, though, the big VM's won't give it to you, and IMO unless you have cables and a bunch of VM brass already on hand or are building a big bore drag bike or road racer (for fast tracks where you spend most of your time at WOT, not tight technical courses), the 38's probably aren't worth messing with.

    Dogger, I've used VM36's on stock motors to baseline them before installing them on big bore motors. True, there's a slight hp loss down low as compared to the 34's, but it's nothing you'll feel in the seat of your pants. The problem is not in your carb selection but in the tuning. Exhaust selection is critical.
     
    Last edited: Mar 4, 2011
  5. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Thanks for the bad news guys. No, really!! Better to hear what I was kind of suspecting anyways. WFO at high RPM on an XS650? Not likely for me. I'd rather have it able to chuff away off the line. Probably send em off on fleasbay. I got plenty of stock carbs to mess with. I checked that number pad on the carb body and got something like :
    B9
    7 11

    If they were a little smaller they might be good on the flat tracker project to be.
    They do have a 2 to one throttle cable and cable operated chokes on them.
     
  6. 5twins

    5twins XS650 Guru Top Contributor

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    You might want to save that cable choke set-up and swap in the lever style if you have some. That set-up might work well on another set if you find some.

    Griz, just out of curiosity, why would larger carbs need larger jets? I would think it would be the other way around.

    My (admittedly) old copy of the Sudco Mikuni manual lists 3 versions of the 36s and also 3 of the 38s. All have different needles and needle jets so it appears there are some differences depending on which model you have.
     
  7. grizld1

    grizld1 Grumpy old man Top Contributor

    Re. fatter jets for bigger carbs--all other things being equal, the larger the venturi, the weaker the vacuum signal. The weaker the vacuum signal, the less fuel is sucked up.
     
  8. angus67

    angus67 Welder's penetrate deeper!!

    you found it in a go-cart? is the chassis any good? Ide like to see it. I always dreamed of making a cart with a cycle engine in it.
     
  9. XSLeo

    XSLeo XS650 Guru Top Contributor

    After my 74 Kawasaki H1 500 tried to kill me, twice, I was young and stupid, I sold it to a fellow that said he was going to put it in a cart. I never saw it after I sold it but if he did it was a fast one. as a bike it could exceeed 140 mph.
     
  10. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    The cart now has a different XS650 engine in it a 78 with BS38s guy says it runs like a raped ape now. The cart is for sale in Green Bay Wisconsin I did not ask how much because, well you know how that goes..... It was fairly big, the engine along side the driver, full roll cage I would guess it weighed 800 pounds at least. I think it had suspension. Kind of an off road cart cool looking, but pointless, a quad would be better in every way. I would PM the guy's phone if anyone is SERIOUSLY interested. I scored a big wad of parts left over from the bike he got the 78 engine out of. Good thing I just had my Prius or I might have bought the cart.
     
  11. angus67

    angus67 Welder's penetrate deeper!!

    of course you could buy a quad, but, so can everyone else. see my point?
    not seriosly interested though. another project, for another time.
     
  12. donald

    donald XS650 Enthusiast

    Put them on a snowmobile site 38s were common on vintage sleds.
     
  13. donald

    donald XS650 Enthusiast

    Looking at the style of throttle cables i would guess their off a polaris tx or starfire
     
  14. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Here I am back on the VM thing. Difference is this time I have them (VM38s on a motor and........... drum roll I put 30 miles on "madness" tonight, 750 web cam #2 (what ever exactly that is) titanium valve components. oil cooler, remote filter. clutch slip under fool power. I think it's a 30 tooth rear sprocket every time I looked down I was at 80 or better.
    So the bike idles and runs fine, great even on the needle jet but flops on it's face when I get the throttle all the way open.
    It's got an electronic tach which is very flaky so my RPMs numbers are unsure right now.


    I have to go check my numbers but 200 main 25 pilots big 3.3 float valve. I think the 6DP1 needle and matching jet set on 3.
    From the chart referenced above the main is tiny for a 38.
    Hint the floats and float rods are fussy about being nice and shiny. One carb was flooding big time till I got those rods cleaned up very smooth and reamed the old varnish out of the float holes with an 1/8"ish drill bit.
     
  15. 5twins

    5twins XS650 Guru Top Contributor

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    The generic VMs listed in that chart were basically jetted and marketed for two stroke dirt bikes, that's why the mains are so big. A 4 stroke wouldn't use mains that large, probably not the same needles and needle jets either. As you probably know, all that stuff needs changing on the VM34s if you're starting with a generic set and trying to make them work on the 650. I'd assume the same applies to your 38 version.
     
  16. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Not sure if this helps, but the mains on my vm36s are #270, which measure at 1.40 mm opening...
     
  17. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Yeah that does help TMB
    I have a set of MM "650 set up" VM34s I will check and see what's in them too.
    Big freaking holes for sure. Good thing these jets come in and out easy.
    The thing is I think the bike makes pretty good (really good?) power, even with a long steep hill it wasn't easy to get the throttle all the way open before it was time to shift. The bike is probably biased towards power way up in the RPM band, with the dodgy tach I gotta be careful about over revving, don't want to leave a trail of engine parts... It's been a while since I ran a non CV carb. Can't just whack em open like a CV. I have to count sprocket teeth I think its got a 17-30 on it.
     
  18. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    A long, long time ago, in a service department far, far away, there was a Mikuni jet reference table, that showed orifice size vs Mikuni jet number. Maybe 5Twins has seen this...
     
  19. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    The mm 34s have 180 mains in them.

    The VM jet "kits" I found start at 240 go to 400

    Freaky big head pipes on that bike OD at 2.25"! 1.6" ID 1.75" OD head pipes "Omars" jet glow coated.
     
  20. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Yep, like mine. You're gonna have fun with this.

    Night & day, Laurel & Hardy, cats & dogs...
     

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