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XS400 single carb setups / tuning ?

Discussion in 'The Garage' started by riotten22, Mar 6, 2012.

  1. How many of you are running a 2 into 1 intake with a single carb? I am going to run a stock single carb on my 81 XS and if anyone else is out there with a similar setup I would love to hear what you are running for jetting / tuning.
  2. bcware

    bcware Well-read Top Contributor

    If I recall I only ever saw one person do this. He provided some photos, but not a lot of detail. I'm not sure there was any longer-term follow up either.
  3. Ya I found the single carb thread where 01ps did this but I have been unable to get a reply from him.
  4. XSLeo

    XSLeo XS400 Junkie

    As far as jetting, the single carb and the dual carbs will be the same, or very close.
    As the engine turns onlly on cylinder draws an air fuel mix from the carbs. One cylinder draws then half a turn the other cylinder draws. This happens with two carb as well as with one carb.
    With two carbs you have one carb feeding one cylinder. On a single carb it feeds one cylinder then half a turn later feeds the other.
    So if one carb feeds one cylinder how can the jetting need changing?
  5. D-Train

    D-Train D-Train

    I just road tested my 81 xs400 with a single carb and to my disappointment it runs way to lean on the left side and to rich on the right, since there's only one mixture screw i don't know how I'm suppost to fix this. Anybody have any ideas.
  6. Did you use the stock carb? What are you running for main jet size? what does your intake manifold look like? Equal length runners?
  7. D-Train

    D-Train D-Train

    I'm usign a VM34 with a 240 main(i think) my intake looks like a "y" and equal length runners. In theorie they it doesn't matter what carb you use what jets are in it both sides should be the same since there's only one carb
  8. JARichmond

    JARichmond XSively Addicted

    It's very difficult/impossible to get a perfect mixture with a single carb on our bikes. Mostly it's because of the way our valve train is set up. The crank on the XS400 is offset 180 degrees causing both intake valves to be open for a very short time in unison (overlap). This causes a huge draw of air through the single carb that no amount of jetting can compensate for, causing a very lean condition for the second cylinder to perform its intake stroke.
    Compare that to the XS650, with a 360 degree offset, which has no problem working with a single carb because there's no overlap. In my opinion if you wanted to seriously move to a single carb you would have to get a crankshaft that fit our bike with a 360 degree offset,a new cam that would match, and mess with your timing to spark the second cylinder at your new top-dead-center. All of which I don't think is worth sinking your money into.
    cbrfxr67 and xschris like this.
  9. Any progress on your single carb xs ? How does it ride at low rpm / speed? Is it mainly leaning out at higher speeds or is it bad all around ? I have been reading up on this more and more and everyone says that it is not a good idea on a 180 degree crank bike but I am doing it no matter what. I would like to get the best carb setup possible for the 180 degree crank.
  10. You always have such smart things to say.
  11. BBS360

    BBS360 XS400 Guru Top Contributor

    Except it's not true on our bikes.
    JARichmond's post above covers it.

  12. It’s a huge draw? Huger than 1/2 of 392cc? Here is the deal, what goes in must come out.

    All engines are forced induction during intake stroke. Every motor is different, tuning rocket science.
  13. Yeah his huge draw theory is wrong
  14. Jeff0133

    Jeff0133 XS400 Junkie Top Contributor

    Why would you go for a single carb anyway?
    Two well tuned/properly jetted VM-34s will work wonders
  15. BBS360

    BBS360 XS400 Guru Top Contributor

    Im not really clear what your point is bringing this all up. Single carb setups are possible, and maybe it’s not a yuuuge draw, but can you explain why he’s wrong?

    As I understand it, out of the 280 degrees of crank rotation that each intake valve is open the two intake valves are both open for 100 degrees. The last 100 of the left cylinder and the first 100 of the right.
    That’s over a third of each cylinder’s intake duration where both intake valves would be open to the same carburetor.
    Now you could maybe look at the duration that’s before top dead Center and after bottom dead Center and say there’s no effective overlap, but it’ll have an affect.

    My math may all be wrong. Please correct me if so.
  16. xschris

    xschris a lifestyle not a trend Top Contributor

    This is why a 180 crank has issues with a single carb. One side will always run hot. A cv carb will hang. A vm carb would be better but still one side will run hot.
  17. Jeff0133

    Jeff0133 XS400 Junkie Top Contributor

    Yes, one lean side/one rich...unless you would find a way to offset this with an uneven manifold and even then it would be a challenge...
  18. xschris

    xschris a lifestyle not a trend Top Contributor

    As one piston intake valve is closing with its charge for a combustion stroke the other is pulling for its charge. That little overlap lets the one pull air/fuel out of the other causing a lean condition. And since there is no way the adjust the others fuel it will always happen. That is way a 180 degree crank motor has two carbs.
    Jeff0133 likes this.
  19. Jeff0133

    Jeff0133 XS400 Junkie Top Contributor

    I might be wrong about this but with different runner lenghts on the intake manifold you may....in theory...be able to give one cylinderbits charge whilst giving some delay for the second cylinder's charge so as to prevent robbing from the first.
    ...but that's thoeretical at best
  20. xschris

    xschris a lifestyle not a trend Top Contributor

    Longer runs will also cause the fuel to "pool" up and the motor will not run well. You want a very fine mist. Better air/fuel mix. The only thing I think that would be a improvement on these bikes would be fuel injection. Perfect mix every time and at all elevations. The only way to go!!!
    TazioliBuilt likes this.

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