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XS400 single carb setups / tuning ?

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

  1. Jeff0133

    Jeff0133 XS400 Junkie Top Contributor

    Which brings me back to my first point....two VM34s
  2. Ok to address all - the huge draw really got to me because you can only draw what’s available in the intake manifold!
    Two variables matter - manifold air density and manifold air temperature. Those variables are needed for the tune. The huge draw thing really just gets me shaking my head. Yes, it is possible because with a air density gauge, you can see this valve or how it changes per each cylinder in their respective stroke. Being conceptual is great, but don’t let it stop you from doing too many things.

    Do y’all know what manifold air density is?
  3. It would be difficult to find a good spot to take a temperature reading. Fuel injection goes great with water cooled because the rich or lean flow is delegated by the coolant temperature sensor because.
  4. BBS360

    BBS360 XS400 Guru Top Contributor

    Again, why are we talking about this six years after the original discussion?
    If you want to do a single carb setup, do it.
    Nobody is actually saying not to.
    There are issues you can attempt to address but you cannot eliminate the simple fact that for a period of time two intake valves will be open to a single carburetor and single intake manifold.

    You may have a valid point here, and I'm curious to find out what it is, but the intake manifold is not a fixed volume of air, it draws from the atmosphere.
    The cylinders will draw what they will draw regardless of whether you have the stock setup with two separate carbs and intake manifolds or a single carb/manifold.
    That's the whole complication.
  5. The cylinder will draw 1/2 of 392cc. Even with dual carbs, the intake is shared. I brought this up because the poster was wrong.

    Again, you are also incorrect and this has to be addressed because people look at this and a 6 year old post stays up and is wrong. Overlap happens in all motors. Nothing to cry about—there is a reason and you don’t understand it.

    Curious to find out what my point is? Do some thinking and it will come to you.

    I don’t want a single carb but I really don’t want this thread to be left sitting with incorrect information that doesn’t tell the truth.
  6. BBS360

    BBS360 XS400 Guru Top Contributor

    We are not talking about a single cylinder. There are two.

    I have thought about it, as have the others in this thread. Perhaps I do not have all the information to come to your conclusion.

    Please explain, possibly with an explanation of why the difference between our bikes, with their 180° cranks, and the XS650s, with their 360° crank, makes no difference.
    Thank you in advance.
  7. Jeff0133

    Jeff0133 XS400 Junkie Top Contributor

    What do you mean by intake is always shared?
    With dual carbs without h-pipe on pods it is not the case...
    BBS360 likes this.
  8. xschris

    xschris a lifestyle not a trend Top Contributor

    we should keep in mind that all British twins with a single carb had
    360° cranks providing a "symmetrical firing order", i.e. same conditions
    for each cylinder. A 180° one,
    causing a 180°/540° firing order, which is unfavorable for a single carb
    (e.g. both inlet valves are open at certain crank angles).

    Found this on do the ton website.
  9. The amount of air available doesn’t change regardless of crank offset. I don’t know why you think it’s a big deal but it’s not. Air density isn’t affected because of crank offset. The goal is to achieve stoichiometry. I have seen zero technical reasons why except the HUGE DRAW hahaha and also valve overlap which most people should know is for valve cooling, regardless of crank or cam offset..... do you follow? A carburetor does NOT know how many cylinders it is being used with. The cylinders don’t know and don’t care where the air fuel mix comes from.
  10. xschris

    xschris a lifestyle not a trend Top Contributor

    It's not about the carb or intake volume but how the motor itself fires and draws air/fuel.
  11. BBS360

    BBS360 XS400 Guru Top Contributor

    Perhaps using the term "valve overlap" is a point of confusion here. There is an "overlap" where both intake valves are open at the same time, but the typical "valve overlap" between intake and exhaust valves is not involved here.

    It's the availability of fuel that is the issue, not air.
    Indeed. A carburetor jetted to supply the stoichiometric ratio of fuel to a single cylinder won't know to adjust when 2/3 of the way through the left cylinder's intake duration has the right cylinder's intake valve open up too.
    If you can get past being hung up on the "huge draw" quote maybe you can see that the 180° crank does have an affect on the intake valves and air drawn through a single cylinder setup.
    Again, nobody is saying it is impossible or not to do it, just that there is a complication compared to other bikes that means these models may not be the best choice for a single carb setup.

    Reasons have been given.
    Jeff0133 and xschris like this.
  12. You guys are really good at quoting but I am seeing less similar information being offered to me than the post from “6 years ago”

    What I’m understanding is that there is a assumption based on information that this 180 degree engine has a diminished volumetric efficiency during valve overlap. OR the “huge draw” causes a lean condition ? Or one cylinder steals air/fuel from another?
  13. JARichmond

    JARichmond XSively Addicted

    Dude. If you're so hung up on me saying "Huge draw" then ignore it. The fact is that our bikes seriously dislike a single carb setup. You can run it, go for it, but one side is going to run lean and one side will run rich. If you want to change your spark plugs, exhaust gaskets, and maybe have to learn how to diassemble your engine to take out soot, go for it! I'm sorry if I put a thorn in your side. Please report back with your findings.
    cbrfxr67 likes this.

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