1. Hello Guest. You have limited privileges and you can't "SEARCH" the forums. Please "Log In" or "Sign Up" for additional functionality. Click HERE to proceed.

CV Carburettors - how do they really work

Discussion in 'XJ Technical Chat' started by Minimutly, Dec 21, 2020.

  1. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    Really struggling with this ipad right now....
     
    Last edited: Dec 21, 2020
  2. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    IMG_2710.JPG IMG_2711.JPG Courtesey of jh haynes publishing, a rather old book.
     
  3. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    Now, I hear you say, what's this got to do with our xj carbs? Quite a bit really. The big difference is that Mikuni and Hitachi decided they needed more accurate low speed/load mixture control, so added an idle circuit.
    To make this work a couple of changes happened - you may see that the SU piston travels to the bottom, effectively closing the venturi off. Our carbs keep it off the bottom. This necessitates a port facing forward on the SU, the Mitachi on the bottom. (I know, couldn't face typing both names over and over).
    The piston seal/diaphragm thing you can ignore, it is a mere detail.
    So the Mitachi carbs have a different starting circuit, then the idle and progression circuit has all it needs to deliver idle fuel via the idle mixture screw, and, as the throttle plate is opened, the progression holes are uncovered, the manifold vacuum,sucking more and more fuel out. It's interesting to see that both companies only added two holes - Dellorto and Weber, both performance type carbs added 3 or more.
    So the plate opens (slowly), and the fuel is added gradually, with air added to the fuel to emulsify it to prevent neat fuel being sucked in.
    Eventually the airspeed under the piston (allong with manifold depression) creates a partial vacuum under the piston, drawing fuel out, and lifting the piston, but at the same time maintaining the vacuum signal balanced according to the spring strength. Full throttle and peak rpm should be achieved before the piston gets to the top. What happens to the mixture if the piston goes too high too soon? Weak fuelling, since the relationship between the vacuum and the needle is lost - you get more air, but no more fuel...
    Lastly, snap opening throttle. I've allready said that idle and progression fuelling is accurately controlled, and that this happens before the piston lifts off the bottom. Someone somewhere here @ chacal? Has worked out how little fuel the idle jet gives per pulse so I won't go into it further, except to say it's tiny.
    So when you snap the twist-grip, what happens?
    Firstly, the two progression holes which may have been partly uncovered now see "full" engine vacuum. I say full, because remember our piston? It's not fully at the bottom is it, so these holes only see a partial vacuum at best, but better than nothing. So they add what tiny amounts the idle jet will allow. Next allong the carburettor the piston sees the engine vacuum, but, remember our port in the Mitachi? It's at the bottom, so needs air travelling under it before it lifts. Add to this someone only drilled a small hole in the bottom. Together these two mean that the piston takes a fraction or two of a second before lifting. This delay means that for a short time the vacuum signal below the piston peaks, and this peak is just enough to drag the extra fuel needed for accelleration out of the main jet.
    There we have it - accelleration fuelling, without a pump.
     
    Last edited: Dec 21, 2020
    JCH, Jetfixer and Franz like this.
  4. Polock

    Polock Well-Known Member

    Messages:
    9,751
    Likes Received:
    2,093
    Trophy Points:
    113
    Location:
    Beaver Falls, PA
    next up "How a I-Pad really works" :)
     
  5. Huntchuks

    Huntchuks Well-Known Member

    Messages:
    950
    Likes Received:
    395
    Trophy Points:
    63
    Location:
    East Rochester, NY
    Mitachi!
     
  6. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    I'll answer @chacal s question here, maybe one day someone will read this drivel..


    b) EACH of these "progression" holes is thus much larger than the pilot fuel jet that feeds them; of course, the forward-most hole (that the mixture screw would fit into during normal operation) is restricted by the taper of the screw tip.

    I found this arrangement to be surprising, and don't quite understand why you would have such a large "total" opening dimension, that could theoretically flow much more fuel than the pilot circuit could provide (although, it seems that fuel would only flow thru the "nearest" exposed hole; but then, why have multiple holes? Once you uncover the "first" (not the mixture screw hole, but the one closest, linearly, to it), what is the purpose of the rest of them?



    The simple answer to this is that it's not neat fuel that comes out of the progression and idle mix screw holes, but emulsified fuel - that is fuel mixed with air. The idle air jet actually has a function..
     
  7. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    Sorry, missed the first bit of this - the "holes" are passed by the edge of the butterfly as it opens, each one adding more fuel than the previous one, until the secondary circuit starts working, and adding to the air and fuel.
     
  8. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    So...they serve the same function as an accelerator pump. Interesting...
     
  9. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    You really must stop putting your words to my replies...
    Read it again, I described the plate passing the holes and allowing emulsified fuel out, at no point did I say this happened quickly, or at a speed that might be approaching the need for acceleration fuel.
    I struggle with your attitude @k-moe , you are an educated person (I believe, yet you are as closed minded as anyone I have ever conversed with - if it doesn't suite you then it can't happen..)
    Rather than putting words to my text, tell me why I'm wrong?
     
  10. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

    Messages:
    8,866
    Likes Received:
    1,780
    Trophy Points:
    113
    Location:
    The room where it happened
    At what speed would you reckon that vacuum exerts a force upon a fluid (especially in a channel that is already full of fluid, due to the idle speed screw port already being active)? Would it be:

    a) more than
    b) less than
    c) or about the same as other universal forces, such as magnetism, gravity (which ain't, but that's another story..), etc.

    Inertia surely plays a role, but in this instance, inertia is already working in "favor" of the fluid.
     
  11. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    I'm sorry, I don't understand this question? The "speed" I mentioned was the speed of the plate passing the progression holes - and this is a function of how fast you twist the grip. In "normal" riding, say tootling allong in traffic, nomsnap accelleration etc this would be a gentle progression - hence the names for those holes, and actually, where the bike spends most of it's time, unless it's doing trans desert courier or racing work?
    Anyway, I would suggest that given the nature and weight of anything coming out of those holes not to be affected by inertia in any way - Bernoulis would be the laws most at work I suspect, after gravity of course.
    Forgive me if this doesn't answer your question, I'm assuming it was a serious one?
    Edit to add - OK, first scenario, as above, the plate passes really slowly, those little holes do exactly as intended, the add fuel to the air going past the plate.
    Second scenario, open the throttle instantly, or as instantly as possible. The plate lifts past both holes in a flash - what comes out of them? The flow rate will be tha same, but the time between no holes, and all holes seeing that vacuum will be minuscule, then they see the remains of engine vacuum applied to the piston - and we know this isn't closed down tight. So right when we want our squirt of fuel to actually increase, we will get less coming out of these holes. Bugger. Tell me something different then?
     
    Last edited: Dec 23, 2020
  12. Dave in Ireland

    Dave in Ireland Well-Known Member

    Messages:
    512
    Likes Received:
    285
    Trophy Points:
    63
    Location:
    Ireland
    I'll stick to my adjustable SU accel pump.
     
  13. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    These work in exactly the same way, but don't have the adjustability of the su - perhaps that's a good thing, the number of times I've had old guys telling me that 3 in 1 oil is the stuff to put in them, then saying they have a stumble on accelleration...
     
  14. Dave in Ireland

    Dave in Ireland Well-Known Member

    Messages:
    512
    Likes Received:
    285
    Trophy Points:
    63
    Location:
    Ireland
    Yeah, there's a huge amount of lore and bullshit attached to SUs.
    Ignore it and crack on. You have to, because all the bollocks will lead you up the garden path.
    I'm now at the stage where it's easily knocking out the same mid-range torque as the factory-fit bank of four.
     
  15. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    Trying to upload a video image here, but apparently it's too large. How big can it be?
     
  16. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    You have to use a hosting site like Imgur and attach it as a direct link.
     
  17. Dave in Ireland

    Dave in Ireland Well-Known Member

    Messages:
    512
    Likes Received:
    285
    Trophy Points:
    63
    Location:
    Ireland
    It's never big enough.
     
  18. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk


    Hopefully this works, and my ancient ipad actually manages it you'll have the proof that answers the question - where does the accel fuel come from?
    Huw
     
    k-moe likes this.
  19. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    Excellent.
    Thank you.
    My opinion has been changed.
    I was wrong.
     
  20. Rooster53

    Rooster53 Well-Known Member Premium Member

    Messages:
    1,852
    Likes Received:
    723
    Trophy Points:
    113
    Location:
    North, FL
    Not to argue, but isn't that a bit meaningless with no load?
     

Share This Page