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2-to-1 carb

Discussion in 'The Garage' started by skillzman1, Jan 8, 2011.

  1. so out of curiousity...now that I've seen it/heard about it...how does one go from a double carb setup down to a single carb?
    I realise theres a needed intake to fab up...but what size carb do you go with for starters and how do you even begin to know where to start jetting etc,.?
    trial and error?
    I know you need to 'read' the plugs to see how they burn and what not...but is there a specific brand/style/model thats typically used?
  2. totalgearhead

    totalgearhead XS400 Enthusiast

    I seen a post on our sister forum where they are deep into this discussion. I am interested in it because my second XS400 is missing carbs! I will post a link when i remember where it is! (found)!

    Last edited: Jan 8, 2011
  3. drewpy

    drewpy Excess twin Top Contributor

    On TCU they have done an xs400 and the stock carb was the one to go for.

    jets will be different for each bike/exhaust combo.
  4. HoughMade

    HoughMade XS400 Guru Top Contributor

    Enlighten me...why a single carb?
  5. On my old KZ440, I converted to single carb. I used a 1991 Honda CR250 carb in place of the stock two. The carb was 34mm. I fabbed a 2-1 intake out of steel tubing. The rubber carb boots have a steel flange inside, so i burned all the rubber away, and welded the 2-1 intake directly to those flanges. I then just used gasket material to seal the intake to the head.

    A single carb over dual carbs has many advantages. The first would be the obvious....less money spent on parts and less time spent on cleaning. Secondly, it is much easier to tune one carb than to have to tune two, especially since the two need to be synced correctly to even start. Also, depending on the carb you choose, alot of times that carb will be from a newer model bike, therefore making parts easier to come by. Lastly, it just looks cool I think. lol
  6. Also, I just thought of another advantage. The stock rubber boots are notorious for cracking. As I'm sure you already know, cracked intake boots make tuning, rejetting, and just keeping the bike running decent, very hard to accomplish. When welding the intake to the steel flanges inside the boots, after burning the rubber off of course, it eliminates the possibility of cracked rubber boots.

    I haven't looked closely at the XS400 boots, so I'm not sure if they have a steel flange inside like the KZ's, but it would still be worth taking a look. If anyone here has there old boots still, maybe they could burn the rubber to see what's inside....
  7. gentlemanjim

    gentlemanjim More Wrenchin than Ridin

    On an engine this small I'd keep the dual carbs. Power is marginal and who wants to lose any? I did a single carb conversion on a virago 1000 and was the best mod I made, but thats a big torquey motor.

    If you do decide on a single set up one of the stock carbs will work fine with proper jetting and needle shimming.
  8. drewpy

    drewpy Excess twin Top Contributor

    single carb will get you more torque at the expense of top end power
  9. A single carb set-up is one of the most popular modifications to do to the KZ400 and KZ440. Engine size affects nothing at all as long as the carb flow matches the engine's needs. It works fine (I did the single carb mod on my first 440, and plan on doing the same on the 400 I just picked up), and usually works better than the stock set-up. Tuning issues that our XS's have get cut in half with a single carb. No leaky carb boots, no need to sync your duals carbs, etc. I noticed no change in power whatsoever.
  10. interesting...I have a buddy thats a wiz with carbs...so I'm not really worried either way...just been seeing it done more and more...wondered how much work it would be.

    not sure my welds are good enough to showcase on a project like that however.
  11. Make a few multiple passes with your welder. The welds can then just be ground down. The only "structure" that the intake needs to have is the ability to hold up the weight of your carb. Just be sure there are no air leaks in the intake. After you grind and clean up the welds, you could always throw some high temp paint on the intake. I have even seen guys wrap the intake runners with exhaust wrap also.

    Fabricating your intake to your own liking is great. You can have the carb angle out to one side or the other, or just have it come straight out of the back of the engine. Frame rail location, battery box location, etc. will sometimes dictate where the carb sits though. It is best to have the intake runners the same length, with the carb centered between the two so the air/fuel mixture doesn't take any considerably sharp turns. Alot of times if the mixture has to take a sharp turn (say 135 degrees or greater), the fuel will seperate itself from the air. This will create a lean mixture in one cylinder and a rich mixture in the other. I have seen a few bikes with close to 135 degree bends in the intake, and they are all said to run fine. I think once your angles get any greater than that is when you start to have tuning problems.

    Let me know if you need any help.
  12. where do you get your bends from?...what diameter tube do you use?
    all I have is a flux core wire feed welder..I really need to upgrade someday...I would love to learn TIG ...but thats a ways off.
  13. I can't remember the size of the tubing on my KZ, and it's been sold a long time ago so I can't go measure. lol I wanna say it was somewhere around an 1 and 3/4in outside diameter though. You can just measure the intake openings in the cylinder heads and get a rough estimate. I just used some steel scrap from a race shop down the street that I picked up out of their "scrap pile". Only had to give the guy a 6 pack of bud light for all of it! haha Some of the pieces already had 90 degree bends in them, so I just cut what I needed and welded it together. I have seen people use copper tubing also, and just solder the joints. You can pick up plenty of different bends in copper.

    Flux core should be fine for welding the intake.
  14. CadXS

    CadXS XS400 Addict

    Cool, a 2 into 1 carb thread. I'm in the middle of doing this myself having been inspired by the guys on the Kawi forums who all seem pretty keen on the idea. I'm not after any performance changes, just doing it for fun really, lol. Picked up a 34mm Keihin PWK for when I get the manifold sorted.


    A lot of the Kawi guys in the USA seem to make the manifolds out of large diameter copper pipe from plumbing suppliers and braze the joints, which is a pretty cool idea.

    There is also a bloke on the TCU forum called Choppersprocket who makes them I think for the XS400. I also found sombody here in the UK who makes a cast 2-1 manifold for the XS650 and asked him if he would consider making one for the 400 but he said no :shrug:

    I was thinking of trying to make it out of steel exhaust bends done with a mandrell but I'm still having a bit of a think about it :)

    Sorry HT you answered the question already...
    Last edited: Jan 8, 2011
  15. yep...another thought on my mind now....I should just turn of the computer and walk away for awhile!!
  16. cturek

    cturek XS400 Addict

    Now I gotta make one too.

    Is it OK to use a CV type carb? I have a spare rack of four Hitachis carbs for the Seca I could pull one from saving the XS's Mikunis to put back on if it doesn't work out.
  17. HoughMade

    HoughMade XS400 Guru Top Contributor

    I'm not going to try to talk anyone out of a single carb. If you like it better, more power to you. However, I can sync my carbs in less than 1/2 hour and once they are in tune, they stay that way for a good long time. I'll stick with my 2.
  18. Not a problem CadXS. I frequent KZ400.com almost as much as I do here. I have a 1981 KZ440, and a 1974 KZ400. I've also had another 440 in the past that I did the single carb on. A guy on that site who goes by the username "Glen" is a God on those bikes. The man is a pure genious!
  19. I heard that the CV carbs weren't the best to do the mod with, but for the life of me, I can't remember the reason. I will run back through a few old threads on the KZ400 forum and see if I can find the post on the CV carb issue.

    Edit: Something about the way a CV carb pulls vacuum and the way that the KZ's pistons rise and fall at the same time, was the reason if I remember correctly. Some kind of big scientific reason that I only understood about half of....lol Now, I have no idea if the XS400 pistons rise and fall together or seperately, but I'm sure someone here knows for sure.
  20. Any two stroke carb that comes from around a 200cc engine should be fine. The two strokes need more CFM than the four strokes. Like mentioned before, I ran a 1991 CR250 two stroke carb on my old KZ440, and it ran like a dream. I would keep around a 34mm-35mm carb though.....definitely nothing bigger. I started with a 36mm Keihin the first time around on my KZ and it was way too big. I switched to the 34mm Keihin and never looked back.

    The two stroke carbs of that size can be had on Ebay all the time for very decent prices. I'm not trying to sway anyone to do it, just something I plan on doing for myself and thought I would share the possibility with others here.

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