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Two-to-one Carburetor Conversion

Discussion in 'The Garage' started by Ramsey Salim, Dec 31, 2020.

  1. Ramsey Salim

    Ramsey Salim XS400 Member

    1978 XS400, 9.5K - restoration project.
    Has anyone done a two-to-one carburetor conversion on an XS400?
    I'm considering doing that and using a single VM34 carburetor.
    Any advice or pointing me in the right direction is greatly appreciated.
    The main question I have is if a two-to-one manifold specific for the XS400 already exists that I could purchase rather than getting one fabricated.
  2. thekinggrim

    thekinggrim Chop Chop Guy

    Theve got a 180 cam so you can't run a single carb sadly as it will run rich on one cylinder and lean on the other because the valves overlap slightly opening.
  3. amc49

    amc49 XS400 Addict

    Virtually every engine on earth has some cam (valve) overlap, it's not that. It's because the firing events are close together based on the 180 degree crank, one cylinder gets to re-energize the air column when it goes down for a while and the other cylinder is fighting the previous carb event which has just happened, the intake impulses are then fighting each other at the carb. The reversion caused with previous carb event then messes up the second one as the fuel/air gets richer from passing over the carb outlet more than once. The single carb intake manifold design can and often does kill you there too. If the intake has a straight section that rams dead into the split with a wall, then that is the worst one to use and how many are designed. The dead end wall then becomes a secondary reversion impulse to mess your tuning up even worse.

    It CAN be done but throwing away power doing it and why do that? Unless you have a custom frame that doesn't allow for the twin carb setup. Look at the typical smallblock Chevy V-8 setup and the tuning around cylinders 5 and 7, one robs the other wildly of fuel in the exact same way but they run fine all day long. The firing order.............18436 (57) 2. 5 robs 7 of fuel.

    Here's the deal, if you can tune one carb you can do two and you simply copy one setting to the other. How hard is that? People are too fearful of details too easy to fix.

    People are always trying the 4 into 2 carb setups along the same lines for four cylinders like the DOHC Hondas I worked on, they get into insurmountable problems doing it too and the quickest way to dump 10-15% of your power there is. It turns a 750 into a 500. Many of the manifold sellers have quit because nobody is smart enough to tune the problems out. Bigger engines like in cars tolerate that idea better because the air action in them is not so violent.

    Yours and do what you will, now you know.
    motoTrooper likes this.
  4. Ramsey Salim

    Ramsey Salim XS400 Member

    Thanks for the reply. I am always looking to make an informed decision. This is the information I am looking for to help me make my decision.
  5. Gra900

    Gra900 XS400 Addict

    Hi Ramsey, I looked into this a few years ago and found a few posts on this forum from people that had successfully converted to single carb and found it ran a whole lot better. I haven’t done it, but if you have the skills to make the 2into 1 manifold I’d give it a go.
  6. amc49

    amc49 XS400 Addict

    You do realize that when people say it runs a 'whole lot better' with one carb only it is a direct reflection on the skills there right? I absolutely guarantee that if the engine was designed with 2 carbs it will always run 'better' like that, or the skills working on it are substandard. Not trying to be mean or insulting at all but it is what it is. There are also very real reasons why they WON'T but getting people to admit it is another thing entirely. The human condition being what it is is always loathe to admit that work done resulted in less than perfection. So you get told it works better when it didn't. It's like people in polls who say they would never vote for Trump then they do secretly. I got buried in that stuff when I sold parts, I had to develop trick questions and methods just to pull mostly truthful answers out of people who it turned out later were lying to you.

    There are also skills in making the manifold that nobody even knows to consider at all. Only a designer could ever know them. I say that having done lots of intake work and mods on car engines, there being whopping more things to consider than just joining up two pipes into one and getting it to fit in the frame. That's not even a beginning. Take the common rich/lean complaint. There are several things that can be done to even that up if somebody knows what they are. The split point, it can freaking kill you if done wrong and it usually is. Should one side tube be bigger or smaller, different size can do several things for you or against you. Should one side run uphill? That one then runs leaner, the low one gets the fuel dropped out of suspension. You can also cut small trenches in bottom of a runner to do the same, richen that one up. How about a divider in the middle of the one main tube before it splits?

    See what I mean?
    motoTrooper likes this.
  7. xschris

    xschris a lifestyle not a trend Top Contributor

    The few on this forum that did this never followed up with any real good results other than wide open running up and down back roads. They also used a home made crazy setup exhaust system. Yes these will run on one carb but they idle terrible and have heat and surge issues at cruising speeds. The entire setup would need to be all custom and like stated many times in other threads power will be lost. Since no long term data in know my guess one cylinder will overheat or wear much fast than the other and the other would foul plugs. One member used huge main jets ( I think 200 or so) to get a good full throttle performance. But again no data long term or at other rpm's. Bikes with a 360 degree crank and firing setup can work well enough with one but I would still think two would perform much better. Our 180 degree bikes are not good for this.
    Last edited: Jan 4, 2021
  8. amc49

    amc49 XS400 Addict

    '...power will be lost.'

    X2. We're talking a big lawn mower engine here. Why I simply don't believe in throwing even 1 hp. away because one is not understanding or patient enough to get one more carb right. And you get one carb right you simply copy the other to it. People make things harder than they have to be, starting with brain work.

    When I hear these stories I immediately think 'big dumb bubba' which is incredibly prejudicial and not fair at all, but nonetheless it is hard to get the idea out of my head. My bad there of course.

    Harleys do it all day long and there you go, backwards tech from the '30s. They work (check out the idle quality-CRAP!) but double the carbs ala XR750 and watch one pick up power instantly. It's silly to me at least to have power potential not tapped by going backwards. Some can live with that I guess.
    Last edited: Jan 2, 2021
  9. Gra900

    Gra900 XS400 Addict

    So if the inlet valve on cylinder 1 opens 30 degrees BTDC, and closes at 70 ATDC, that means there is 90 degrees of rotation until the inlet valve on cylinder 2 opens. Basically the carb is in almost continuous use for 290 degrees of rotation, then has a pause of 430 degrees before opening again. See photo. A single carb should be no different to twins. The real key is tuning the inlet duct length so that pressure reflections work, but any benefit is in the few % range. Higher cams, bigger valves have the most effect.
    I would always say the best way to go faster is to get better brakes, tyres and suspension, or failing that a faster bike!

    Attached Files:

  10. Lou Ranger

    Lou Ranger Former xs400 Luddite Top Contributor

    Before I bought my 250 after 37 years of not riding, I tested a Honda 200 "Twinstar" which had one carburettor, as designed and built by Honda.
    Maybe, since it was such a tiny, light bike, Honda deliberately sacrificed "unneeded" power in favour of simplicity and production cost. Hitting price points was pretty important in those days.
    I'm curious about something though. The xs400 has opposing pistons, which make is smoother than the xs650, however, could someone discuss the difference between 180* and 360* in this context?
    Does the xs400 fire twice during one 360* cycle and then do 2 exhaust strokes for the second 360* cycle?
    Or does cylinder #2 do it's exhaust stroke right after #1 does its power stroke, so that the power strokes are evenly spaced?
    Never thought about it before.
  11. amc49

    amc49 XS400 Addict

    'Does the xs400 fire twice during one 360* cycle and then do 2 exhaust strokes for the second 360* cycle?'

    Yes, kind of, except on the latter half of statement. Sort of a power-power; then nothing for a while. The stagger timing is what messes up a single carb. On a 360 crank the intake pulses are essentially spaced the same, what works on carb on one then works on the other side too. That design evolved from the old CB160 and CB 175 which were 360 cranks too. The older Honda 250-305 used in the Superhawks started out being 360 cranks to make the single carb Dream model too, they then changed to 180 cranks when they dumped the single carb setup.

    You usually do 360 crank on smaller motor because they are harder to mechanically balance even though the power pulses are then dead even. Yamaha did the bigger 650 in 360 because they were copying the idea of the bigger British twin and they used that setup too, it was all about getting that 'sound', the two types sound different.
  12. BBS360

    BBS360 XS400 Guru Top Contributor

    You're not reading the valve timing correctly.
    Double check the acronyms and redo your timing chart.
  13. amc49

    amc49 XS400 Addict

    X2, I never paid any attention to the numbers there. You always have at least 180 degrees plus the overhang on both sides there. 24 + 180 + 60 is the 250/360 total intake timing. 264 total degrees but a lot of it is clearance ramps where no real flow has started yet. XS400 is 280 degrees.

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