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2-to-1 carb

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

  1. 16VGTIDave

    16VGTIDave XS400 Guru Top Contributor


    I'd love to see photos. And test results of performance and efficiency that proves such a conversion is at least as good as stock. I'm still curious how physics have been overcome to allow one carb to work better than 2 on this engine. Or is it just running better than it had been with dirty mis-tuned carbs?
     
  2. olgar

    olgar XS400 New Member

    my thoughts on the dual carbs vs the single ,, the motor is really only running on one carb at a time anyway,,, the cyl's don't fire at the same time '' and the single is feeding each cyl the exact same amount of fuel ''' xs400 will run fine on a stock carb
    olgar
     
  3. 16VGTIDave

    16VGTIDave XS400 Guru Top Contributor

    Welcome Olgar!

    Do you have anything to back up your thoughts? I ask because, as I previously stated, there is a lot of physics involved in this. I know enough to know that I don't know enough. What do you know?
     
  4. JARichmond

    JARichmond XSively Addicted

    Indeed, 2 carbs are better than one, mostly because of the way our crankshafts are set up as opposed to the 650s, which have been documented to work well. Our crankshafts operate on a 180 degree journal offset which gives our bikes a characteristic exhaust note (fire, fire, pause, pause) as opposed to the 650's 360 degree journal offset (fire, pause, fire, pause). On paper, we ideally should not be affected. In the real world however, due to exhaust scavenging, valve float (regardless of magnitude), simply the way the camshaft operates, and many more factors, a 2-1 carb set-up is not the best way to go about stripping weight and simplifying your motorcycle. Rather, consider losing weight around your carbs, keeping them tuned with safe chemicals and regular maintenance, will result in a more reliable and more powerful fuel system, which will lead to longer-lasting plugs, valves, pistons, and oil.

    Source: 5.5 years of automotive engineering classes, 3 years of sketching and simulation, and 4.5 years of bike tinkering.

    Sorry Olgar, but your claims are just not true.
     
  5. xschris

    xschris XS400 Guru Top Contributor

    The intake strokes will overlap a bit causing one piston to run lean. With only one carb there is no way to compensate for this. I think this has been covered before. The bike will run. But will have hanging idle issues and one side will run hot.
     
  6. tayarimoto

    tayarimoto XS400 New Member

    The problem with the long intake tubes is that you are limited by the volume, compression, and stroke of the pistons. As the pistons lower after the exhaust stroke, they create a vacuum in the cylinder, which draws air in through the carbs (and any leaks).

    The ability of the piston to create this vacuum depends on the length of the stroke, how well it is sealed around the rings, and the physical size of the piston. With all else equal, the larger the piston, the larger the vacuum/pressure difference; the longer the stroke, the larger the vacuum; and the lower gap between the piston and cylinder by way of the rings, the larger the vacuum. So for the most power, you want a large diameter piston, a long stroke, and high compression; however, there are limits due to cost, ability to manufacture, the amount of heat through the combustion process available in the fuel, and just the shear size and weight of the engine...but I digress.

    So basically, there is a limited amount of air-fuel mixture that can be sucked into the cylinder, and you want as much of that mixture as possible. If you go and make a long intake, you will have to suck up all that "empty" (fuel-less) air in the tube before you get any air-fuel mix. This is OK (but not good or great), and you can compensate by setting your carbs rich so that it will mix with the stagnant air in the intake to create the proper mixture once the intake stroke is done, however, you will end up with fuel condensing on the intake pipe.

    The real problems come when you use a single carb for two or more cylinders. Especially with the "out the side" examples shown in this thread. Now one side sucks more fuel-less air than the other, so as someone already said, you will have one side running richer than the other because the ratio of air to fuel will be different.

    The compression might (will) be different in each cylinder. Sometimes a little, sometimes a lot different, so there again, even if you have a symmetrical intake using one carb, you will be running one side rich and one side lean relative to each other. If your compression is very close in each cylinder, this will work fine.

    To stop fuel from condensing, to keep each side running at the same air-fuel ratio, and to keep the most stable running condition that depends very little on ambient humidity and temperature, you want your intake to be as short as possible, as symmetrical as possible, and you want the same ratio of fuel and air going to each cylinder. That's why stock boots are so short and why there's one carb for each cylinder on stock bikes.

    If you want more power and a ram-air effect, attaching U-shaped pipes on the inlet side of the carbs that face forward on the bike is your answer, not doing it on the outlet side.

    Source: Engineering, Thermodynamics, Physics
     
    Last edited: May 25, 2015
    cbrfxr67 likes this.
  7. Curseduth

    Curseduth XS400 Enthusiast

    So just a question as this is intriguing, by changing the runner lengths instead of the symmetrical lengths as seen on most of the designs wouldn't this help compensate for the rich or lean condition? I.e. Slower moving for the rich cylinder introducing more "stagnant" air and a shorter runner for the lean cylinder? It's been awhile since I took hydrodynamics and I have had a few so thinking might be askew.
     
  8. Mickey85

    Mickey85 XS400 Enthusiast

    Curseduth, theoretically, yes. This is why on straight 6 car motors, the first and sixth cylinders run leaner than 3 and 4 - they're farther away.

    I'm working on the design phase of a single carb system using a stock carb and a log style intake. By putting the carb next to the left cylinder, and running equal length runners off of a straight log style manifold, it should in theory lean out the right cylinder and enrich the left cylinder, relative to each other. Then, even if it doesn't work perfectly, I should be able to fine tune it by extending the carb runner and/or increasing/decreasing the main jet.

    I'm not sure how significant the difference would be on a motor this small, or with cylinders right next to each other, but given that it IS so small, and the effective runner lengths would be so much different, I can't help but think that it wouldn't cause at least a mild leveling out of the fuel delivery.
     
    Last edited: Sep 27, 2015
  9. 16VGTIDave

    16VGTIDave XS400 Guru Top Contributor

    Don't forget to calculate and adjust runner diameter. This will have a larger impact on velocity than runner length. Runner length is more for pulse tuning than anything else.

    Once you have invested thousands of hours building and running statistical models, you can let us know how the stock CV carb setup is better overall than a single carb could ever be. ;)
     
  10. Wolfe_11B

    Wolfe_11B XS400 Guru Top Contributor

    Weird, on all the power sports vehicles I've ever owned, the first thing to toss is the single carb setups. Just don't quite understand why you'd want a single carb....
     
  11. xschris

    xschris XS400 Guru Top Contributor

    Look up 360* crank and 180* crank twins with single carbs setups.
     
  12. Mickey85

    Mickey85 XS400 Enthusiast

    I will be the first to admit that I don't have a whole lot of experience with motorcycles in general, and Japanese multi-cylinder bikes specifically, so what the heck do I know, but I do have quite a bit of experience drag racing cars and building performance car engines (everything from an AMC 258 I6 to a 500 HP Olds 425).

    In an automotive setup, rarely do you find performance gains from running multiple carbs, even though multiple carbs effectively shorten the runner length from carb to valve. What instead is best is to run an intake manifold with a single carburetor and long runners. This increases low-end torque, and, at least on a bigger motor, provides tolerable high end losses. For instance, on the 258 I built, the intake had runners that were twice the length of the stock intake. This not only had a benefit of straightening out the runners, helping to eliminate dead spots, but it also increased torque due to the runner length.

    Now, I realize that the 360* crank on English bikes, and Xs650s and the like would theoretically be better with a single carb, as there is absolutely no overlap, but (and again, I'm just theorizing, as I don't have much effective experience with multi-carbed motorcycles) with the 180* bikes, since there is a bit of overlap, you're going to run #1 lean while #2 is richer. With a 90* intake that would resemble something like a zoomie header on a hot rod, the length of the runner, given an equal diameter, would effectively slow the velocity of the mixture in #2 so that you could more evenly modulate how rich and/or lean the mixture is.


    But then again, having multiple carburetors strikes me as a backward and silly design that was a gimmick in the automotive world, and was unnecessary for 70 years in the motorcycle world before people decided to reinvent the wheel.
     
  13. Wolfe_11B

    Wolfe_11B XS400 Guru Top Contributor

    One carb per cylinder will always win in efficiency over a single carb, and that's enough for me. That is assuming the carbs are tuned properly and sized correctly for the cylinder. Very similar to MPFI being more efficient than SPFI.

    I suppose on something that just gets paraded around town on the weekends, a single carb setup wouldn't be so bad.
     
  14. xschris

    xschris XS400 Guru Top Contributor

    Also look up cv (vacuum operated) carbs which these bikes use and vm ( cable operated) carbs. If you run a cv carb on these there will be a hanging idle and a weird throttle response because of the overlap. Also one cylinder will be leaner than the other as we already know. A vm carb would help with the hanging but you would have a response issue. Jetting and plug temp combos might help a lot of it but would I give you a better performing bike? I don't think so. Only a dyno sheet of the two setups would answer that question.
     
  15. amc49

    amc49 XS400 Junkie

    The major difference in the Kaw and Yam engines is the 360 vs. 180 degree crank. The Yamaha will NOT like the single carb as the power and thus induction events are too lopsided in 720 degrees of crank rotation making one cylinder always have an inherent advantage over the other in catching moving active air in the intake. There will be a dead air period for the cylinder that has to wait the longest, it then cannot pull as much air and the natural pulsations that always occur in that type of offset manifold will make it likely worse. The same idea works better on the Kaw as the intake events are evenly spaced so the problems are relatively equal in the manifold. Anybody thinking twists and turns in manifolding does nothing is crazy, I've seen a turn slightly changing straighter to greatly increase power in some cases. On early Honda DOHC 4 valve fours the ports are relatively straight but making them 100% so is worth up to 30+ hp. at max effort engines, before that they are limited to about 150 hp. Anybody ever heard of the Ford Boss 351 exhaust port plate that largely corrected the exhaust port kink in the last 3/4" of the head to make well over 100 more hp.? AMC had a similar issue with the head change from '69 to '70, the newer 'dogleg' head flowed more than 50% better than the old one and everybody thinks it was the dogleg which helped some, but the true improvement was the port floor got raised to remove a slight curve.

    Those bends will kill you.

    Not being able to sync a twin? I feel for you. I do fours all day long. Can do a twin by ear without removing a single cover or engine part.

    Somebody mentioned air with no fuel in it higher up and impossible. If the air has passed the carb discharge point there IS fuel in it, and pulsation in the intake can reverse it to go back and come across the discharge point again to be doubly rich.

    I do NOT agree with multiple carbs not increasing power, it means somebody did not know how to tune for them. I say that after building and racing AMC V-8 up to 600 hp. and higher. I did plenty of tunnel ram work and the more carb you have the better, we ran 1150 Holley Dominators on our 395 inch AMC race engines and they were modded to flow 1400 cfm each. That car ran in the mid 9's 1/4 mile at 140 mph. Later 800 hp. ones ran in the 8's at 150+. I also worked on lots of pro stock big block Chevy and the difference in the ports on those heads made for lots of fuel distribution issues, we ran engines up to over 700 inches at over 200 mph.

    Small bike motors are more 'air active' as air like any other matter has inertia and why car engine rules may have to change for bikes. The smaller engines will tolerate much bigger carb bores relative to the cylinder size and going to a common intake where there was a one for one before is simply a power throwing away mistake.
     
    Last edited: Jul 17, 2020

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