Why is there 1 carb per cylinder when a car has one carb for multiple cylinders?

Discussion in 'Motorbike Technical Discussion' started by markc, Jun 11, 2006.

  1. markc

    markc Guest

    I'm an amateur just beginning to work on my own bikes (GS850L & CB450)
    and I'm wondering if there are any multiple cylinder bikes that operate
    with one carb like most cars do.? Thanks for any input
    markc, Jun 11, 2006
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  2. markc

    Leon Guest

    It's done on cars to save money. It doesn't do much for power output,

    Leon, Jun 11, 2006
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  3. markc

    Les Guest

    Harley's use one carb, as well as some of the older British bikes. A
    buddy has a Honda Goldwing that's running a 4 bbl Edlebrock carb made
    for a car. Runs pretty good but has a slight hesitation when the 4 bbl
    kicks in. He's rode it this way for several years now.

    Les, Jun 11, 2006
  4. Lots of twins did/do.

    Gold Wings.

    Kawasaki Z1300 six used three twin-choke carbs.

    Early Suxuki GSX550/750 fours used a p[air of twin-chokes.

    But not that many, agreed.
    The Older Gentleman, Jun 11, 2006
  5. markc

    SAMMM Guest

    bikes often run at higher speeds than cars and the 'tuning'
    of the intake tract becomes important then.
    the pulse of the intake valve opening creates a 'wave' which
    can increase the amount of air the cylinder will intake.

    it's more complex than that but that's the general idea.
    one carb per cylinder makes 'tuning' that 'wave' easier.
    note some pricier cars have intake tracts that change length at higher rpm.
    some older hondas such as the CA77 and CA72 used one carb for two
    cylinders. these were the lower performance bikes.
    they also had crankpins that were side-by-side so the intake pulses
    were evenly timed.
    the nearly identical engine in the CB77 et al. had 180 degree cranks
    and needed two carbs.
    SAMMM, Jun 11, 2006
  6. It is rather difficult to fit a single carb with an intake manifold on a
    R. Pierce Butler, Jun 11, 2006
  7. (And not for the first time he posts this reply...)

    Complete and utter nonsense.
    The Older Gentleman, Jun 11, 2006
  8. markc

    FB Guest

    Getting frustrated by the complexity and mysteries of multiple carbs,
    are you?

    It's a lot easier to get to understand motorcycle carbs than it is to
    re-engineer a single carburetor on a manifold and have it *work* on a
    motorcycle engine.

    Motorcycle inline 4's are like automotive inline 4's and inline 6's in
    that the cylinders are all in a row.

    Older automobile designs used simple "log" manifolds that were just a
    simple large tube that bolted to the side of a 4 or 6-cylinder engine
    and the carburetor bolted to the large tube. The middle two cylinders
    would get a richer mixture if a simple log manifold was used. The end
    cylinders would get a leaner mixture because the fuel/air mix had
    further to travel.

    Harley Davidsons use one carburetor for two cylinders, but it doesn't
    work all that well.
    At some speeds one cylinder hogs the gas from the other cylinder
    because of the firing order. As I recall, there are only about 70
    crankshaft degrees between the intake strokes of the two cylinders, so
    one cylinder gets starved with only one carburetor.

    More modern intake manifold designs paired the intake runners for

    It's easy to see what is being done when there are two carbs on two
    individual intake manifolds on the side of an inline 4 car engine.

    My Triumph TR-4 had two long tuned manifolds with 1 carb on each
    manifold. Each manifold fed two cylinders. The # 1 and # 2 cylinders
    were fed by one manifold, the
    # 3 and # 4 cylinders were fed by the other manifold.

    Each intake track was about a foot long from the valve head to the
    carburetor mouth.

    The cylinders paired by the two intake mainfolds were 90 degrees apart
    in their firing stroke. If you study the motions of the pistons and the
    positions of the intake and exhaust valves, you will see that only one
    intake valve is open at a time and that air is only being sucked
    through one leg of one of the two manifolds.

    If that's the case, why have two manifolds with two legs? It makes more
    sense to use two carburetors on two intake tracks.

    There just isn't much room behind a lower RPM motorcycle engine, if you
    are going to use paired mainfolds like cars use.

    Americans were inspired by the screaming high RPM European grand prix
    racing during the 1960's, so racer styling was important to American

    Really high RPM engines don't need the long intake manifolds like my
    TR-4 used. They can use really short intake tracts. The actual length
    of the intake passage would be from the head of the closed intake valve
    to the mouth of the carburetor. So it could be six or seven inches

    Italian motorcycle engineers began building inline-4's back in the
    1950's that had
    one carburetor and one exhaust pipe per cylinder. MV Agusta inspired
    Honda to
    follow the scheme. MV lent Honda their technical expertise.

    The early one carb per cylinder Hondas didn't really produce a lot of
    power in spite of their use of really high RPM. Some smaller Honda
    grand prix engines turned an incredible 20,000 RPM. But the horsepower
    output was just as incredibly low, compared to modern engines.

    The individual grand prix racer cylinders did not "breathe" all that
    because they didn't take advantage of intake and exhaust pulses from
    the cylinders that were firing 180 degrees later.

    George Kerker and a few other California hotrodders figured that if a
    pair of tuned headers worked well on American V-8's, a single 4 into 1
    header would do the same for an inline 4 motorcycle engine.

    The problem was that the engines had a frustrating flat spot between
    5000 and 7000 or 8000 RPM, and then they made great horsepower from
    8000 RPM to whatever RPM the engine could stand. Up until just
    recently, that was about 13,000 RPM, but
    there is a 600 cc Yamaha engine that turns 18,000 RPM now.

    It has 4 individual fuel injector bodies.

    Motorcycle engineers have taken another trick from the automotive
    performance book.

    They use tuned air boxes called "Helmholtz Resonators" to feed the
    individual carburetor. The air boxes overcome adverse pressure waves
    that try to drive air backwards out of the carburetors or throttle
    FB, Jun 11, 2006
  9. markc

    markc Guest

    Your in depth explanation is very much appreciated.
    markc, Jun 11, 2006
  10. (The Older Gentleman) wrote in
    Cites please.

    I would love to see someone put an intake maniford and a single carb on
    something like a KZ-1300 and then tell me that it was easy.
    R. Pierce Butler, Jun 12, 2006
  11. markc

    James Clark Guest

    R. Pierce Butler wrote:
    Cites please.

    Boss Hoss uses a single carburetor, and it has 2 more cylinders than the Kawasaki.
    James Clark, Jun 12, 2006
  12. markc

    FB Guest

    I saw a neautiful old 1970's era Triumph Trident fitted with a manifold
    and a single carburetor.

    The installation was sanitary, but horsepower output was severely
    limited by the single
    ~30mm Amal carburetor. The owner wanted good low RPM throttle response
    because the Trident was powering a sidecar rig.

    Like Sammm said, it's necessary to keep the intake port velocity within
    certain limits.

    When you stick any kind of a manifold onto a small displacement engine
    whose cylinders don't fire all that often at lower RPM, you can never
    get the intake velocity up high enough to suck enough gasoline out of
    the float bowl and the fuel droplets mix poorly.

    The log type manifold just has far too much volume in the plenum, and
    the engineer would have to employ the flask resonance formula to see
    what frequency the manifold would resonate at.

    I won't repeat the flask resonance formula, as I have posted it before.
    But, even if you installed two carburetors on two, 2-legged manifolds
    like my old Triumph TR-4 had, you would run into a problem with excess
    volume of air in the manifold leg that went to the cylinder that wasn't
    on the intake stroke.

    A smart designer could probably make use of that air to give him a
    boost at some RPM, but the kind of guy who is wondering why motorcycles
    don't have a log manifold with one Stromberg 97 is unlikely to be a
    "smart designer"...

    There are *enough* problems inherent with getting good throttle
    response out of a small displacement motorcycle engine that uses one
    carburetor per cylinder, feeding through one intake port.

    Gas velocity through a port = piston speed X (port area / cylinder bore
    area )

    Modern high performance engines need port velocities around 350 to 450
    feet per second to have good throttle response.

    Old British engines like the AJS-7R would hardly run with port
    velocities down around
    220 feet per second, and the almost-competitive Harley V-twin road
    racers that were discontinued had such large intake ports they lacked
    throttle response with intake port velocities around 300 feet per

    The Harley was fuel injected, so getting fuel into the system was no
    problem. But the designer of the inlet ports was a NASCAR engine
    builder (Roush), and he was designing the ports as if the Harley was
    going to run wide open on super speedways.

    The upper limit for intake port velocity is around 1100 feet per
    second. Anything faster than that causes sonic shock waves that reduce
    air flow.
    FB, Jun 12, 2006
  13. markc

    Les Guest

    Settle for a CSR-650 inline 4 when I start working on it? I'll more than
    likely set it up with a single carb, just to be different.
    Les, Jun 12, 2006
  14. Well, after I posted, I realised you were probably talking about fours,
    but the OP referred also to a CB450 and they're twins.

    As you know, just about every brit twin made for years had a single-carb
    version. So did many Hondas.

    Kawasaki Z1300s made two with three carbs before they went to FI.

    But, agreed, I've never seen a four-cylinder bike with a single carb,
    except possibly a Nimbus or Henderson.
    The Older Gentleman, Jun 12, 2006
  15. markc

    FB Guest

    Major clue ahead:

    The Chevy's single carburetor has *accelerator pumps* to overcome low
    engine vacuum when the butterflies are suddenly opened. The Kawasaki
    uses smaller inlet port runners to keep vacuum high for quicker
    response. Motorcycle carbs have gotten along without accelerator pumps
    for decades because the whole induction design concept is different.

    The Boss Hoss has 5 times the displacement of the KZ-1300 and turns a
    lot slower to put out half the horsepower per cubic inch. It probably
    has a complicated manifold with long U-shaped runners feeding banks of
    cylinders.It doesn't matter much to the Chevy engine that inlet port
    velocity is low, since the mainfold runners are feeding from a larger
    plenum chamber under the carburetor.

    NASCAR tuners are even taking advantage of the fact that fuel droplets
    condense and puddle in the bottom of the manifold. They regard the
    puddle of gasoline as a sort of fuel reservoir to help the engine
    accelerate when the butterflies first open.

    Car drivers can put up with cantankerous throttle response because the
    car has four wheels and it isn't going to throw the driver out the
    window. But a motorcycle rider needs the smooth response that the
    engineers have achieved using multiple CV carbs
    and single intake runners.

    If you compared the venturi area of the Kawasaki carburetors vs. the
    Chevy carb, you'd probably find that the Kawasaki has more venturi area
    per cubic inch, so it uses a different manifold/carburetor arrangement
    to keep inlet port velocity high.

    There's no room behind the KZ-1300's engine for long intake runners,
    the space is needed for the airbox, which acts as a Helmholtz Resonator
    to help smooth out the
    midrange flat spot.
    FB, Jun 12, 2006

  16. Jesus, in every bit of wisdom you post there's a huge steaming pile of

    Like this: multiple-cylinder bikes manage to achieve smooth response
    using ordinary slide carbs. There's one sitting in my garage right now.

    I'll not argue that CV carbs manage to compensate for a lot of nasties,
    but slide carbs still have a lot to offer.
    The Older Gentleman, Jun 12, 2006
  17. markc

    FB Guest

    You don't have to call me "Jesus", you can call me "LeRoy". ;-)

    And, there's no secret that some users surf through Usenet all day,
    just looking for opinions to disagree with.
    Fine. You can use smaller bore, old Amals, or you can use larger bore
    Mikuni and Keihin slide valve carbs and get good throttle reponse
    because Mikunis have 5 circuits instead of 3.
    Yes, I have a little GSXR-750 in the garage now. It has a set of 33mm
    Mikuni roundslide smoothbores that I bought used. With a Yoshimura
    competition pipe it probably has 20 horsepower more than stock and I
    have startled GSXR-1100 riders with the accleration.

    The old gixxer is a monster that takes careful throttle control, and is
    no fun to ride slowly.

    In order to tame it down, I jetted it lean and advanced the timing 5

    But, if I was going to do a carb swap on a later model GSXR, I would
    spring for the RS-series radial flat slide carbs with accelerator pumps.
    FB, Jun 12, 2006
  18. markc

    Scott Guest

    My CB900F has an accelerator pump, but only one, on the #2 carb. FWIW.

    Scott, Jun 13, 2006
  19. markc

    James Clark Guest

    Or the occasional ATP equipped UJM.
    James Clark, Jun 13, 2006
  20. Why doesen't my bike have a midrange flat spot, I use individual pod air
    filters, not an airbox?

    Ted Mittelstaedt, Jun 13, 2006
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