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

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

 

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

Leon

 

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

 

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.

 

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.
sammmm

 

It is rather difficult to fit a single carb with an intake manifold on a
bike.

 

(And not for the first time he posts this reply...)

Complete and utter nonsense.

 

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
cylinders.

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
buyers.

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
long.

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
well
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
bodies.

 

Your in depth explanation is very much appreciated.
Thanks

 

(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 wrote:
Cites please.

Boss Hoss uses a single carburetor, and it has 2 more cylinders than the Kawasaki.

 

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
second.

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.

 

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.

 

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.

 

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.

 

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

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.

 

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
degrees.

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.

 

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

-Scott

 

Or the occasional ATP equipped UJM.

 

Why doesen't my bike have a midrange flat spot, I use individual pod air
filters, not an airbox?

Ted