Post by firstname.lastname@example.org
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
Getting frustrated by the complexity and mysteries of multiple carbs,
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 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
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
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
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