The Mikuni VM28 carburetor was an
interesting choice by Suzuki engineers for the production RL250 Exacta.
Contemporary bike manufacturers typically fitted a 26mm or even a 24mm carburetor to their
250cc trials motorcycles. Mikuni, according to their own engineering charts,
recommended no larger than a 26mm carburetor on an 18HP two-stroke engine.
The TS250, from which the RL250
engine grew, had been using the same 28mm carburetor for several years. Suzuki
really liked the idea of using common parts so it may have been no more than an accounting
decision for the RL250. What effect, though, does the 28mm carburetor have on the performance
of the RL250?
The internal combustion engine is
often referred to as simply an air pump. The swept volume of the cylinder is a
specific size and, disregarding inertia, heat, scavenging, and etc., the
engine will take in so much air and then expel it. That mass of air is pulled
from outside the air filter and over, around, and through the components that
make up the intake system.
Mr. Bernoulli determined for us many
years ago that a mass of air will speed up as it passes through a venturi with a
corresponding decrease in pressure within the venturi. This difference in
pressure allows an airplane wing to produce lift and our carburetors to meter
gasoline into the air flow on its way to the cylinder. The relatively high air
pressure outside the carburetor pushes the fuel out of the float bowl, through
the jets in the carburetor passages, and into the relatively low pressure air
stream within the venturi.
The amount of pressure decrease
within the venturi is dependent upon the difference between the diameter of the
venturi inlet and the diameter of the venturi itself. Consider two carburetors
with the same diameter inlet but different venturi diameters fitted to identical
engines. Each engine will pump the same amount of air through each carburetor.
The air mass moving through the smaller diameter venturi will have a higher
velocity and a lower pressure inside the venturi than the carburetor with the
larger diameter venturi.
Or, said another way, the difference
in pressures creates the motive force to move the fuel while jets determine the
amount of fuel that reaches the intake air stream. The cylinder displacement and
crankshaft target rotation speed determine the air volume requirement to be
passed through the engine. The size of the carburetor determines the velocity of
that air as it passes through the venturi.
Most of us are probably more
performance oriented than anything else. We 'get it' that installing a larger
carburetor on an engine can result in more power and/or speed. We can also agree
that if one installs progressively larger carburetors on an engine, there will
be a point where no additional gains in performance are realized. The larger the
carburetor, the slower the air mass moves though the venturi. The slower air
mass velocity means that the pressure difference becomes smaller and the job of
pulling fuel out of the float bowl and into the air stream becomes more and more
difficult at slower crank speeds. Useable power moves up through the RPM band
making low speed performance the biggest loser.
Generally speaking, the opposite
would be true if one installed progressively smaller carburetors on an engine.
Performance would suffer first in the higher RPM ranges because the resistance
of the air moving through the smaller venturi would become an ever increasing
obstruction to the air flow. Limiting the air to the cylinder would limit the
RPM and the useful power band would move down in the range.
The TS250 and RL250 have a 28mm
carburetor, while the TM250 has a 34mm fitted. A TM250 flat tracker will most
likely have a 36-38mm carburetor and the porting and exhaust chamber to match. They
are the same size engines but with different performance expectations.
Does the RL250 suffer because of
the 28mm carburetor? In fairness, it is difficult to distinguish distinct
carburetor issues because the carburetor was set up with the intention of using
a 20:1 fuel/oil mixture ratio. There will naturally be some difference in
viscosity between that mixture and a mixture with a modern synthetic lubricant. The
viscosity will influence how easily the mixture moves through the passages and
jets. In addition, these carburetors are now 40 years old and have varying
degrees of wear and damage from corrosion and abuse.
It is well established that the original carburetor setup operates too rich in all phases of operation. The jetting recommendations from Matrix Motor Sport will bring the mixture into normal range by specifying new idle and main jets. Once installed, the changes will allow clean operation from idle through high speed with a 50:1 synthetic oil mixture.
Would a smaller bore carburetor on
the RL250 provide better performance? The answer would technically have to be,
yes. Mikuni publications place the 28m carburetor outside the range of its own
recommendations. The chart specifies carburetor choices in a range of 22mm to
26mm. Carburetor bores in this range
would have an air velocity in a range considered to be optimum. Once correctly
jetted it would be tuned for more precise fuel metering, better distribution of
the mixture, and more predictable response to throttle movement.
Will a few revolutions be sacrificed from the maximum RPM? Probably. Unless an RL250 owner tries to establish a new land speed record I don't see how losing a few RPM at the upper end will make much of a difference.
I have been running a
modified version of the original
carburetor on my personal bike for years. This has been
achieved by placing an insert in the outlet of the carburetor reducing the
opening from 28mm to 25mm. In addition to being a very clean running engine, it
has exceptional low end response and the spark plug consistently shows a light
Beamish recognized this aspect of the RL250 and made this carburetor insert part of their early package of changes.
In addition to this modification, Matrix Motor Sport also can supply a new and fully tested 26mm carburetor kit. This would apply to situations where the original carburetor is not in good condition or for those owners who desire the most performance from their RL250.
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Matrix Motor Sport