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Carburetion

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 brown color.
 

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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Last modified: 05/26/14