Yamaha have produced a multitude of different carburettors since their foundation in 1955. Carburettors are used to regulate the delivery of the fuel mixture into the cylinders of internal combustion engines. The GT1, DT125C and DT100 B/C models are three examples of Yamaha produced carburettors, and illustrate the variations in specifications very well.
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The important specifications to a carburettor are the size of the main jet, the position of the jet needle, pilot jet, air screw and float level. The main jet is where fuel is drawn from into the airstream. These are attributed a number in carburettor specifications; a larger number indicates a wider jet hole. The jet needle is the long tapered rod that controls fuel intake. The position of the jet needle is designated by a number which refers to the notch where it is clipped into place, counting down from the top notch. The pilot jet supplies fuel when the throttle is only slightly open. The rating of the pilot jet in carburettor specifications refers to the size of the jet's aperture. The air screw regulates air flow before it reaches the pilot jet; the air screw's number in carburettor specifications refers to how many turns of the screw are required to restrict air flow and enrich the fuel mixture. The float level refers to the level of the float that regulates the level of the fuel reservoir.
The GT1 carburettor is a Y18P-3 type carburettor. The number 3 at the end of the carburettor type refers to the diameter in millimetres of the venturi tube's constriction. The venturi tube consists of a tapered entrance cone leading to an aperture where fuel flow is constricted and then delivered into a diffuser where it returns to its original pressure. This can be used to measure the fuel rate in a pipe. The GT1's main jet rating is 94, its jet needle position was 1 (meaning it was clipped at the first notch from the top), its pilot jet rating was 40 and it required 1 ½ turns of the air screw to restrict air flow. The GT1's float level was not adjustable so that specification is not applicable.
The Yamaha DT125C carburettor is a VM24SH type carburettor. It does not use a venturi tube and therefore does not have a diameter value. Its main jet was much larger than the GT1 and is rated at 140. The DT125C's jet needle position was 3, its pilot jet aperture is rated at 25 and its air screw requires 1 ¾ turns to restrict air flow. The DT125C's float level was 21.0mm.
Like the DT125C, Yamaha's DT100B/C carburettor did not use a venturi tube and therefore had no diameter rating; its type was simply expressed as VM32SS. Its main jet rating was larger still at 160, its jet needle position was 3, its pilot jet rating was 50 and it required 1 ½ turns of the air screw to restrict air flow. The DT100B/C had a float level of 17.3mm.
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