The Mikuni BDST Carburetor

The Mikuni BDST carburetor is a variable venturi type, whose venturi cross section area is increased or decreased automatically by the piston valve (1) which moves according to the negative pressure present on the downstream side of the venture (A). Negative pressure is admitted into the diaphragm chamber (2) through two orifices (3) provide in the piston valve (1) Rising negative pressure overcomes the spring (4) force, causing the piston valve (1) to rise to increase the said area and thus prevent the air velocity from increasing. Thus, air velocity in the venturi passage is kept relatively constant for improved fuel atomization and for securing optimum ratio of fuel/air mixture.



Slow System
This system supplies fuel during engine operation with throttle value (1) closed or slightly open. The fuel from the float chamber (2) in metered by the pilot jet (3) where it mixes with air coming in through the pilot air jet (4). The mixture, rich with fuel, then goes up through the pilot passage to the pilot screw (5). A part of the mixture is discharged into the main bore out of the bypass port (6). The remainder is then metered by the pilot screw (5) and sprayed out into the main bore through the pilot outlet.



Main System
As the throttle valve (1) is opened, engine speed rises, and this increases negative pressure in the venturi (A). Consequently the piston valve (2) moves upwards. Meanwhile, the fuel in the float chamber (3) is metered by the main jet (4), and the metered fuel enters the needle jet (5), in which it mixes with the air admitted through the main air jet (6) to form a emulsion. The emulsified fuel then passes through the clearance between needle jet (5) and jet needle (7), and is discharged into the venturi (A), in which it meets the air stream being drawn by the engine. Mixture proportioning is accomplished by the needle jet (5); the clearance through which the emulsified fuel must flow is large or slam, depending ultimately on the throttle position.



Starter System (Choke)Pulling out the starter shaft (1) via the choke cable/lever, fuel is drawn into the starter circuit from the float chamber (2). Starter jet (3) meters this fuel, which then flows into the starter pipe (4) and mixes with the air coming from the float chamber (2). The mixture, rich in fuel content, reaches the starter plunger (5) and mixes again with the air coming through a passage extending from behind the diaphragm. The two successive mixings of fuel with air are such that proper fuel/air mixture for starting is produced when the mixture is sprayed out through the starter outlet (6) into the main bore.



The Float System
Floats (1) and needle valve (2) are associated with the same mechanism, so that, as the floats (1) move up and down, the needle valve (2) too moves likewise. When fuel level is up in the float chamber (3), floats (1) are up and the needle valve (2) remains pushed up against the valve seat. Under this condition, no fuel enters the float chamber (3). As the fuel level falls, floats (1) go down and the needle valve (2) unseats itself to admit fuel into the chamber (3). In this manner, the needle valve (2) admits and shuts off fuel alternately to maintain a practically constant fuel level inside the float chamber (3).