Carburetor Overview (Keihin)
Although intended for Keihin non-pumper type carb, this info may be helpful for other carburetors as well.
The carburetor on your dirt bike should perform suitably with the standard recommended settings under average load, climatic, and barometric conditions. However, to fine tune the engine’s power output, the carburetor may require adjustments for specific competition needs. Optional main jets and slow jets are available for your dirt bike. Any engine or air-box modifications or the use of an aftermarket exhaust system may require jetting changes. The function of the carburetor is to atomize fuel and mix it with air in proper proportions to suit engine operating conditions. In operation, the carburetor meters gas into the fast moving air passing through it. The atomized gas (a mist of liquid fuel) is then vaporized (changed from a liquid to a gas) by engine heat and the heat of compression to provide a uniform and efficiently combustible air/fuel mixture.
In theory, the perfect air/fuel ratio is 14.7 parts of air to one part of gas, by weight. A uniform air/fuel ratio of this proportion allows the mixture to burn completely without leaving an excess of either fuel or air. Rich (excessive fuel) or lean (excessive air) mixtures both result in loss of power. An excessively lean mixture can also cause engine damage. An intentionally rich mixture (from applying the choke lever) is used for starting because a cold engine reduces vaporization. A throttle valve (carburetor slide) controls the amount of air/fuel mixture delivered to the engine, regulating the engine’s power output. When the throttle valve opening is increased, engine speed (rpm) also increases and air rushes through the carburetor bore at a greater rate. Unfortunately, the rate of fuel flow through a fixed jet does not increase proportionately with an increase in air speed through a fixed venturi. At high speeds, the air/fuel mixture tends to become richer. For this reason, it is desirable to vary the venturi size and meter the fuel flow to maintain correct air/fuel mixture ratios over a wide range of operating speeds. This is achieved by using compensating jets and air-bleeds (air jets). Each of the carburetor circuits affects the delivery of the air/fuel mixture over a given portion of the throttle valve opening. These circuits overlap as shown on the graph.
Idle & Low Speed System
An adjustable pilot screw controls the idle mixture. The pilot screw is located in the passage between the low speed jet and the idle fuel discharge orifice to control the rate of flow of aerated fuel delivered to the carburetor bore. An adjustable throttle stop screw controls the idle speed by raising or lowering the slide position when the throttle is closed. A replaceable slow jet, located next to the main jet, controls the amount of fuel entering the idle and low speed system.
Opening the throttle valve (carburetor slide) permits a transition from the low speed system to the intermediate system which meters fuel from the main fuel discharge (needle) jet. A tapered fuel metering rod (jet needle) connected to the throttle slide, extends down into the main (needle) jet. The jet needle position, which is adjustable, maintains the correct air/fuel mixture ratio through most of the carburetor’s operating range, just short of fully open throttle. At that point the jet needle is fully raised, and fuel flow will be controlled primarily by the main jet.
High Speed System
Fuel delivery is controlled by the size of the replaceable main (needle) jet and the thickness of the jet needle.
The float system is designed to maintain a constant and correct level of fuel in the carburetor’s float bowl. A float rises or falls with the fuel level in the float bowl. The correct float bowl fuel level is established by the carburetor manufacturer.
Float System Vents
Float system vents are necessary to ensure a smooth flow of fuel through the carburetor. The externally vented float bowl has its vent tubes routed to atmosphere so that atmospheric pressure can maintain pressure on the fuel inside the float bowl. Vent tube routing is critical. Any change in the stock routing of the tubes may pinch the tubes. Improperly routed tubes may also be exposed to low pressure when the motorcycle is in motion, which could change the pressure in the float bowl and alter fuel delivery.
Air Metering Systems
Air under atmospheric pressure is bled, into the carburetor fuel passages to improve fuel atomization, to stabilize fuel height in the jets, and to provide corrections in the air/fuel mixture ratio. Air jets and/or air-bleed adjustment screws control the relative amount of atmospheric air drawn into the fuel systems. This system is factory pre-set and should not be altered.
Main Jet Air-Bleed System
Low venturi pressure, which causes fuel to rise through the main fuel jet, also causes atmospheric air to flow through the air jet. Air and fuel meet and mix together in a perforated (emulsion) tube above the main fuel jet. The aerated fuel released into the venturi is more easily atomized than a dense un-aerated stream of fuel. Aerated fuel also has less tendency to fall back down the jet tube between intake strokes, thus stabilizing fuel height in the jet tube. The same effect can be observed when drinking beverages through a straw. When you remove your mouth from the straw, a frothy beverage tends to remain in the straw, but an un-aerated beverage will fall back down the straw into the glass.
An air-cut valve is used to prevent popping in the exhaust system during deceleration. The valve enriches the air/fuel mixture during deceleration. A diaphragm in the air cut valve is activated whenever high manifold vacuum is present, such as during deceleration. The movement of the diaphragm causes a partial blockage of the air bleed system in the low speed circuit. This reduces the aeration to the low speed jet which creates a richer mixture. The air-cut valve is factory pre-set and should not be altered. It may have to be replaced eventually, because the rubber diaphragm in the valve may deteriorate over time. If the diaphragm is deteriorating, you may notice leanness or a popping in the exhaust during deceleration.
Cold Starting System
Fuel does not vaporize well in a cold engine. For this reason, the carburetor must deliver a richer mixture. The mixture must not be excessively en-richened, however, or the combustion chamber can become flooded with liquid fuel. Your motorcycle uses a choke valve that en-richens the mixture by obstructing the carburetor bore. When the choke lever is closed (choke lever ON), it reduces the volume of air that can flow through the carburetor bore to fill the vacuum created in the engine cylinder. Atmospheric pressure in the float bowl then forces more fuel into the carburetor bore.