Carburetor - KEIHIN 2-Bbl: Description

Carburetor is a 2-barrel downdraft design incorporating electronic and automatic compensating devices to maintain proper air/fuel mixture under all operating conditions. The fuel flow from the accelerator pump is regulated by a temperature sensitive by-pass valve within the carburetor. When cold, by-pass valve is closed, so the accelerator pump receives maximum fuel. When engine is warm, by-pass valve opens and allows excess fuel back into float chamber.

Carburetor components include an electrically-heated automatic choke, choke opener diaphragm, throttle controller diaphragm, idle controller diaphragm (A/C), dashpot diaphragm, fuel cut-off valve, primary slow mixture cut-off solenoid valve, fast idle unloader, power valve, air vent cut-off diaphragm, air temperature sensor, intake air control diaphragm and air jet controller (some models).

The Air Jet Controller (AJC) is an atmospheric pressure sensing device, controlling the amount of airflow into slow and main air jets of primary carburetor venturi and slow air jet of secondary carburetor throat. As outside pressure is reduced by increasing altitude, the bellows expand to open valve in AJC, increasing airflow to the jets to maintain optimum air/fuel mixture.

Both choke valve setting and fast idle position are controlled during engine warm-up by automatic choke. It consists of the choke valve, linkage, choke heater electrical circuit, choke opener, fast idle and fast idle unloader system.

On some models, a heat riser, utilizing engine coolant, is built into intake manifold to prevent carburetor icing and improve cold driveability, thus improving air/fuel mixture atomization. A by-pass valve, with thermostat, shuts off coolant flow at normal operating temperatures to prevent fuel percolation. Other models are equipped with an Early Fuel Evaporator (EFE) heater to improve engine operation while the engine is warming up.

All models are equipped with a power valve. On Calif. 1.3L and 1.5L HF engines, the power valve is open when intake air temperature is less than 59°F (15°C), or when vehicle is going slower than 10 MPH and intake air temperature is greater than 59°F (15°C). When vehicle exceeds 10 MPH, power valve is controlled by vacuum switch signal to power valve solenoid valve. The solenoid valve controlling the power valve is energized by low manifold vacuum to the vacuum switch.

On all other 1.5L engines, the control unit energizes power valve control solenoid to cut-off manifold vacuum to power valve, which supplies supplementary fuel to primary main fuel passage. The power valve control solenoid valve is on, under any one of the following conditions:

• Coolant temperature greater than 149°F (65°C) and engine speed less than 2800-3300 RPM.
• Intake air temperature less than 59°F (15°C), coolant temperature less than 149°F (65°C) and vehicle speed less than 10 MPH.
• Engine speed greater than 2800-3300 RPM and vacuum switch "B" is off.