Carburetor: Description and Operation

Fig. 1 Holley model 1945 carburetor assembly. 1985 shown, others similar:

Fig. 2 Holley model 6145 carburetor assembly. 1985 shown, others similar:

The single barrel carburetors, Figs. 1 and 2, utilize dual nitrophyl floats to control the fuel level, permitting high angularity operation during the most severe operating conditions. The float construction eliminates the possibility of a malfunction due to a punctured float.
An electric choke system is incorporated to open the choke at approximately 60° F.
The accelerator pump is of the piston type and is operated by a rod and link connected to the throttle lever. The power enrichment system on all units consists of a power valve installed near the center of the carburetor body and a vacuum piston located in the bowl cover. A spring loaded modulated power valve is used. A vacuum passage in the throttle body transmits manifold vacuum to the vacuum piston chamber in the bowl cover. Under light throttle and load conditions, vacuum acting on the vacuum piston is sufficient to overcome the spring tension. When the throttle valve is opened to 55°, vacuum acting on the vacuum piston is bled to the atmosphere and manifold vacuum is closed off. The throttle shaft is provided with a small hole which aligns with a port in the base of the carburetor when the throttle valve is opened to 55°. This vents the vacuum piston chamber to the atmosphere, allowing the spring tension to open the power valve. The 6145 electronic feedback carburetor incorporates a duty cycle solenoid which provides a limited regulation of air/fuel ratio in response to electrical signals from the spark control computer. The solenoid meters the main fuel system and operates in parallel with a conventional fixed main metering jet. When there is no electrical signal applied to the solenoid, the valve spring pushes upward through the main system fuel valve, fully uncovering the solenoid controlled main metering orifice so that the richest condition exists within the carburetor for any given airflow. When the electrical signal is applied to the solenoid, the field windings are energized, causing the armature to move the push rod and main system valve downward against the valve spring. This movement will continue until the main system valve bottoms against the main system valve seat. In this position the solenoid controlled main metering orifice is fully sealed so that the leanest condition exists within the carburetor for any given airflow. This condition will remain unchanged until the signal from the spark control computer to the solenoid is switched off. The main system fuel may be regulated between richest and leanest limits by controlling the amount of time that the solenoid is in the power position. Under normal operating conditions, 12 volts at a frequency of 10 Hz is applied to the field windings. By controlling the duration of the voltage signal, the power on time to total time, referred to as the duty cycle, is established.