Holley Model 6520 2 Barrel Carburetor

Fig. 2 Holley model 6520 carburetor. 1984-86:

The Holley model 6520, Fig 2, is a staged dual venturi electronic feedback carburetor. The primary bore is smaller than the secondary bore. The secondary stage is mechanically operated by linkage connecting the primary and secondary throttle levers. The primary stage includes a curb idle and transfer system, diaphragm type accelerator pump system, main metering system and a fuel regulator solenoid responsive to oxygen sensor. The secondary stage includes a main metering system and power system. Both the primary and secondary venturi draw fuel from a common fuel bowl. The electric automatic choke incorporates a two stage heating element. On manual transmission carburetors, only one choke valve is used, while on automatic transmission carburetors, two choke valves are used.
The 6250 electronic feedback carburetor also 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 air-flow. When the electrical signal is applied to the solenoid, the field windings are energized, thus 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 on 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.