M/C Dwell-Mixture Control Solenoid Dwell

The M/C solenoid regulates fuel flow through the carburetor to control air/fuel mixtures based on operating signals from the ECM. When the ECM completes the solenoid ground circuit, the solenoid is energized and fuel flow is reduced, providing leaner air/fuel mixtures. When the ECM opens the ground circuit, the solenoid is de-energized and more fuel is allowed to flow through the carburetor providing richer air/fuel mixtures.
The solenoid is cycled on and off 10 times per second. The amount of on time versus off time (duty cycle) for these 10 pulses determines the air fuel mixtures, and the duty cycle is measured in degrees of dwell. A low dwell reading indicates more solenoid off time per cycle, or a rich mixture command, while a higher dwell reading indicates more solenoid on time or a lean mixture command. With the ignition on, engine stopped and the SCAN tool connected, most ECM's will output a median command of 30 degrees dwell.