Semi-Automatic Choke
The semi-automatic choke differs from fully automatic choke in that choke is manually activated, but electrically deactivated. When choke knob is pulled out and engine temperature is below 159°F (°C), choke linkage will mechanically close choke valve and electrically activate electromagnetic switch.
Voltage to switch is supplied through engine temperature sensing switch. See Fig 1. As engine temperature increases, less voltage is supplied to the electromagnetic switch and the force used to hold choke knob out is reduced.
At the same time, choke return spring is exerting a constant pull against electromagnetic force, which gradually decreases choke valve angle. When engine temperature reaches 159°F (°C), electromagnetic switch is deactivated and choke knob is pulled by return spring force to the off position.