Wiring diagram of the carburetor electromagnetic valve control system: 1 – ignition switch; 2 – ignition relay; 3 – ignition coil; 4 – control unit;5 – electromagnetic valve; 6 – carburetor limit switch.
The carburetor solenoid valve control unit turns off the valve (interrupts fuel supply) when the crankshaft speed increases to 2100 min⁻¹ and turns on when the speed decreases to 1900 min⁻¹, if the carburetor limit switch is shorted to ground (the "gas" pedal is released). When the "gas" pedal is pressed (open circuit breaker) the valve is on regardless of the crankshaft speed (fuel is supplied). Power is supplied to the control unit only when the ignition is on, so when the ignition is turned off, the valve is also turned off (regardless of the position of the carburetor limit switch), which prevents the engine from self-igniting (solenoid valve normally closed).
To check the operation of the control unit, connect a voltmeter in parallel with the winding of the electromagnetic valve (the valve operation can be determined by its knocking, without connecting a voltmeter). Disconnect the wire from the carburetor limit switch and connect it to ground. Start the engine and, gradually increasing the crankshaft speed, monitor the voltmeter readings. After starting the engine, the voltmeter should show at least 10 V (the valve is open), and at about 2100 rpm the voltage should drop sharply to a value of no more than 0.5 V (valve closed). After that, slowly reduce the engine speed, at about 1900 rpm the voltage should increase abruptly to the previous value (valve open). Set the speed within 2200–2300 rpm (the valve is closed) and disconnect the carburetor limit switch wire from the ground – the valve should open.
See Removing the Solenoid Valve. Removal and checking the electromagnetic valve.