Connection diagram of the carburetor solenoid valve control system: 1 - ignition switch; 2 - ignition relay; 3 - ignition coil; 4 - control unit; 5 - solenoid valve; 6 - carburetor limit switch.
The carburetor solenoid control unit shuts off the valve (interrupts the fuel supply) with an increase in the frequency of rotation of the crankshaft up to 2100 min–1 and turns on when the speed is reduced to 1900 min–1, if the carburetor limit switch is shorted to ground (pedal «gas» released). When the pedal is depressed «gas» (open circuit breaker) the valve is turned on regardless of the number of revolutions of the crankshaft (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 at the same time (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, we connect a voltmeter in parallel with the winding of the solenoid valve (valve actuation can also be determined by its knock, without connecting a voltmeter). Disconnect the wire from the carburetor limit switch and close it to «mass». We start the engine and, gradually increasing the speed of the crankshaft, we monitor the readings of the voltmeter. After starting the engine, the voltmeter should show at least 10 V (valve open), and at a speed of about 2100 min–1 the voltage should drop sharply to a value of no more than 0.5 V (valve - closed). After that, slowly lower the engine speed, about 1900 min–1 voltage should increase abruptly to its previous value (valve - open). Set the speed in the range of 2200–2300 min–1 (valve closed) and disconnect from «masses» carburetor limit switch wire - the valve should open.
Removing the solenoid valve, see Removing and checking the solenoid valve.