The only wire going to the carby is from the no 3 contact on the power switch to the solenoid on the bottom of the carby. The other wire from the solenoid goes to earth. When the ignition is on, power is supplied to the solenoid which allows fuel to flow. Turn the power on and off while feeling the solenoid, if it doesn't click, it could be seized.
Gazza, May 2009