Connect a test lamp between the carburetor and the positive pole of the battery.
If the lamp lights up, and the idle shutoff valve does not turn off despite the flow of current, replace the valve.
If the lamp does not light, unscrew the ground wire (see arrow) on the carburetor and clean the contact well, reconnect the ground wire and repeat the test. If the lamp does not light, then find a break in the ground wire and eliminate the malfunction.