Tighten up the throttle cable. There are adjusters on top of each carb. You loosen the stop nut and then just twist (counter clockwise to tighten the cable).
Sometimes you can look at the cable splitter (where it splits 1 cable into 2) and see which cable is not adjusted properly.
What is going on is the throttle has two sensors in it. One to tell when the throttle is being pushed, and the other to tell when it is not. When you let off, if there is any slack in the cable, it should turn off. This is a safety feature.
Technical.... The two switches are connected in series with each other and in parallel with the kill switch. When sitting there, the throttle close switch is being pressed and it OPENS the circuit. When you push on the throttle, is CLOSES that switch and OPENS itself.
The switch for no-throttle is usually visible, little plastic button near where the cable goes through. The switch to tell when the throttle is being pushed is hidden inside and is activated by the pivot pin.