Yamaha's Raptor ATV (All Terrain Vehicle) uses a carburetor to mix fuel and air. Problems with the carburetor can include uneven idle. Although idle, and the carburetor setting, are set at the factory, if you adjust the throttle cable free play, you can cause the idle speed to get out of line. In any case the idle should be checked and if necessary adjusted following the owner's manual maintenance schedule. Troubleshooting a Yamaha Raptor carburetor can be accomplished by following some steps.
Start the engine and warm it up. The engine is warmed when it becomes highly responsive when you adjust the throttle. Listen to the idle for unusual speeds. It shouldn't sound as if the engine is racing when out of gear. It also shouldn't stall when idling. Listen for a happy medium.
Remove the spark plug lead and attach a diagnostic tachometer if the idle doesn't appear right. Measure the idling speed. It should be between 1,500 and 1,600 rpm (revolutions per minute) on the tachometer you attached.
Adjust the stop screw on the carburetor if the rpm reading from the previous step is out of range. The screw is on the side of the carburetor, and turning it clockwise increases the idling speed. Turning it counterclockwise decreases it.