Place the chainsaw on a flat work bench.
Check to see if the spark plug wire is exposed. The spark plug wire is black and is connected to the spark plug. If its not exposed, remove the air filter cover located directly above the carburetor. This will expose the spark plug wire.
Turn the tachometer into the on position. Wrap the external lead from the tachometer around the spark plug wire.
Start the engine and let it idle. Check the reading in the idle position. Most chainsaw operate between 2500 to 3000 RPM's. Check your model's service manual for the proper range.
Advance the throttle to the full throttle position. Check the reading. Normal RPM's in the full throttle position are between 11000 to 15000. Check your model's service manual for the correct operating range.