Tighten the RPM screw. It that doesn't work out, get a mechanic from nearby, he won't charge you much 
Copy and paste from Uncle Google:
Adjust the frequency by stopping the engine and loosening the bolt that clamps the governor arm to the governor shaft. Then stick a screwdriver in the slot in the governor shaft and turn it so it changes orientation with respect to the governor arm. I believe it will be counterclockwise to speed the engine up and clockwise to slow it down. Tighten up the clamp, start the engine, and measure again. Repeat until it's right.