The GCV 160 does not have any adjustment on throttle speed, so you need to check the governor spring and make sure it's not bound or twisted up and over-pulling the governor arm. If it's still over revving, then you may have an air leak in or around the carby causing it to run lean and over rev. If all is in order and you can't find a logical explanation for the over revving then you may need to stretch the governor spring a "LITTLE" so it's not pulling on the governor arm as much.
A few pics in and around the carby and governor area may help us identify any issues.
A "how to" for posting pictures can be found HERE