Small Engine Troubleshooting Problems? Ask a Mechanic Now.
You are on the right track, fist thing to check is the carburetor throttle shaft and the butterfly in the carburetor. I have seen them broken or screws fall out!
Can you control speed manually at the carburetor? If you can, check the governor link. See if it has push/pull feel when you throttle engine manually. If no movement or feel from the governor linkage, the internal governor parts are broken!