Check to see that the throttle plate in the carb is free to rotate.
I had a similar problem with a tiller. While stored over the winter, the shaft had gummed up enough to prevent movement. Since the governor "defaults" to wide open when the engine is off, that's the position it sticks in.

Where is the plate and how do I adjust it?

It's in the carb. It moves when the throttle linkage/governor moves. Don't confuse with the choke plate which will be closer to the air cleaner end of the carb.