Outdoor Power Equipment and Small Engines - B&S Governor Problem

It is a 20HP B&S Twic Cylinder Model 406777 Type 0115 E1 Code 000310YG. The engine is running at full throttle and will not idle down. There is nothing jammed in the linkage. It appears to be a governor problem. I tried to do a static adjustment however I ran into a problem. I loosened the nut that holds the linkage on the governor shaft. I tried turning the governor shaft by inserting a screwdriver into the slot but it wont budge. Am I doing something wrong or maybe the governor is jammed internally. Supposedly the owner hit a big bump in the field and that is when all this started.

Perhaps the shaft is siezed in the block.

Yep absolutely a possibility although I never thought of that -- would you suggest PB Blaster or something similar ?? There is so much linkage at the front of the block that it is difficult to get in there.

Does the governor arm move? If neither moves, I'd say the throttle shaft in the carb is stuck or there is some sort of bind in the linkage, rather than the governor shaft. The clamp bolt may be loose, but the arm may still be squeezed tight to the shaft. You might have to spread the clamp a little to get the governor shaft to turn. If the arm moves, but the shaft doesn't, then it's a good chance the shaft is stuck in the block.

For what it's worth, I had the same problem with an MTD riding mower and it was what Cheese had said, it was a stuck throttle shaft in the carb.

Well I was able to spread the clamp and do a static adjustment on the governor. I then noticed that one of the vertical linkages to the carb had a bow in it. I straightebed the bow, readjusted the throttle cable and the motor returned to idle. The motor is still running way too fast at full throttle. I will dig deeper into adjusting top end. Thank you everyone for your help. Fell free to suggest something about the top end overspeed.