Briggs and stratton 375 throttle wont work

Hi, I have a Briggs and Stratton 35 sprint and 375 sprint that both have the same problem. I have replaced the air filter, spark plug and governor springs on both of them. They both start-up fine but as soon as they do they seem to be stuck wide open and nothing I do with the throttle has any impact.
Any help would be greatly appreciated.