My Coleman 5000 generator surges while it runs and won't settle down to its operating RPM. I replaced the carb and it worked well one time and the next time I started it the surging is back. The repair shop tells me it is the carb, can it be anything else?

Crud in the fuel system. Water in the gas. Check the gas tank for any rust. Clean the entiire fuel system. You might have to take apart and clean the carb again.

Did you use the same fuel tank to transport the fuel to unit after the new carb was installed? If so, it could be nasty fuel.