I was thinking along the same lines as Ted but if you manually hold the throttle just open (idle position with Choke off) and the motor is obviously running too fast then there must be a fuel or air pressure problem .
When the engine idle suddenly runs high that usually means you have a fuel or air pressure problem, have a dirty air filter or you're running bad fuel. Cross the dirty air filter and bad fuel off your checklist after inspection and if the idle remains high then you might want to conduct a compression test to rule out an air leak.
An easy way to check the carby is not the problem for me is to bolt the carby to another motor and see if it runs as it should,
It could be just a governor problem as Ted suggested but you will need to check a few things.
Leaking crankcase seals can be a common cause of of excessive no-load engine speed ,there are a few methods for checking for worn crankcase seals ,I usually check the lower crankshaft seal by removing the spark plug lead and spark plug then with an oil can give the spark plug hole about 10 squirts of oil then refit the plug and turn the motor over ,most of the time this causes the motor to hydraulic lock when turning over so I slowly turn the motor over about 20 times from the blade plate then spin the motor with the starter a few times ,if the lower seal is excessively worn you will have oil dripping down the crankshaft if the seal is slightly worn the crank is usually dripping oil after a few days of sitting idle.
Another way is to get a 1.5 thou feeler gauge and push it between the seal and crank ,when the old seal has gone hard and is leaking the feeler gauge will have very little resistance when inserting between the seal and crankshaft ,with this method you need to be careful not to damage the seal with the feeler gauge if the seal is good.
Not long ago I had a powertorque motor, I rebuilt the carby then fitted and ran the motor and the engine ran excessively high in the idle position so I removed the carby then after fitting on another motor it ran perfect so I knew the other motor must have an air leak .
