Small Engines (Lawn Mowers, etc.): 12.5hp Briggs not running good at full throttle, simplicity tractor, float bowl

Question
Hi,
I have an older model Simplicity tractor with a relative new 12.5hp Briggs on it, the problem I am having is it will start and runs fine when idling, but when I go full throttle, it will hesitate and then speed up then hesitate then speed up, then when I try to increase rpm's using the governor it acts as if it has no power, then I can turn it back to idle then turn the key off and the engine will back fire, what could be going on??

Answer
Bsckfireing was common in some of those engines but the surging is usually a sign of a dirty carburetor.
Is there an adjustable needle on the bottom of the carburetor float bowl?
When was the last major engine tune-up by a repair shop?
Let me know.
Eric