engine surging of my craftsman riding tractor

Surges while I am driving the lawnmower. Had new carburetor , new sparkplugs, oil change, gas filter , and even emptied the gas tank and put new gas in it. Can you tell me what could it be. Does the carb need adjusting, like gas and air mixture. model 917270781 It is a 19 horsepower Briggs and Stratton engine . 15 yrs old