One of the mowers we use around our farm yard is a little no frills mower with a Briggs and Stratton engine. It's use around trees and equipment, to get into the little places. Anyway, using it one day and just as I caught site of a piece of baler twine, the mower hit it and it wrapped around the shaft. So I stopped the mower,and lifted up the back end, tilting it quite a bit to get at the twine and unravel it. As soon as I started it again, the **** thing wouldn't run right. It starts right up first pull, but the engine keeps surging back and forth, and seems to have slightly less power. I know it's something simple but what?