In 2011 I purchased a Craftsman model # 917288561 semi-zero-turn lawn tractor withe 22hp B&S 2-cylinder engine, which has performed flawlessly mowing and treating our 3/4-acer irregular, hilly lawn, until now. When I brought it out this spring I wanted to change the oil, clean the deck and change the blades before cranking it up. So I removed and inspected the mower deck and performed the stated tasks, noting that one of the mandrel shaft housings was developing stress cracks at two of the four attachments and should be replaced asap. Following reassembly I started the engine, observed it was running smoothly, tested the blade engagement, also smooth, and proceeded to mow the lawn.
After a few passes there suddenly began a chucking and clattering that seemed to be associated with the brake pedal. I checked underneath to see if anything was amiss but all appeared perfectly normal, apart from what seemed to be some extra slack in the brake cable-rod assembly behind the mower deck, and I found I could minimize (but not eliminate) the noise by keeping very light pressure on the brake. Since the lawn was badly in need of mowing I proceeded, but then observed an additional problem in that, while the engine was running smooth as silk with the mower engaged, it would start surging whenever I disengaged the mower, regardless of the throttle setting.
I have removed and re-installed the mower deck in the past without incident. I purposely chose not to crank the engine until it had new oil, so have no way of knowing if these symptoms developed over the winter, resulted from my repairs, or started spontaneously after running the machine a while, although it seems most likely the latter. I also don't see how the two symptoms could be connected, or why the engine would run smoothly with the mower engaged, but surge when it is idling or moving the tractor with mower disengaged. It is also puzzling that these symptoms only appeared after a few lengthy mowing passes. Any suggestions would be greatly appreciated.