I have a yard machines (mtd) riding mower that started to act up just as I finished my last mow last year. It is a model 13A1660F000 with a Tecumseh 13.5 hp engine model OHV135-206904E. It ran great all last summer. Then, after finishing mowing, I disengaged the blades, and it started to act like it would if it was choked too much. It would run but would keep acting like it wanted to stop. The funny thing was that, when I engaged the blades, it ran fine. As soon as I disengaged the blades, it acted up again. In other words, it ran normally while under load, but ran erratically under no load. I ran the gas out and left it until now. The only thing I did different last year from previous years was that I added Seafoam to the gas at every use, even though I was not having any issues. I had bought Seafoam for my other mower that had issues and I just wanted to use the stuff up, thinking it wouldn't hurt anything. I just started to look at the mower now and am wondering if anyone has any suggestions on how to fix this problem. I did just notice that the gas cap is missing. I don't know if it might have fallen off as I was mowing, or if I just forgot to put it back on after I emptied the gas last year. But it doesn't seem like that would cause this problem. Does anyone have a suggestion? Will appreciate all help.