I picked a 2005 Exmark 48" w/15hp Kawasaki that was on Craigslist. The transmission was replaced with a new transmission. The seller said it wasn't used for commercial use. I saw a brief demonstration that the mower runs, got it home and cut the back of my yard (about 1/2 an acre) and it seemed to run okay other than bogging down in thicker patches of the yard. I went to cut the front yard (about half an acre) and it seemed to just lose power. I raised the blade, dropped down to first gear, and still the mower struggles anytime the blade is engaged. It is also smoking from the muffler. I checked the oil and it was low so I added oil. I called and left a message to the Craigslist seller and told him that the mower just isn't running the same as when I saw it run at his house. The Craigslist ad reads: 05 walk behind with 48" deck and velky new paint runs great just put and new tranny in it needs a $15 drive belt When I was at his house, I saw some smoke and he said that in the winter he puts a little diesel in the gas and that the smoke was the diesel burning off. He also said that it needs an air filter. Could an air filter cause problems like this? Bring it back or bring it to the shop???? Is it possible that an air filter would cause this much trouble?