I have a Husqvarna lawn tractor with a B&S 24 HP Intek-V Twin engine that I bought seven years ago and use to mow about one acre. Last week it ran fine and when I was done cutting I parked it in the shed. Yesterday it started right up and seemed to be running at full speed, but was making a consistent pow/putting sound, not as loud as backfiring. I ran it for maybe 30 seconds and then shut it down to search for mouse nests. After clearing some mouse debris I started it again and it made the same sound. Thinking it might go away if warmed up I ran it for a few minutes, but the sound persisted and I shut it down to investigate. I am not a mechanic, and only removed the top cover and air filter but could find nothing unusual, so I cranked it again. This time it would not start and I noticed it was puffing air out of the air inlet where the air filter mounts. Suspecting valves I removed the valve covers and found one push rod had fallen out of place and was severely bent. The other push rod from the same cylinder was intact but also bent, just not as bad (see photo). I then pushed on each of the four valve stems (two cylinders) to see if one was stuck and three of them depressed at least a good half inch. The fourth valve that had the severely bend push rod would only depress about a quarter of an inch and hit metal. I rotated the engine by hand several times but that valve always stopped against metal at about a quarter inch. The only metal in the cylinder I could think of is the piston, but it shouldn't be that close to the valve. One item of note. I had rented this house for 19 months and the lady tennant cut the lawn with this tractor for two seasons. She left last December and when I went to start cutting the lawn in April I found the oil level was just touching the tip of the dipstick. She apparently never checked or added oil. I filled the crankcase for that first cut, and then changed the oil and filter for subsequent cuts. I cut seven times so far before it went kaput yesterday. Any ideas as to what caused this and how severe of a problem it is? It ran fine when parked and simply sat for a week in the shed. I suspect that simply replacing the push rods will not fix it.