The tractor been running great for 14 years (550 hrs on the clock). My wife is the operator and reported to me that it had a little rattle. I found the front tire bushings and blade spindle bearings were worn. I replaced the bushing with bearings, replaced the front axles and blade spindles. I started it up, ran it for 2 minutes, shut it off and it would not restart. I found it was very low on oil (I am embarassed). Pulled the carb and cleaned it. Sanded some rust off the alternator and flywheel magnets. Found the bolt on the intake rocker arm had backed out and the valve wasn't moving on the Kohler single piston engine. I tightened the bolt (no locktite) and it fired up and sounded good. Now it takes 75 minutes to cut the grass instead of 45 minutes. The mower drives slower and doesn't completely mulch the grass. I found the drive belt (with previously replaced pulleys) was a little eaten up. I replaced the belt and blades but it didn't fix anything.
So, in a nutshell... Mower was fine before I started working on it. Found it was low on oil, had the carb off to clean, rocker arm was "off", replaced the drive belt, engine sounds good, plug/filter are good. I have not done a compression test. Any ideas what is causing my problem?