I have a Sears Craftsman 18 HP tractor that has been running very badly. It backfires when running and sometimes does not even start up when I press down on the clutch/brake. It has taken a beating the past two years with all the snow plowing I have used it for and has left me stuck when in the middle of plowing at times. Where do I start to look for the problem? The only maintenance I have done is the usual oil changes and air filters. It seemed to start giving me problems when I plowed snow. Any ideas?