The biggest problem I see when I go to someone's house to repair their mower that isn't working is they don't service it or didn't do it right. OIL= Oil selection is VERY important. You shouldn't use oils that have paraffin in them.  How do you know? Pour some oil out of the bottle and look at it.  If it is clear or light yellow, it does not have paraffin.  Brown oils do. Paraffin in a great lube in water cooled engines like your car because they run the same temp all the time.  But an air cooled engine will get hotter as you work it. And on a summer day, with you mowing grass that is a foot tall, the inside of your motor can reach 500 degrees.  That's when paraffin will smolder and cause engine failure.  I always recommend Castrol or Valvoline 30-HD for Briggs and Tecumseh motors. Kohler and Kawasaki require 10w-30 because the have hydraulic lifters. NEVER use Rotella in a lawnmower because it contain's graphite and sticks to your aluminum rod surfaces and will tighten the required gap between your rod and crank, Then BOOM! no more motor.  AIR FILTER= Change your air filter at least once a year.  If you have a paper filter, DON'T CLEAN IT. This only imbeds the dirt deeper into it. Foam filters should be washed in gasoline, squeezed out and dried with a clean rag, then saturated with no more than an ounce of clean oil. OIL FILTERS= This is simple, change it with your oil.  Don't use auto filter's just because they are cheaper. Your car runs with 30 to 80 PSI of pressure and a car filter has finer paper for that pressure. Mowers run around 7 to 12 PSI and the oil won't pass thru a car filter freely enough. SPARK PLUGS= ALWAYS put the same number plug back in. Don't clean you plugs to save money. Cleaning causes hot spots which cause early detonation of the fuel and make's the motor run hotter.  New plugs only.  Gas filters have to be changed too. But, if you don't have a fuel pump on your motor, don't use a paper filter. Paper filters will cause vapor lock without a fuel pump.

The best rule of thumb is: service every spring. Replace your blades if they show too much wear or are missing pieces. Many people I know have had pieces come off a blade and stick in a tree or somebody.  A flying blade part is like a bullet only bigger.

One more important thing: when you shut your mower off, return it to idle and let it do so for about 5 seconds before you kill it.  If you don't, you are dumping raw gas into a red hot muffler and this can be a bomb.  I know people who limp now because they shut their lawnmower off at full throttle and the muffler blew up, and pieces hit their leg. Imagine being hit with apiece of metal.  Now imagine it is RED HOT. Ouch.  I hope all this helps......Tommy