Ever wondered what might be hiding under the engine covers. Well here's a photo of just how much grass can collect inside a engine that hasn't been serviced regularly.

Lawnmower engines are air cooled. Air is drawn in from the top of the motor and blown across the aluminium cooling fins by the flywheel. Obviously air can not cool an engine if it can not flow across it.
How can you tell if your mower engine is running hot?  Have a close look at the cylinder head bolt heads and the spark plug. If they are all rusty with a very slight blueish tinge, then you can bet on having a HOT engine. A motor that is running hot will burn all the shiny zinc coating off the spark plug and bolt heads allowing them to rust. The lack of air flow will also mean that the engine speed governor won't work well, causing the motor to over rev.
Continuing to use your lawnmower will cause damage in three ways. 1: Over revving leading to a broken con rod. 2: Piston ring damage causing high oil consumption and possible seizure. 3: High wear on valves and guides.
So while you may think your mower is going well, all may not be well at all, hidden under the engine covers. Remember small air cooled mower engines need a service every 25 hours of use.