When you've finished your lawn, shut off the mower's fuel tap and run at full revs until it stops (runs out of fuel after 15 seconds or so). This will leave the carby nice and clean for the next start.
When you stop the plug firing (in a typical stop), the remaining fuel/oil mixture evaporates and leaves some of the oil lining the carby, making it harder to start after a few months.
Works a treat!
Brett
August 2007