I have a 2002 Exmark LazerHP with a 23HP kawasaki. Machine and motor have 500hrs. In the last two weeks, on three seperate occasions when I have tried to start the motor for the first time, the motor backfires during start up. The only other symptom is that on all three times, the motor has had a longer than normal time before it starts...until after it backfires. Any ideas? BTW, once it's started, it usually starts fine the rest of the day. Also, I always use the choke with the first startup every day.