Small Engines (Lawn Mowers, etc.): kawasaki fh21v wont start

Question
I have a 25 hp Kawasaki that only backfires when trying to start, any ideas

Answer
What is the engine mounted on?  Lawn mower, generator, pump?

Has the engine sat for a while or did the problem suddenly happen?  
Have the valves been checked?
Have you checked the flywheel key?

Did you download the service manual?
Take a look at the address below and see if it is your engine service manual.
http://www.parish-supply.com/documents/574.pdf

Let me know the answers to the questions.
Thanks.
Eric