Small Engines (Lawn Mowers, etc.): Craftsman 6.25 hp lawnmower wont start..., compression test, gas cap

Question
I am perplexed by my lawnmower.  It ran great until recently, but now when I try to start it I do not get good ignition and when it is catches it seems to backfire.  I changed the plug and checked the wire, both are operating properly.  I also took the gas cap off to see if the engine is being starved.  There is only one adjusting screw on the carb, which I think is for choke adjustment.  Is there any way to adjust timing? Does this kind of motor have a fuel filter?  Any suggestions?  

Answer
Hi Kim,

If an object was hit with the blades, the timing might be affected by a sheared flywheel key. A loose blade would cause damage to the key also.
Carb adjustment should not have suddenly gone out of adjustment.  The only other thing I can think of is a valve problem.  A compression test would confirm that.  There may be an in-tank filter but I don't think that is the problem.

Let me know what you find.
Michael