Engine stalls when the air cleaner is installed!?

I just fired up my 18 year old Craftsman Eager1 lawn mower and it would run for a few moments then stall. I put in fresh gas, and a new plug but still no luck. It ran fine all last year. When I pulled off the air filter it started and ran fine. I bought a new filter and after I installed the fresh air filter it still stalls. I am thinking this might mean that an engine valve is leaking and the extra resistance with the air cleaner installed limits the air that can be pulled through the carb thus starving the engine for fuel. Is this possible or is there a more likely and cheaper resolution? Thanks!

Bill Thompson
April 2006