90% of poor running on most newer (< 5 years old) non-side valve mowers (either overhead cam or overhead valve design) is fuel-related, usually the carburetor. As others have said, consider replacing the carb itself AND THE GASKETS, and I'll bet it will run like a top. The carb for most GCV engines is pretty inexpensive, out the door with gaskets is probably <$30, and be sure to get the springs, anti-surge and governor, back in the right holes. Also confirm the choke linkage is working right. Share the serial number off the ENGINE (not the mower, unless it's a Honda mower), and I can get some service details for you.