Have a Z HP 48" with Kaw21, 1 year old, with 300 hours on it. From time to time the engine will sputter and lose power and will eventually "clear its' throat" and run fine. During one of these "episodes" I removed the cover off the air filter and noticed that I had exhaust blowing back out the air filter. Took it to the dealer and they said I had fuel in my oil due to a bad seat in the carb. So they replaced the carb, oil, fuel and air filters. Ran it for a couple hours and had the same problem with the engine sputtering. Took it back to the dealer and they don't know what is wrong with it. I was going to guess, I would think there is something wrong with the intake valve...but what do I know? Machine has worked great before this.