Sears replaced the first tractor. The second tractor started sputtering and missing like the first. They sent a tech, who replaced the carb. This lasted for two mowings. As advised, changed fuel filter, air filter and bought ethanol-free gas at $5 a gallon. I even bought a B&S gas can! Not as bad, once warmed up. Sputters at all RPM's. Is this the nature of the beast? Any suggestions? 725cc, 22 HP has 38 hours on it. Still under warrantee, but perhaps someone knows a better fix than replacing the carburetor.