That is a 120000 series engine, so the valve clearances are intake 0.005-0.007", exhaust 0.007-0.009". As you said, any problem that matches your symptoms is more likely to be with the exhaust valve than the inlet. Remember that one possibility is a loose exhaust valve seat, which may not show up in the clearance measurement.
Remember that exhaust popping can be due to lean mixture, incorrect ignition or valve timing, or weak ignition. Bear in mind also that side valve mower engines usually miss an occasional shot, especially at light load or idle, though this should not be accompanied by an exhaust pop.
