I have a GXV620 on a Hustler that is giving me problems. It started missing and popping under load. It seems the left cylinder is the problem. I've pulled the plugs and replaced them with new ones. It will run good for a while and then start sputtering again. When I pull the plug out it is sooty, but not wet. I've changed the fuel filter and air filter also. The engine has 285 hrs. on it. Could misadjusted valves cause this? They've never been checked. I don't have a shop manual, so I don't know the procedure/clearances. Any recommendations?