Gravely Compact Pro with Kawasaki fx 481 engine has popping sound

Hi, I bought this mower use with 703 hrs on it. When using mower it keeps popping. I have tried adjusting valves and took carburetor off and cleaned all ports with carb cleaner. The one adjustment on top of carb (air adjustment doesn't make any difference). I just ordered the 102 size main jet for carb but have not received yet to see if that will help. I worked for years on small engines when I worked at Sears as a technician but this has me baffled. One more thing it seems to get worse as the engine gets warmer as you mow the lawn. I would gladly take to a Gravely dealer but the closest one to me is 50 miles away and I'm leary to drop it off and if they do not have a good mechanic I will be constantly hauling it back and forth several times. Any help would be greatly appreciated. Thanks