I had a similar issue, popping out the exhaust only at idle. Bring up the RPM's and it went away. I checked the ignition pulled carb apart, adjusted valves, checked for intake and exhaust leaks nothing worked. In the end..bad gas. Emptied the tank, cleaned it and put in new fuel and it ran like a champ. Gas of today isn't anywhere near as stable as fuel even from a few years back. Something else for you to consider.