Misfire at cruise, popping/afterfire at 3000RPM

1 st Put your hand over the tailpipe and see if it sucks in pulse on your hand. Exhaust valve/s may be hung open... otherwise it night be ignition related...but sounds like exhaust valves are open , why is the thing to find out.
2nd, check carb for fuel dumping through mains.
3rd Leakdown test it, or try pulling valve covers and tapping the exhaust rockers with a rubber mallet.
Report back.