Backfire through Carb

Tell you how you can instantly determine if it is an exhaust valve that's not opening. Remove the air cleaner. Start it up, drop it in drive with the parking brake set. Put your foot on the brake pedal good and tight. Ease into the gas and bring the RPM up to about 1500 loading the transmission in drive. If you get a steady rhythmic pop out of the carburetor that changes with engine RPM, you have an exhaust valve not opening and it is still firing on that cylinder, but blowing the exhaust back up through the intake and carburetor. At that point, you need to remove the valve covers and report back.