Reply to "Backfire thru Carb and popping out the Exhaust"

Given the work you've already done, popping out the exhaust will probably be due to either a bad sparkplug or wire, a misrouted plug wire or a burnt valve. It is also possible that you have a bad rocker, a worn cam lobe or a burnt piston. First, try touching each exhaust pipe just after starting: an intermittently firing plug or wire will give a noticably cooler pipe and help localize the trouble. Pull the rocker cover on the 'bad' side and watch the valve action for differences- this will show a bent pushrod or bad rocker. Finally, I would run a leak-down test to see which cylinder is bad and how bad it is; this will also tell you if its a valve or piston.