[richnct]

Check for closed choke plate (as suggested) and attach clear tubing to the carb drain nipples. Loop the tubes up and open the drains to compare the gas level in the float bowls to see if the problem side is noticeably higher. It seems like a "too rich" condition, as poor ignition would likely give poor performance from the get go, not only after the plug carbons up. Then if no problem is found, disassemble the carb to check for incorrect placement of jets or jet unscrewed.