Possible burnt valves???

hi all,
i think i may have 1 or more burnt exhaust valves in my 1600 xflow. to give the full story, drove fine, pulled up in a car park. when i restarted it instantly struggled to idle and ran on 3 cylinders. pulled the plugs out and no. 3 cylinder was burnt (white plug). changed plugs, tuned the carby so it would run. have also done valve clearances and run carby/ upper cylinder cleaner through it. this made it better. put a piece of paper over the exhaust and it sucked in. but, occasionally it starts to run good, and one of those times i jumped out and did the paper test, and it didnt suck in! so, is it definately a burnt valve, or are there other possibilities???
p.s. im also looking for another xflow to rebuild