if you swapped the coil and plugs around and it was the same cylinder then you know its not electrical.
my banshee did this to me and it turned out the glass on the right side carb had popped out and was just leaning out like crasy. look for an air leak. also check your compression.
check the obvious first, is there anything wrong with the intake boots, exhaust seal?, airbox boots?
if those all clear the inspection then go into the carb and make sure its 100%.