my dads nc30 is firing on only 3 cylinders, its had new spark plugs and he's checked the coils. is there anything else that could be the problem?

carbs blocked?
which cylinder is it?

can you swap a working plug into the non firing cylinder? that would eliminate a lot...

yeah he's swapped plugs round and taken one from mine which i know works and it still isnt working. its the rear left side cylinder, and we are checking the carbs now