wilky

hello to all im new to all this i have recently bought a r reg thundercet not run for two years done all that needed doing but its only firing on 1 2 4 it was firing on 3 4 at first but after blasting air through carbs it now runs on 1 2 4 i have spark on three and carbs have all been cleaned so theres no reason why fuel shouldnt be getting through any ideas