very easy to clean the carbs.. you have choices.. but some fuel addative which will go through the carbs and clean them, failing that you can remove the carbs, bit of a task but easy enough for any home mechanic with simple tools.. carb cleaner or compressed air will do the trick once dismantled.
If it had been stood have to replaced the plugs and oil?? Also would suggest removing the ht caps of the plugs and spraying wd40 into the caps.. let them sit for 10 mins, then reconnect(dont bother drying them out) just let the wd40 soak in. That may also cure the problem as the ht lead plugs oxidise..