My bet would be carbs in need of a good sync, though it could be a number of things. Valves in need of adjustment, partially clogged fuel line, bad petcock diaphragm, fouled plugs. I would personally start by pulling the carbs off/apart for a good cleaning, check float height (should be 17mm), and bench sync them. I would also drain the tank and GENTLY pull the petcock apart to check for tears in the diaphragm. GOOD LUCK