From the experiences ive had with carburated bikes, it sounds to me like you have a varnished or rusty gas tank. It clogs up the bowls in your carbs, fuel lines, and everythings in between. The reason it runs well after it sits for so long is because the fuel has time to slowly fill everything up. shine a very bright LED flashlight into your tank and check for rust. If there is any significant amount of rust on the walls and/or floating in the gas then you must remove the tank and coat it. There are various products around that usually involve a 3 step process to achieve this goal. After the tank is 110% clean, dry, and coated then you should rebuild or have your carbs rebuilt.