I also have N's and a 340U. If you decide to get a carb kit, the same kit is used. You will probably need carb gaskets and needle/seats.
The wet spring has been a huge problem here as well. Ethanol gas has been sucking water vapor from the atmosphere and forming an organic acid in the gas. In real bad cases, you can see a separation of gas and water/acid if you draw fuel from the bottom of tank and let the sample settle in a glass jar. This polluted gas will not fire, but may fire if the engine is started with ether of carb cleaner (my preference). The engine will have low power until you get this crappy gas completely out of the system. The water phase of this old gas is likely to have a lot of rust in it. Your floats will probably be covered by a slightly green layer which gives them a heavier weight and you lose the correct fuel level. I gently scrape the green powder off the floats and then use a fine rotary brush on my Dremel tool to clean them further. The green layer is from the acidic water leaching the copper out of the brass, and can be found on any brass in the carb like needle and seat and the main fuel flow needle and tube. It can be cleaned up enough to get the tractors running and usable, but it takes patience. The fuel bowl of the MS carb will show rust, but that can be cleaned up well enough with the Dremel tool and wire brush.
For all above reasons, I hate ethanol in gas!!
Paul in MN