I don't think you need a carb cleaner with SUs, they don't have any ports in them that are small enough to clog up. I would, take the bung out of the bottom of the tank & see what water & dirt or rust comes out the bottom, if the fuel looks clean as it comes out, put the bung back in & use it, but use as much as possible before getting a fresh tank. The fuel tank drain bung on carb cars has a 7/16"AF head & is tapered pipe thread, so it's very easy to put back in while fuel is coming out of it.