My 49 had fuel problems from the tank all the way through the carb. I had to drain the tank and have it boiled and coated. You can check other threads about the different ways to this yourself or have it done. I chose to have it done at the local radiator shop. On my 49' there is a small valve on the bottom of the tank that has a very small orifice that is easily clogged. Again this is on a 49' I dont know how it differs from a 51'. Always start at the beginning I say. I'd plan on a carburetor rebuild as well. I added a little in line filter until I knew I had clean fuel.
Good luck