My 331 was having an issue where Stb fuel gauge stuck at “full” and the Port gauge kept on going down even when the fuel tank lever (under the steps to the stern cabin) was permanently over to the Stb tank.
The reason (I believe) was that fuel was cross-draining from the Port tank into the Stb tank, via the Generator fuel pipework, either when the boat sailed on a Port tack, or just due to some kind of pressure.
The solution, (and thanks to Piers for suggesting this), was to close one of the two valves located each side the generator fuel filter on the pipe between the generator and the two fuel tanks. In accordance with the note on the valve, I closed the valve to the Port tank, leaving the valve to the Stb tank open.
Enclosed is a detailed diagram of the 331 fuel system (thanks to my friend Toby who was an RN engineer). The valve in question is next to the fuel filter marked “B” and it is located at the stern, Stb, upper side of the engine bay.
If you would like to discuss this please phone me 07817845775