Please help with OMC stringer fuel issue

Just bought a 76' Glassmaster that has been garage kept for the last 15 years. The marine I bought it from replaced all spark plugs and did the annual maintenance on it. Told me it had bad gas (had been in tank for 15 years), and it had been drained and flushed. We took it out to the lake, after filling up both the port tank and starboard tanks... There is a switch in the back of the boat for the gas tanks, line running from the starboard side a line in the middle, and a line running from the port side. When we have the fuel line switched to starboard, the boat runs like a champ.. however, when switching it over to the port side, the boat dies after a few seconds. It seems to not be getting fuel from the port side. I have wondered if the switch for the fuel lines is incorrect. The switch diagram states that when the switch is to the RIGHT, which SHOULD be the port side, the fuel is OFF. When the switch is on the left side, it's of course, the starboard side. The switch directly up, which seems should be the OFF side, is the port side. Does this make sense to anyone? It's difficult to explain.

Hello, is the switch you are talking about a manual valve type? If so can you disconnect it and remove it? If you can remove it I would and then use compressed air throttled down to low psi and test the position of the valve. As a last resort remove the hose going to the tank with the issues and see if you can blow on it...if it has alot of resistance you might have a clog in that line. Hope this helps!