M_Boats: nissan outboards

HELP, I have a 4hp Nissan OB on my M-17 "Little Breeze". The engine has both and internal fuel tank as well as provisions for an external tank. I recently added an external 3 gallon tank. I cannot figure out the correct position of the fuel cock on the port side of the engine when using the external tank. Nissan's manual is unclear....at least to me. I have run out of fuel in the internal tank twice now for lack of the correct position of the fuel cock.....not fun when you are docking. Any ideas out there...thanks. Arnold on M-17 "Little Breeze" on the river in Portland Oregon.