When I first setup the external fuel, I was frustrated by the lack of fuel pump on the Yamaha... but all is not lost. Here is what works for me, maybe it will help others.
Prep) Run generator out of fuel completely... with the factory cap, let it run until it dies.
Step 1) The bottom of the external tank needs to be above the bottom of the internal tank. A few inches is probably all that's needed, a few feet works for me. Fuel does not leak out; when the generator is stopped, the fuel flow stops, as long as your cap seals properly and your fuel hose is in good condition.
Step 2) Open outboard tank air vent, attach hose to generator cap, attach cap to generator but don't tighten all the way yet
Step 3) Prime fuel hose using bulb, until you can hear/feel the air is out, then tighten cap on generator
Step 4) Start generator as normal, enjoy uninterrupted run-time for hours / days.
2013 KZ Sportsmen Classic 200, 20 ft TT
2020 RAM 1500, 5.7 4x4, 8 speed