HOnda 2000 extended gas tank

I bought aluminum caps for my twins several years ago, and have had no problems at all. If you are seeing bubbles, you might have a vacuum leak. It doesn't take much to lose the vacuum, then it will not draw fuel. Also, the more full the Honda tank is, the sooner it will start drawing gas from the auxiliary tank. There is less room to build vacuum with a full tank.

I bought the caps, then a plastic boat gas tank with fittings like yours, put a tee on the tank so I can run one or both twins on the tank and it works flawlessly.