Yeah I've opened the tank before, not the easiest but I have it open again. Which one of the two lines is the overflow line? I see them both but neither seems to be corroded.
I was thinking something along those lines but I wasn't totally sure. And thanks for the picture! It really helps me visualize how I'm Going to tackle this project. Now, I'm going to go run her out of gas (Good excuse to go for a ride :D) and I'll let you know what it looks like once I get in...
I am experiencing this exact problem, except I am able to hold about half a tank. I thought it could be the same cause that you thought, the corroded overflow line inside the tank. Did you ever solve the issue?
I recently bought my 05 for just $1500 at 5k miles. It had sat for a year or two, which is why I got the price. FZ6 wasn't my first option, but looking back I would 100% buy it again.