I was just doing routine maintenance on my BF2D Honda (16-years old).  I noticed that the forward side of the fuel tank was "sucked in" around a half inch.  This was early this morning and it was around 70F, and not hot, so when I opened the cap, it popped out a little bit.  I put a big screw driver in the tank and could push most of it back to be rounded out, instead of pushed in.

It sure looks like it is going in and out, like a bladder would, with changes in temperature.

Do they all do that?  Has anybody else seen that?  I'm not sure I ever looked for it before!  Bob