On Jul 12, 2022, at 11:30 AM, Jove Lachman-Curl <jovelc87@...> wrote:



I've had this happen with plastic gas cans in my garage, if they sit for a few months mostly empty they suck in and create a vacuum.
My first thought is that the fuel is oxidizing and consuming the oxygen to create a vacuum.... We've all done the water into a glass with a match trick right?

My second thought is that it's breathing from hot to cool and creating a vacuum like a canning jar does when it cools....... heats up, forces out the gasoline vapour, cools and creates a vacuum. The cap acts like a one way flap valve.
Solutions could be leaving the tank full during storage. OR leaving the cap cracked open a little.

Maybe someone knows which answer is correct.

-Jove

On Tue, Jul 12, 2022 at 11:02 AM Bob Larkin <bob@...> wrote:

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