This happened the twice in the last week before I realized what was happening. I commute to work, top off my gas tank across the street at 7:30am (cool gasoline), then the bike sits out in the blazing sun (80-90 degrees) all day. By the afternoon, fuel expanded and came out of the vent overflow. This part I understand. I'll get gas after work from now on.

The weird part - it a dark, tar like stain on the ground. Gas usually evaporates without leaving much residue. What's on the ground looks more like oil than fuel, but it's dry to the touch. I checked the bike, no evidence of motor or primary oil leaking.

Any ideas? The stains are covered with oil dry, didn't get to take a pic. It's definitely from my bike. EFI, no carb.

 

You're not saying what the "ground" is. I'm assuming asphalt. Both Asphalt and gasoline are petroleum products. You may be seeing the by product of some reaction between the asphalt and the spilled gasoline. Perhaps the gas acted as a solvent, partially dissolving the asphalt.


Otherwise, not being a petroleum chemist, I haven't a clue.

 

Wow, you guys are good. I was just talking to our Maintenance guy at work about it. The ground is tar & chip, and he says the fuel is reacting with that. Mystery solved.

 

your bike should still be not leaking gas.

 

If you fill the tank with cold gas and leave it out in the hot sun, the fuel is going to vent out the overflow, it's got to go somewhere. That's why the event is their. But like I said, I won't be feeling as in the morning anymore.

 

Or stop over-filling the gas tank. There should be an inch of air space in there.

 

I'm filling the tank same as I always do. It's never been an issue until 3 specific things together: fill up across street from work; bike sits out in very hot sun all day; bike is parked on slight incline (fuel tipped even closer to vent).


In any case, the main question I had was what is on the ground, and you helped me solve that, so thank you.

 

It's either that or blinker fluid!

 

This^^^. It happened to me last year and now I make sure to put the gas nozzle into the tank about an inch to inch and a half. Once the pump auto kicks off do not top it off. This will leave plenty of room for fuel expansion.

And as for the stain on the ground, I still have a spot on my asphalt driveway from where it overflowed last year.