Fuel bubbling

[johnb9]

This past summer while traveling on a very hot day on my 82 GS 650 (24k on it) I filled up the gas tank and then went into the store for water. When I returned the gas was seeping out of the top of the tank around the cap. I opened the cap and the gas was welling up slowly like water out of a natural spring. The bike was on the kick stand so it was tipped to the side. I put the petcock to the main position from reserve and the gas bubbled or belched and the level went down an inch and stopped leaking. Since then I have filled it up less and haven't had that problem again.

The leaking gas emulsified the paint which already had streaks on it from spills from the previous owner. I would like to eventually repainit the tank but I wonder if this problem will ocntinue and damage the paint again.

The petcock is not new as far as I know (I bought the bike in 5/08). Otherwise, the bike performs well after the carbs were professinally cleaned in June. Thanks for any thoughts on this.

[bwringer]

This is perfectly normal -- the gas comes from a cool underground tank, then expands when it's sitting in a tank in the sun atop a hot engine.

The gas wasn't boiling or leaking -- it simply expands when warmed like any other substance.

The solution is simply to not fill it as full.

[Larry D]

Never fill your tank completely. Gas expands when warm. Not an uncommon occurance. No more than gas expanding and had nowhere to go but out of the vented cap.

[physics-teacher]

If you have quality paint the occasional overfill and spill onto your paint won't damage the paint if wiped off promptly. Don't ask how I know this.

[psyguy]

it is a worry that the petrol level went down an inch when you set the fuel tap on a different position!
it means it probably leaked into the cylinders/engine and got mixed into the engine oil

a new fuel tap and an oil change may be in order