I have a 91 GS500, my question is, how is the gas tank vented? When I open the gas tank I hear a WHOOSH sound which leads me to believe the tank isn't venting properly. The only hose exiting the tank is the over flow hose at the bottom of the tank. The tank does have a Pingle fuel valve, stock fuel valve and hoses have has been eliminated. It has one fuel line with a  "T"
going tp a pair of Keihin CR special round slide carbs.

Hi there,
I have a 1991 GS500E, too, and have taken my fuel cap off the tank (The allen head bolts hold it on -- if I recall, three are real bolts and the other three are decorative) to rebuild it.
There is a small ball bearing and a spring that are used to vent the cap. If they get dirty and clogged, the venting is harmed. So take it off and carefully disassemble it (the parts are small and easily lost, so be careful and do it over a cloth so they don't roll away), then put it back together. I may have had to use some light grease to hold the ball bearing in place upon reassembly. It's been a while so I'm not sure how I did it.
Good luck and let us know if that helps.
yours,
Todd

Todd: Thanks  a great deal! Just returned from my little shop and have completed removing the cap, disassembling and re-assembling it. Yes, the grease helped retain the tiny spring and check valve ball and also all those tiny springs on the cap. Now I can blow air through the two holes in the cap, so, I'm sure it's venting. Again, thanks!
Wally