been battling an issue with my 1976 xs500. after the bike sits i'll come out to it and gas will be on the ground under it. I believe its coming out of the over flow for the carbs. i've had the petcocks rebuilt with new parts, had the carbs gone through (just a good cleaning) and replaced needles / seats and floats with n.o.s. parts. any ideas? I was thinking maybe pressure builds in the tank and pushes gas through the carbs while sitting?

I had sticky floats that caused this to happen. Leaving the petcock open would make it pour out the overflow. When it was warm, if I had it too full it would push past the cap gasket and pour out onto the tank (which is why mine looked like shit). But after I tore the carbs apart and made sure everything was clean and moving freely, it worked perfectly. Clean them out for starters, and see where it goes from there.

Turning the petcock off (if you can) is always a good idea.

I would recheck the carbs for cleanliness and float height. The main reasons this happens is:

Too much fuel pressure which pushes the floats open.

Float height not set correctly so that the fuel bowel is filling when the needle valves should have closed the flow of fuel.

Needle valves not seating properly. This can be due to grit, fitting or badly made.

thanks for the responses. what could cause too much pressure?

I just shut the fuel off when parked. seems to be working well. thanks again.