1972 T100R Gas Tank Issue

[saig55]

The bike runs good until it runs out of gas, with gas in the tank. Cleaned the carbs, drained the tank again and cleaned it again. Same problem…
This time I pulled the tank and sat it up level and opened the petcocks… gas drained perfectly and then didn’t, then after a few seconds started to drain at a slower rate. I loosened the cap and everything worked great as normal. What am I doing wrong here? Is the cap bad? Do I need to loosen the cap when I ride?
I just don’t get it….

 

[Greyfell]

Is the cap bad?

Yes. There is an air vent hole in the cap to let air into the tank as the fuel is drawn out of the tank (pressures have to be equal). You might be able to clean the hole, but if that doesn't work it's time for a new cap.

 

[saig55]

That makes sense. I couldn’t see the vent hole because of the flaking of the chrome off the cap.
Just ordered a new one. Thanks