Well, I knew it was on reserve as I'd switched the tap not 10 miles before Problem with those tanks is there are 'wings' on either side and so at some point the petcock side runs dry and there's still gas in the other 'wing'. Tipping the bike over etc. doesn't work so I was stuck. I now carry one of those siphon thingy's so I can move gas from one side to the other. I also adopt a 'fill it whenever the opportunity presents itself' approach on rides.