Today I had the fuel tap set over to primary and knew it was low but thought I would see how far it went before I had to change over to reserve. As I rode along it started coughing and spluttering so I changed over to reserve. I would have made the change within 5 seconds of it begining to splutter however it still continued to splutter after changing and then when I pulled over it stalled. I thought that maybe I did not get it to reserve in time so I tried to start the engine again but ti still stalled. Tried a few more times with the same result but no luck. My darling wife brought me out some fuel which got it going again. Any ideas as to why their was no fuel in reserve.