Bike will only run when fuel tap is in the prime position, plenty of fuel in tank, anyone any ideas why?

1. An intro in the New Members Area would go a long way to getting help - would you just walk into a club meeting in a pub and start asking questions without introducing yourself?

2. Vacuum - either the vacuum hose isn't sucking properly (Leaky? Missing?) or the tap internals are goosed. Switch the tap on and apply some suction to the take-off point and see what happens.

Now, about that intro ......