Fuelling issues

[Slartibartfast]

Hi all

I have a question for the collective wisdom of the forum with respect to the fuelling on my bike.

Basically what is happening is that the only way my bike will run is with the fuel tap to prime. It seems to run pretty well once its warm and pulls well through the gears returning about 45 mpg. Its a bit grumpy until its warm but fine after.

If I put it to either normal or reserve, after a quarter of a mile or so you can hear the engine note change and you lose power (opening the throttle just makes more noise) and as soon as you roll off the throttle it dies completely.

Its just had a tank of super unleaded fuel and a redex fuel treatment which made no difference.

What difference does the fuel tap make from normal/reserve to prime? I wasnt expecting it to even have a fuel pump as I thought the carbs would use vacuum to draw fuel? does the prime position just bypass the pump, in which case do you think it could be the fuel pump just not working? I've tried the fuel tap in normal and reserve and its the same in both positions.

The bike had been laid up for the last 7 years, to be honest I'd initially put it down to that and hoped that some proper use would sort it out but its not changed and I've put 500 miles or so on it since then.

Any thoughts?