Piaggio Sfera - Carb Problem?

[CaptainJag]

Will try to describe in more detail. The scooter starts with some persistence. However it only runs for about 10 seconds max. When it does run it smokes very much. (Note - I have just replaced the piston rings) After 10 seconds it stalls. I realised that when I turn the throttle the fuel valve stops supplying fuel. This machine has a vacuum type fuel tap. Without turning the throttle, the valve works fine pushing a good amount of fuel out of the tank. As I turn the throttle no fuel is discharges from the tank. This is what I think causes my scooter to stall. I tried to take out the black pip from the fuel tank. When i crank the engine without turning the throttle i feel a good ammount of vaccume. as i turn the throttle it get very very little.

However now I need to know how to solve this problem. Is there something wrong with my Carb? I have tried to clean it several times, have also made sure it has no air leaks and have assembled it with a film of silicon on the joint to keep it air tight. The jets are clean the whole carb seems very clean.

Could it be a problem with the choke?

Thanks