Modern Vespa

Hi, my skipper engine runs well but cuts out , like its run out of petrol at obout 45 mph, I stop ...choke on and it starts again..it is standard .. i thought it was old carb,renewed with dellorto and it does the same, I removed inline filter to see if this worked and it didnt...any ideas? It runs well at lower speeds??

Does it have a vacuum operated fuel cock just under the fuel tank? That could be malfunctioning.

No its just connected via fuel pipe , basic 2 stroke auto

Ok......just remembered there was a thin pipe connected to old carb , didnt think it was doing anything ...is this vacuum assisted ?.....I should mention this is a Lambreta auto conversion...still standard engine but basic tank with tap at bottom with just fuel line to carb..