Getting a bit geeky - I measure the outlet as being 4mm ID. using a conservative velocity of 1.5m/s - this would give a flowrate of around 1.13 l/m. Looking at Puma Race engine website (
http://www.pumaracing.co.uk/carb01.htm) they reccomend about 0.7 l/m/100BHP
That gives the fuel tank outlet being good for about 160BHP.