needing help! engine dies after 5 mins of driving

hi, so this is the issue on my 1.8 weber carb 1991 cabby.

turn the car on and it fires up fine, i then proceed to drive for 5-10 minutes before what can only be described as a complete loss of engine power occurs, the longer i leave my foot on the accelerator the closer the car gets to cutting out. however if i put my foot to the floor so i go passed the progressive valve the car drives like it should.

so jobs i have done to find the fault:
removed the fuel sender in tank and checked for rust or particles in tank… none there, no blockage of pipes in sender unit.
blown through fuel return and pressure lines to check for blockage… none there
pressurised fuel and return lines to check for air leaks… none found
checked fuel pressure at pump and at inlet to carb… fuel pressure ok
removed exhaust from down pipe to check for blockage and potential choke of engine… no blockage found.
fuel return line blocked off… no change to fault
carb, removed, cleaned, serviced, put back together (this was done by a specialist)… no fault found

Problem lives on!

can anyone help me with ideas of what is causing the issue? i have absolutely no idea anymore thanks in advance

So for those who are interested in what the fault was; 
It turns out the distributor was a tooth out, causing the inlet manifold to get hot and therefore evaporating the fuel in the inlet prior to getting to the piston. Strange how it took 6 months to come about but then again these cars have minds of their own! 
So on the rolling road at the flywheel my BHP has gone from 65 to 95 it's like a different car! Fallen in love all over again! 

The inlet manifold is meant to be hot enough to vaporise the fuel  though that's why they are heated with engine coolant.

Correct, however this was so hot it boiled spit on it. The valves wernt seated correctly at the point of the spark allowing the mixture to burn inside the inlet, heating it to a very high temp.

If it was that far advanced then you should have noticed symptoms other than dieing after 5 mins. As you say you were down on power close to 40%. You also should have had difficulty starting and back fires from the carb not to mention pinking when driving.

The specialist who removed and serviced the carb should have been able to spot the above if no one else.

very true, just glad it was fixed!