Crap in the tank ?
Has it had a new Fuel Filler pipe, or is the old one rusty ?
Flushing the tank and fitting a new fuel filter might be the solution.
What is the state of the circular plate in the metering head that lifts up with increased air flow ?
Does it lift the lever it is attached to easily, the pivot might need a drop of lube.
Is it cental to the cone of metal it sits in, there is some pre-adjustment by a little bracket underneath that the plate sits on at rest, there should be the tiniest of gaps all the way round, about the width of a paper business card.
If you have a Haynes, it details a procedure for checking the fuel delivery system, it involves removing the injectors (they just pull out) and checking the fuel mist pattern etc.