If your classic is having difficulty drawing fuel, it can manifest in symptoms from not starting to regular cutting out particularly under load as fuel runs out in the carburettor float. If you suspect this to be the case, there are a few things you can check.
Flakes of rust or even a disintegrated gauze filter can be sucked up the pipe, causing a blockage at a corner or kink. If you suspect the blockage is in the main supply pipe from tank to the engine compartment, renew it – it’s likely to be corroded. If the riser pipe inside the tank is the problem, this can be more tricky. Some simply unscrew or unclip from the top of the tank, but others are soldered in position. In the latter case, the tank will need specialist attention and it may be cheaper to simply obtain another one. There’s usually some method of getting at the pickup pipe’s gaze filter – often the drain plug is situated directly below the end of the pipe. This may provide a point of access for probing with wire or an air-line. Incidentally, also check that a jack hasn’t been placed under the front-to-back metal pipe, crushing it flat.