In December last year, I used a pressure washer to wash my van.
After washing, my van developed a profound misfire and shortly after that stopped running all together. Shortly before Christmas I had the van towed to a garage and it has remained there ever since. During the last four months the van has been fitted with new electrics (coil, distributor cap, plugs, leads etc.) and the carburetor has been stripped, cleaned and rebuilt. The fuel lines have been checked and in-line filter/s fitted.

The van will now start on the choke, but refuses to tick over. Once the choke is closed the engine dies. The engine also cuts out under load, i.e. when the engine is running with the clutch in and first gear engaged, the engine is unable to develop sufficient revs to allow the clutch to be let out for the van to move off. In other words, the engine dies under load.

My mechanic is completely stumped by this (as am I) but is determined to get to the bottom of it so any suggestions will be much appreciated.

Brian.

Earth strap?

If it dies as soon as the choke flap is opened it's a sure sign of a carburetter fuel blockage. I don't know the details of the carb fitted to your van but in general, in a bog-standard multi-jet carb, closing the choke creates enough suction to draw fuel through the main jet. Idling and slow running is handled by a separate small jet or maybe two jets. The main jet (sometimes called the power jet) doesn't normally come into play until the throttle is nearly fully open. Although you say the carb has been stripped and cleaned I would nevertheless suspect that the smaller jet/jets, or the body drilling(s) to it/them, has been blocked and was not cleared when the carb was stripped.

It's been quite a while since I've worked on a carb but I would get a can of brake & carb cleaner, remove the jets and poke the tube down the jet drillings to make sure they are clean and that cleaner flows back into the float bowl. Also use the tube to spray cleaner through the jets, blow them dry and hold them up to the light to check they're clear.

Assuming all that work has been done properly I would suspect a slipped timing belt.