[Nasher]

Good point, perhaps the vent tubes have been joined together, kinked or similar.

Strangely I had a neighbour round only a few weeks ago who'd put a new/refurbished pair of carbs on his 1936 Riley Kestrel. I normally do a lot of that sort of thing for him, but the specialist who'd supplied them assured him they just needed bolting on and possibly the mixture adjusting slightly. He'd planned on getting me to adjust them and the timing once he'd got it started.
The 1.5l 4cyl just would not start, so I went and had a look and found completely dry float bowls despite the fuel pump obviously pumping fuel.

I found the 'Brand New' float valves full of a sticky jelly type substance. It started fine after we'd cleaned them out.

Nasher.