Thanks to all who sent suggestions. The important clue to the problem was
the fact that after the engine was turned off, the rear carb, (and to a
lesser extent the front one), would send a little fountain of fuel up
through the jet. This obviously indicated a pressure build up in the float
chamber. Since there are only two places for air/fuel to escape from the
chamber, the jet and the overflow pipe, and the jet was not blocked, the
only remaing candidate was a blocked overflow pipe. Sure enough when I
disassembled the carb and put some compressed air through the overflow, I
blew out a lump of crud, (looked like paper).
So reconstructing what was happening: after shutting off the engine the
pressurized fuel flooded up through the jet. If a restart was attempted
before the fuel evaporated the engine was hard to start due to the gobs of
fuel sitting in the intake. Also the pressure build up in the bowl prevented
any more fuel from being admitted, hence the problem with no further fuel
being drawn through the carb.
So I had the rather interesting situation where a problem caused the
carb to both flood the engine and also to starve it for fuel.
Cheers,
Simon
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