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If the air temperature you are experiencing is unusual for your region, the fuel you get may be formulated for colder conditions and be too volatile-- it boils too easily now in your 80 deg.F.
The 444 and 544 models seem to be built in a way that traps hot air high in the engine bay like in a hot-air balloon. There is lots of hot cast iron manifold below the carburetors. You might try opening the hood (bonnet) part way when stopped, to let the hot air escape upward and let the carburetors cool off.
If too much fuel in the manifold is typical of vapor lock after a "hot soak" happened, then it may be due to thermal expansion of fuel trapped between fuel pump check valves and carb float valves, and leaking past the float valves into the carbs, filling them too full. If there were a small passage to drain off this expansion pressure to the tank side of the fuel pump, forced fuel drainage into the carbs might be prevented. The least fuel volume in this part of the plumbing, the better: no filter and small pipes.
Fuel boiling in the float bowls will push liquid fuel into the carbs and manifold if the vent provision does not let the pressure escape.
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