[Rubbond]

I can't follow this reasoning at all, Bellerophon.
Massive heat sinks? Glowing manifolds? (Do you mean inlet?).
If the engine is running well normally, and the car has a manual advance and
retard, I wouldn't have thought the fuel quality could be a major factor
contributing to overheating?
Fuel droplets travelling through the inlet manifold will have a cooling
effect as they evaporate.
When the engine is switched off the carburettors however obviously shouldn't
get so hot that you can't touch them.
I would have thought it much more likely to be a cooling problem.
The water temperature you see on the dashboard gauge is only the temperature
where the sensor is located.
Maybe some channels in the head are blocked (or even the head gasket is
missing some holes!) and there is no circulation at all near the
carburettors.
I have messed around with very old outboard engines, and overheating is
ALWAYS caused by blocked channels, some of which are almost impossible to
find.
Good luck!
Rubbond.