Can anyone tell me what the spacer/insulator under the carb on a 2CV does, and why it needs to be so thick?

Is it to keep the carb cooler than it would be otherwise? If so, why is the exhaust run in such a way as to warm up the carb?

Dennis,
at a guess, the carburettor does need to be kept cool, whereas the inlet manifold needs to be heated to ensure good atomisation of the fuel mixture as it passes through the manifold.
Those conflicting requirements are resolved by using that insulator block.

Removing or blanking off the manifold preheat pipes, as done on 2CV race engines, may give a shade more power, but that's at the expense of heavier fuel consumption and an increased tendency for carburettor icing to occur.

My understanding is that it stops the carb vapour locking on hot days when the float chamber would otherwise get hot enough to evaporate its contents.

Apparently an old trick used to be to replace the spacer for a visa one which was a tad thicker, this increased the effective plenum chamber volume for a bit more low end torque.

just replaced the origional one on the engine we can,t get back into the gearbox the origional had a gasket on the top the new heat insulator one was thicker and came with 2 paper gaskets btw the origional had a bit of the paper gasket missing therefor sucking in air