Side mounted carburetors are much closer to the cylinder heads than the standard carburetor. The carburetor then becomes warmer and it can happen that the temperature rises so high that the fuel has already evaporated before it can do its job. To counteract this, this flange is mounted between the carburetor and the intake manifold, it allows the intake manifold to emit less heat to the carburetor.
These flanges are used by Volkswagen as standard in the side-mounted carburetors of 1600cc Type 3 and 1700,1800 and 2000cc Type 2 engines.
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