Page "Carburetor" Paragraph 59

Usually, special vent tubes allow air to escape from the chamber as it fills or enter as it empties, maintaining atmospheric pressure within the float chamber ; these usually extend into the carburetor throat.

Placement of these vent tubes can be somewhat critical to prevent fuel from sloshing out of them into the carburetor, and sometimes they are modified with longer tubing.

Note that this leaves the fuel at atmospheric pressure, and therefore it cannot travel into a throat which has been pressurized by a supercharger mounted upstream ; in such cases, the entire carburetor must be contained in an airtight pressurized box to operate.

This is not necessary in installations where the carburetor is mounted upstream of the supercharger, which is for this reason the more frequent system.

However, this results in the supercharger being filled with compressed fuel / air mixture, with a strong tendency to explode should the engine backfire ; this type of explosion is frequently seen in drag races, which for safety reasons now incorporate pressure releasing blow-off plates on the intake manifold, breakaway bolts holding the supercharger to the manifold, and shrapnel-catching ballistic nylon blankets surrounding the superchargers.