In many marine motor installations, the engine is often set at some angle from the horizontal. This is quite unlike, for example, an automotive installation where the engine is basically level most of the time.
Since gasoline engine carburetors are intended for level operation, the tilt shim is used to bring the carburetor back to its correct plane of operation. Tilt shims are available for many different angles depending on the installation angle. They are also available for various carburetors and number of barrels or throats. If the engine is mounted horizontally, a tilt shim is not necessary.