The self-supporting frame of the classic hand-operated Vespa models has always been used to accommodate the wiring harness and Bowden cables. The engine also draws in fresh air through the hollow frame interior.
To prevent the frame from acting as a resonance body, the front part of the frame is separated by this plastic part. This reduces the intake noise considerably, especially if the motor has been converted to diaphragm operation.
To further reduce the intake noise, the frame hole can be extended inwards with a pipe in the area under the seat where the fresh air is drawn in. This will also break the sound without sacrificing engine performance.
NOTE: The cover should always be fitted. Otherwise, the engine will not be able to accept the gas properly after full load phases. If the engine is swallowed despite meticulous tuning work on the carburettor, the lack of this simple plastic part is very often the cause.
A simple test is to close the frame tunnel behind the fork (under the cascade) with a cloth cloth. This saves the removal of the fuel tank for testing.
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