Seal the vacuum hose to the carburetor with a ball or similar so that the engine does not lose weight due to air leakage. The advantage of the manual operation is a much lower susceptibility to defects (torn diaphragm). Therefore this variant is also used in racing.
If the float needle valve is intact, the cock can also remain open permanently without any problems.
Fits almost all plastic tanks with a vacuum petrol tap, e.g. Aprilia, Piaggio, Derbi, Gilera etc.
(US)