The fuel supply of the Vespa models with SI carburettor type is always a bit difficult. Due to the very small difference in height between the fuel level in the tank and in the float chamber of the carburettor, there can always be an undersupply of fuel. Especially when the tank content is approaching the reserve, the pressure difference is often so small that this is noticeable in a lean engine. Beside the usual suspicious like a functioning tank ventilation also the fuel tap should not be blocked and the fuel hose length should be as short as possible without kinking the hose. To prevent the latter we use fuel hoses with a wire inlay, this effectively prevents the hose from kinking. Another measure to optimize the fuel flow are the float chamber covers of the Vespa Cosa models. These have a larger float needle seat with a larger flow hole. In addition, the float needle is equipped with a spring to prevent excessive pressure on the sensitive rubberized needle tip. This way the valve stays tight much longer because the pressure is not only distributed over a larger area but is also cushioned.
The drilling of the fuel channel in the carburettor between the float chamber and the nozzle block has proven to be a recommended measure. Here the drilling on approx. 1.8mm extend to guarantee always a sufficient flow.
The float chamber cover is available from DRT, as well as from Dell'Orto or Pinasco. All of them are constructed in the same way, but cost differently. The DRT kit contains a float needle which has to be added to the Dell'Orto lid.
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