The SI carburettor with its low overall dimensions, good parts availability and short intake travel is a favourite on the Vespa engines. To increase the power output even more, Polini has developed a funnel top. The funnel, with a larger air intake surface at the inlet, allows a higher flow rate and therefore a higher power output. In order to avoid increased wear, the foam filter is mounted between the carburettor sump and the frame instead of the rubber knee. Through the funnel the inflow behaviour as well as the premix changes massively. Therefore the carburettor must be retuned. Various main jet sets and other tuning parts are available for this purpose. A fresh float needle, new gaskets and possibly a COSA float chamber together with an extended bore between float chamber and nozzle set make sense for an adequate fuel supply.
Note: It is important that the suction adapter for the foam filter is fitted, even if the set is driven to the frame via a suction hose. The adapter provides with its passage taper a defined negative pressure without which the funnel is otherwise difficult to tune under full load.
For engines with a very high power output, please note that the main nozzle area can move in size 150+. Here our BGM nozzle set with main nozzles up to 162 is highly recommended. Please also note that a reduction of the main air correction nozzle is considered accordingly. Otherwise overgreased by the oversized main nozzles the middle speed range.
Please note: Secure the fastening screws of the funnel with screw adhesive!
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