Vespa Main Jet 90 - 138
-BGM & SIP-
(type: 4576) SI-
Description
The main jet of the SI carburettor determines how much petrol is made available to the so-called premix or pre-foaming. The higher the capacity and the higher the litre capacity, the greater the need for spraying. The available range is from 90-160. 102 main jet has a bore of 1.02mm, and thus provides less fuel than 104 main jet (1.04mm bore). That means the bigger the nozzle is numbered the richer the mixture becomes.
function nozzle block SI
The main nozzle should not be considered isolated, because it is only responsible to keep the fuel level in the nozzle block area on a sufficient level. Due to the vacuum at the atomizer tube the fuel is sucked in from the nozzle block. The fuel flowing into the nozzle block from below is limited to a maximum by the main nozzle. Air is also sucked in from above, whereby a fuel/air mixture is already formed inside the carburettor (mixing tube). This pre-mixture lets the mixture burn more effectively since it can be atomized more finely than pure gasoline.
