Vespa Si Carburettor Jet Dellorto 4576 Main Jet - Push In

Fits SI Carb etc.- push in type jet

Ranges intermittently from 72 to 155

Please select size you require

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.

If the HmainLairKcorrectionDnozzle is dimensioned the pre-mixture becomes too rich or too lean. If the HLKD is e.g. very large (190), the premix receives a high proportion of air whereby a disproportionately large main nozzle is required so that the engine runs properly even under full load. However, the result is often a mixture that is too rich under partial load and the scooter bucks. Nevertheless it can die off under full load in the upper speed range because the fuel level drops below the delivery height of the exhaust tube and the fuel supply is cut off. Often this phenomenon is tried to cure by mounting a so-called float chamber spacer. It is assumed that the float chamber will be sucked empty and therefore the engine dies off.

Normally a change of the air jet is more effective because the spacers increase the filling level in the carburettor, but they also provide a higher level in the nozzle block area, so that the premix can no longer work properly. Under certain circumstances gasoline can then also run (flood) from the atomizer directly into the engine. The real problem is therefore not a too small volume of the float chamber but a tuning of the HVACD.

A frequent problem which causes very large main nozzles is a modification of the air filter or the replacement by an intake funnel. In the original filter, a hole is often drilled into the filter bottom above the nozzles to allow a better flow to the HVAC. This measure usually improves the pre-foaming, but with heavily modified engines it can lead to a too low negative pressure in the nozzle block. As a result, the fuel level drops below the threshold of the atomizer/petrol outlet tube and the engine is only insufficiently supplied with mixture. Apart from a too lean full load operation, which usually no larger main nozzle can catch, it can come in extreme cases also to a complete stop of the gasoline delivery.

No more extreme is this problem when using air funnels. Here, with a used HVAC of 160, main nozzles are often used far beyond size 150. Here, too, problems often occur in the partial load range because the system of fuel delivery and premixing is out of balance. An adapted, smaller main air correction nozzle also helps to bring the main nozzle requirement back into an appropriate range.

TIP
The main nozzle for a new tuning of the engine is always as large as to throttle off under full throttle (four-stroke). Starting from this, reduce the main nozzle until the engine takes full throttle and turns off without jerking. A swallowing at throttle acceptance or a four-stroke in the partial load range is then carried out afterwards by an appropriate selection of the secondary nozzle, gas slide and possibly another mixing tube.

Automatic Jets*