BGM PRO Dellorto Si 24/24G Carburettor Main Jet Set Pack (120-180)

BGM PRO Dellorto Carburettor Main Jet Set Pack (120-180)
Main Jet 120-130-140-150-160-170-180

Air jet set specific for the Dell'Orto SI24/24G carburettors (e.g. Vespa T5). The carburettors specified with the suffix 'G' have a lower overall height than those with the suffix 'E' or 'D'. The
'G' carburettor types also
use
shorter side and
air jets, and the T5 models are almost exclusively equipped with a size 120 air jet. Other sizes are no longer available from Piaggio/Dell'Orto. The BGM PRO set fills this gap and offers with the sizes 120, 130, 140, 150, 160, 170 and 180 a very large range for the individual tuning of the carburettor.

FUNCTION AIR
JET One main jet has a linear delivery characteristic. This means that the main jet would overlubricate the mixture more and more with increasing speed. This is
exactly where the air jet (exactly: main air correction jet)
comes in.
This eases the mixture with increasing speed again. The bigger the air nozzle the more it reduces the mixture.
The available range for SI carburettors is from 120-190. 120 air jets have a bore of only 1.2mm, so they provide very little air and are accordingly greasing. A 190 air jet provides a lot of air and is therefore very lean.

FUNCTION Nozzle block SI
The working range of the main nozzle block starts at approx. 3500 rpm. Before that the engine is only operated via the auxiliary nozzle (depending on speed and slide opening). In the nozzle block area of the SI carburettor there is always a defined level of fuel (depending on the level in the float chamber). I.e. below the atomizer tube (which opens into the intake area of the carburetor) there is a petrol column. For this reason we also advise against the so-called float chamber spacers. These increase the filling level in the carburettor, but they also provide a higher level in the nozzle block area, which means that the premix can no longer work properly and in some circumstances petrol may run directly from the atomiser into the engine (flood). The actual problem is also not a too small volume of the float chamber but a e tuning via the air nozzle (see below).

From the very bottom, the main nozzle defines the amount of fuel that runs after it. In the middle is half in the fuel, half in the air, the so-called mixing tube (e.g. BE3). From above the air nozzle defines the amount of air flowing in.

If the air nozzle is too large (e.g. 190), no sufficient negative pressure can be created and the fuel supply under full load will become dangerously lean or break down completely (often confused with sucking the float chamber empty). In this case even increasing the main nozzle will only help to a limited extent or not at all.

Often this phenomenon also occurs with modified series air filters, where the subsequently drilled hole above the air nozzle is too large for the engine's requirements (e.g. 8mm).

The same happens when using air funnels from Polini or Pinasco, which are mounted instead of the air filter. In order to compensate for these modifications, not only the main nozzle has to be dimensioned considerably larger, but also a smaller air nozzle has to be used (e.g. 140 instead of 160).

TI
P Near-series engines with series filters can usually always manage with a 160 air nozzle. Tame tuning variants such as a Pinasco 213 can also be operated with the lean 190/BE4 combination (orig. PX200 10HP variant) (but then never combine with a modified air filter). An engine tuning with a lot of power may also need an air nozzle with only 140 or smaller to avoid a lean one under full load. An exception is the 125cc T5 engine which already uses a carburettor with a Ø24mm cross-section ex factory and uses an air correction jet of 120 for this purpose.