Idle jet -DELLORTO (type: 9892) SI- (45-120) 2.67

The side jet of the SI carburettors does not have the simple function of a conventional jet, i.e. the pure quantity regulation of the fuel quantity. With the introduction of the PX 1977 the SI carburettor type and its side jets were modified. The old carburettor types, such as those still used in Rally engines, had a separate air duct for the side jets. These were therefore equipped with nozzles which could only regulate the amount of fuel (simply named nozzles, e.g. Rally 200 ND=50)

The new carburettor types have a closed air hole for the side jet. Since then Piaggio has been using side jets with double function, recognizable by the double designation (e.g. 55/160). For this purpose, the side jets are hollow drilled from above and thus themselves ensure the appropriate premixing of petrol with air.

D.i.e. the mixture preparation in the side jet area works in the same way as in the main jet section via an air jet and a main jet, here only combined in one part.

In the past, for easier comparison of the various nozzles with each other, a quotient was formed from the values for the air nozzle (160) and the actual nozzle size (55). The resulting value should then help to classify the nozzle in a rating system from lean to bold. Unfortunately this consideration is only partially correct as the effective flow rate of the respective nozzle cannot be linearly calculated from a simple divider as two different media gasoline (non-compressible) and air (compressible) are processed here.

A nozzle with 50/140 (2.8) has a smaller bore for the gasoline (50) compared to a 55/160 (2.9), but at the same time a smaller air nozzle (140). The smaller air nozzle greases, the smaller gasoline hole is thinner. According to this, a delivery characteristic curve can result which behaves contrary to the intended purpose, i.e. although it would be classified as richer according to the conventional evaluation system, it is minimally leaner.

Leider no general statements can be made here since the transition area between the sole working area of the secondary nozzle and the start of delivery of the main nozzle block is already dictated by the air consumption of the engine. Changed inlet control angles can already have a significant influence here. It is important to know that the range of influence of the secondary nozzle extends over almost the entire speed range. At higher engine speeds and full slide valve stroke the proportion of mixture of the secondary nozzle in the total mixture is relatively small but still measurable.

As a rough orientation the old system can be used by means of a quotient. Accordingly, the smaller value is richer, the higher is leaner. Please also note the influence of the different gas slides on the flow rate/time of the auxiliary nozzle (see slide SI)

A 55/160 ND is sufficient for almost all engines, the next fatter ND is the 52/140 then the 50/120. A swallowing of the engine when suddenly opening the throttle can often be cured by a fat side nozzle..

(US)

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