05/10/2026
🇬🇧 BE1 to BE6 – What the Numbers Tell Us About Their History
Anyone who spends time looking into the classic SI atomiser tubes will sooner or later come across the designations BE1, BE2, BE3, BE4, BE5 and BE6. This quickly raises a question: where does this numerical sequence actually come from when the atomiser tubes differ so significantly in their geometry?
The answer lies less in any technical ranking than in their development history.
At the beginning, two distinct lines can initially be identified. On the smaller 125 and 150 cc Largeframe engines and the smaller SI carburettors – such as the SI 20/15 and 20/17 – we encounter the E1 early on. This line remains in use for many years and can still be found on the later Sprint with the SI 20/17.
In parallel, the designation E2 has also been passed down from this early period. A corresponding relationship between E2 and the later BE2 appears plausible on the basis of the nomenclature, but so far it cannot be traced as clearly as the relationship between E1 and BE1: reliable geometrical data for E2 is lacking. For this reason, E2 is shown in brackets in the later historical numerical sequence.
On the larger engines and carburettors, we find the atomiser tube already designated BE2, for example in combination with the large SI 27/23. In simplified terms, this initially places the smaller engine and smaller carburettor with E1 alongside the larger engine and larger carburettor with BE2.
However, something interesting already happens within the early small-carburettor family: the geometrically closely related BE1 emerges from the E1 environment. The basic three-level structure is retained, but the drillings within this structure are altered. This reveals, very early on, a principle that we will encounter again later: not every further development requires an entirely new level geometry – different atomiser tube variants can also be created within the same basic structure.
With the larger engines, a different development step follows. Within the GS 160 series, Piaggio switched to BE3 during production. This marks the first appearance of BE3 as the next distinct atomiser tube geometry – not with an entirely new model generation, but already during the development of an existing series.
The Rally 180 subsequently provides a particularly interesting intersection. With 180 cc, its engine clearly belongs to the larger family, yet Piaggio uses the smaller SI 20/20. Despite this, it does not receive the E1/BE1 of the small-carburettor family, but BE2. Here, therefore, a large engine and the smaller carburettor class meet.
With the Rally 200, the development then moves completely into the 200 cc family: SI 24/24 and BE3. In the subsequent development, this family is divided into a 10 hp and a 12 hp version. For the 10 hp version, BE4 extends the atomiser tube geometry by another development step, while the more powerful 12 hp version uses BE3.
The T5 represents a second exceptional case. Although, as a 125, it represents a completely different and considerably more rev-oriented engine concept, Piaggio adopts the BE4 from the 10 hp 200. In this respect, the T5 forms something of a counterpart to the Rally 180: there, a large engine meets the small SI 20/20 with BE2; here, a small engine meets the large SI 24/24 G with BE4.
The contrast becomes particularly clear when looking at the main jet stack: the Rally 180 uses 160/BE2/109, with the main air correction still located beneath the filter, whereas the T5 in the oil-injection version considered here uses 120/BE4/112 with an exposed main air correction.
BE5 finally appears in later Piaggio concepts. We find it, for example, on the PX 125 Lusso and the Cosa 125, and later also on the PX 125 with catalytic converter. With BE6, the development eventually returns to the large engine and carburettor family on the Cosa 200.
And with the Cosa, we once again encounter something already seen on the GS 160: changes to the atomiser tube within an ongoing model series. On the Cosa 200, BE6 and BE3 can be found in different factory configurations before Piaggio returns to BE6 and additionally changes the main air correction from 160 to 140. Here too, therefore, not only the atomiser tube itself but also its air correction is used as a development variable. At the same time, this brings us back to the T5, where Piaggio had taken this approach considerably further with the exceptionally small 120 main air correction jet.
Over several decades, this creates the striking historical numerical sequence:
E1/BE1 → (E2)/BE2 → BE3 → BE4 → BE5 → BE6
The brackets around E2 deliberately indicate the point that remains unresolved: E2 is historically documented, and its relationship to the later BE2 appears plausible, but its geometry has not yet been documented sufficiently to trace this transition with the same confidence as the one between E1 and BE1.
From a technical point of view, however, this sequence is not a simple scale. This becomes apparent as soon as we look at the geometry rather than the numbers.
As a basis for understanding this, I use the BE2 with its four possible drilling levels, which I refer to from bottom to top as levels 1 to 4. This numbering is my own explanatory model and not a historical Dell’Orto designation.
Within this level model, both the basic geometries and variants within the same geometry were changed over the decades. This becomes particularly clear with the three-level structure: E1, BE1, BE3, BE5 and BE6 form a geometrically related family. What begins early on with E1 and BE1 therefore continues through BE3 to the later BE5 and BE6 variants.
Development consequently proceeded along two paths at the same time: between different level geometries and within a shared geometrical family.
Looking back, the Rally 180 is a remarkable Piaggio reference example of a basic principle that can also be found in my own development work: a large engine, a comparatively small carburettor and a fuel-oriented atomiser tube geometry – with the aim of creating a torquey, agile engine with a broad usable rev range.
LEMARXON carries this principle forward under modern tuning conditions. Engine capacity increases, engine speeds rise, and the air supply to the engine is optimised and freed from unnecessary restrictions. As a result, the airflow through the comparatively small carburettor increases. This is precisely where my X234 and XX34 atomiser tubes come into play. With the ###4, this geometrical principle is ultimately taken into a particularly extreme range.
This also explains the seemingly mysterious numbering: the designations BE1 to BE6 do not describe a geometrical or functional ranking. Rather, their numbering reflects the historical development of the atomiser tubes.
The numbering shows the historical path. The geometry shows the actual relationship and how they work.