A carburettor marked SI does not automatically mean that every part listed for a Vespa PX will fit your scooter. Owners asking which Vespa models use SI carburettor units are usually about to order a gasket set, float valve, jet or replacement carburettor. Getting the engine family and the exact SI size right first avoids a common and expensive mistake.
The Dell'Orto SI is the familiar box-shaped carburettor mounted directly to the inlet on many classic Vespa largeframe engines. It is compact, fed through the airbox, and uses a removable jet stack in the carburettor body. It was fitted across several 1960s to 1990s Vespa ranges, but specification changed with engine capacity, market and production year.
Vespa models that use SI carburettors
In broad terms, SI carburettors belong to Vespa largeframe scooters. The following model families commonly use them:
- Vespa GS160, SS180 and related high-performance 1960s largeframes
- Vespa Super 125 and Super 150, Sprint, Sprint Veloce, GT and GTR models
- Vespa Rally 180 and Rally 200
- Vespa P-series and PX models, including PX125, PX150, P200E and PX200
- Vespa Cosa 125, 150 and 200 models
Early largeframes: GS160 and SS180
The GS160 and SS180 are among the earlier Vespa models associated with the SI design. These scooters used larger, performance-oriented SI carburettors, commonly referred to as SI 27/23 units. They are not interchangeable with a later PX 20/20 or 24/24 simply because all carry the SI name.
Their airbox arrangement, carburettor top, fuel supply parts and jetting requirements differ from the more familiar P-series setup. If you are restoring one of these models, buy parts against the exact carburettor version and engine rather than treating it as a standard PX application.
Super, Sprint, GT and GTR models
Many 1960s and 1970s 125cc and 150cc largeframe Vespas use an SI 20/20 carburettor. This includes numerous Super, Sprint, Sprint Veloce, GT and GTR applications. The letter after the size, such as D or E, matters because it identifies a particular carburettor version.
An SI 20/20D is widely associated with earlier largeframe engines, while later versions may use an SI 20/20E. The basic layout is recognisable, but items such as the air filter, float chamber cover, fuel tap connection and jetting can vary. For service work, match the component to what is physically fitted rather than relying only on the scooter’s badge.
The word “Sprint” also deserves care. Vespa naming and export-market specifications are not always consistent, and a scooter may have received an engine swap during decades of use. Check the engine prefix and carburettor body before ordering.
Rally 180 and Rally 200
The Vespa Rally range is another key SI-carburettor application. Rally 180 models generally use an SI 20/20D setup, while Rally 200 models are usually fitted with an SI 24/24 carburettor. The greater choke size of the 24/24 suits the 200cc engine, but the carburettor should still be paired with the correct airbox, jet stack and filter.
Rally 200 parts are often confused with P200E and PX200 parts because both use 24/24 SI carburettors. Some service items interchange, including many internal consumables, but not every external part does. Fuel elbow arrangements, top covers and airbox details can differ. A carburettor rebuild is straightforward when the correct components are on the bench; it becomes less so when parts from several generations have been mixed together.
P-series and PX models
The P-series and PX are the models most people mean when they refer to an SI carburettor. Standard 125cc and 150cc P-series and PX engines generally use an SI 20/20 carburettor, while 200cc P200E and PX200 engines use an SI 24/24E.
There are exceptions caused by regional specification, later emissions equipment and previous modifications. A 150 may have been upgraded with a 24/24 during an engine rebuild, for example. That can be a sensible performance modification when combined with suitable cylinder timing, exhaust and jetting, but it does not make every 24/24 service setting correct for that engine.
PX carburettors are also often identified by the presence of an oil feed drilling or spigot on autolube engines. A non-autolube carburettor can be used on a premix engine, but an autolube carburettor fitted to an engine without oil injection needs the unused oil connection dealt with correctly. Never leave it as an uncontrolled air or oil path.
Vespa Cosa models
Vespa Cosa models continue the largeframe SI arrangement. Cosa 125 and 150 models normally use an SI 20/20 type, while the Cosa 200 uses a 24/24 version. Cosa carburettors share much of their internal design with late PX units, but the airbox and surrounding engine components should be checked carefully.
Cosa parts are frequently catalogued separately for good reason. A gasket, float, jet or choke component may be common across several SI types, whereas a complete carburettor, cover or air filter may have model-specific differences.
Which Vespa models do not use an SI carburettor?
The SI was not fitted to every classic Vespa. Most smallframe scooters use Dell'Orto SHB carburettors, including the Vespa 50, 90, 100, Primavera, ET3 and PK ranges. These are smaller round-bodied carburettors and their jets, cables, manifolds and rebuild parts are not SI parts.
Earlier widebody and early largeframe machines may use MA, MB or other Dell'Orto designs. Automatic Vespas use different induction systems again. If the carburettor does not sit beneath a largeframe airbox and does not have the distinctive rectangular float chamber body, do not order SI components on model name alone.
How to identify your SI carburettor before buying parts
Start by looking for the Dell'Orto stamping on the carburettor body. The size is normally marked as 20/20, 24/24 or, on earlier sporting applications, 27/23. The first figure relates to the carburettor bore and the second to the outlet size. The suffix letter is equally useful when sourcing complete units and certain external parts.
Next, establish whether the engine is 125, 150, 180 or 200cc and confirm its engine prefix. On a Vespa, the frame number identifies the chassis, but the engine number tells you what engine is actually installed. This is particularly valuable with restored scooters, where engines, cylinder kits and carburettors may have been changed more than once.
Then check whether the engine runs premix or oil injection. Look at the carburettor body and oil tank system rather than assuming from the model year. Finally, inspect the existing airbox, filter and fuel pipe connection. An SI carburettor relies on the correct airbox seal and filter arrangement. Air leaks here can produce weak running, inconsistent tickover and misleading jetting symptoms.
SI carburettor parts: match the part to the job
For a routine rebuild, the usual requirements are a gasket set, float, float needle or valve, top gasket, fuel elbow seals and possibly a choke plunger. If the carburettor has been standing with old fuel, inspect the jet stack and atomiser carefully rather than merely spraying cleaner through it. Blocked passages and worn float valves are common causes of fuel starvation, flooding and poor starting.
Jets need more care. The main jet, air corrector and mixer tube work as a set, and the idle jet must suit the engine’s state of tune. A larger main jet cannot correct an air leak, blocked filter, poor ignition timing or a worn crankshaft seal. Likewise, a carburettor conversion from 20/20 to 24/24 should be jetted from a known safe baseline and tested properly, not guessed at after one short ride.
Scooter Vista stocks specialist Vespa carburettor, fuel and engine service parts for owners working on classic machines. When selecting an SI item, use the carburettor marking, engine capacity and oiling arrangement as your three main checks.
A clean, correctly jetted SI carburettor makes a largeframe Vespa feel exactly as it should: easy to start, steady at tickover and willing under load. Take five minutes to identify the unit before ordering, and the parts on your bench are far more likely to be the parts your engine actually needs.
