Description
The SI carburettor series from Dell’Orto has been used on Vespa’s rotary vane engines since time immemorial. Their big advantage is the very simple construction and the good tuning possibilities. They differ strongly from conventional carburetor superstructures and have neither carburetor needle nor a mixing tube in the conventional sense.
They captivate by an extremely short intake path, which guarantees a fast response of the engine. Furthermore, despite the relatively small diameters available, they are always the first choice when it comes to fast and touring engines based on rotary vane technology. Even rear wheel powers of more than 20kW are already possible with a SI24.
The G-carburettors of the SI series play a special role. These are shortened versions of the usual D and E types. Thus a larger air filter with integrated funnel could be realized. Unfortunately these carburettors need different air jets for main and secondary air correction, and they are difficult to tune on the 200cc engines because there are no air jets available.
A super alternative here are our SI 26/26 carburetors. These have with Ø25mm passage already clearly more surface than a SI24 carburettor. In principle, this type of carburettor can be drilled even further, but then the guiding of the gas slide and its sealing function is no longer 100% guaranteed. Therefore we limit ourselves here to a measure which has been successfully tested in practice. The SI26 carburettors work very well even on very powerful engine superstructures.
In addition you can of course modify them as described below.
All SI carburettors can be completed with the intake manifolds manufactured by Polini. In this case the serial filter including the carburettor sump cover is removed and replaced by a nice polished funnel. The increase in power is limited and results only from the removal of the original filter and cover, which reduces the mixture. This must be greased massively when using such a funnel, because the main air correction nozzle is flowed differently.
All new SI carburettors are manufactured under licence by Spaco in India. These carburettors have been and are used in the series production of Piaggio for a long time. They are of the same quality as the original Dell’Orto carburettors. They also pass the quality assurance at Dell’Orto and are therefore partly packed in Dell’Orto cartons.
SCK Technology Tip:
The fuel supply of the Vespa models with SI carburettor type is always a bit difficult. Due to the very small difference in height between the fuel level in the tank and in the float chamber of the carburettor, there can always be an undersupply of fuel. Especially when the tank content approaches the reserve, the pressure difference is often so small that this is noticeable in a lean mixture. Beside the usual suspects like a working tank ventilation the fuel tap should not be blocked and the fuel hose length should be as short as possible without kinking the hose. To prevent the latter we use fuel hoses with a wire inlay, this effectively prevents the hose from kinking. Also the used tank shape and the used spacer at the rear shock absorber have an influence. Both influence the height difference between tank and carburettor.
flow in the carburettor:
As a recommended measure the drilling of the fuel channel in the carburettor between float chamber and nozzle block has proved to be a good idea. In case of problems with the full load tuning, the bore should be widened to approx. 1.8mm to always guarantee a sufficient flow.
Spacer for float chamber:
In addition there is the possibility to increase the float chamber volume by a spacer. However, this has proven to be problematic with regard to the resulting increase in petrol levels in the supply area around the mixing tube. Often the petrol is so high here that the premix in the mixing tube no longer functions properly and the petrol drips through the atomizer into the engine when the engine is at a standstill. This can lead to the engine literally sinking and is very difficult to start. Furthermore, a larger float chamber volume does not help to compensate for an undersupply. If the original chamber is not filled fast enough, then a larger volume will not help under continuous load. If it is filled fast enough, no larger volume is needed…
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