A peculiarity of the SI carburettor is the fact that it is difficult to tune without an air filter. It reacts sensitively to changing pressure conditions. This has to do with its mode of operation as a downdraft carburettor, where the nozzle block is supplied by a so-called main air correction nozzle. A hole is often drilled in the filter bottom above this nozzle to improve the air flow to the air nozzle. This measure usually improves the pre-foaming, but with heavily modified engines it can lead to a too low negative pressure in the nozzle block. As a result, the fuel level drops below the threshold of the atomizer/petrol outlet tube and the engine is only insufficiently supplied with mixture. Apart from a too lean full load operation, which usually no larger main nozzle can catch, in extreme cases it can also come to a complete stop of the fuel delivery. This often leads to the assumption that the float chamber was sucked empty. This is usually not the case (assuming a proper fuel supply through an appropriate fuel tap), but merely a matter of adjustment via a smaller main air correction nozzle.
Differences of the SI filters
short version Only usable on the small old SI carburettors like 20/15 or 20/17 (e.g. VBB or similar).)
Long version Used on all SI 20 - SI 26.
Fine filter Indian version with double filter fleece for a long engine life. If this filter is used the spraying should be reduced accordingly, since the throughput is clearly smaller compared with a standard filter.
With hole over main air nozzle Standard filter of the Cosa models which have additionally a plate air filter. Has two large holes above the nozzle bars. If it is installed on PX/Rally/Sprint models, it reduces the mixture significantly compared to a filter without holes. This means that here you should replace the secondary nozzle, main nozzle and possibly the main air correction nozzle with a more powerful one to prevent engine damage.
Without hole above the main air nozzle The standard filter of all PX/Rally models. Can be well tuned and offers sufficient filtering. Perfect to add a small hole (start with 3 millimeters) above the air nozzle to improve the pre-foaming.
T5 Special filter version which fits only on the T5 specific low SI24/24G carburetors due to the bigger height. With built-in air funnel. Requires special mounting screws. Can only be removed by unscrewing the idle screw, with a hole above the main air correction nozzle (T5 uses a 120mm air nozzle).
Most engines are easiest to tune with an unmachined filter. A classic 160/BE3 air nozzle/mixing tube combination in combination with a 55/160 or 52/150 secondary nozzle is a good starting point and often available as it is the basic nozzle of a PX200. If the filter bottom is perforated (start with a 3mm hole above the air nozzle) at least the main nozzle should be enlarged, depending on the engine and the size of the hole in the filter bottom also the main air correction nozzle.
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