Differences of the SI filters
Only usable on the small old SI carburettors like 20/15 or 20/17 (e.g. VBB or similar).)
Used on all SI 20 - SI 26.
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.
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.
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.
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.