back
Piaggio Item number: 9021002
Piaggio

Air filter -PIAGGIO- Vespa Dellorto SI20/20D, SI24/24E, SI24/24H (long version, with hole above the main air jet) - Vespa Cosa (with adjustment of the jetting also for Vespa PX80, PX125, PX150, PX200)

26 Ratings
Manufacturer: PIAGGIO
Carburetor: Dellorto SI20/20D, SI24/24E, SI24/24H
Version: long version, Cosa type
Feature: Holes above the jet block (main air correction area)
Vehicles: Vespa Cosa, PX after jetting
OEM comparison: suitable for Cosa filter version (hole above main air jet)
39,90 €*
available immediately
Shipping weight 0,12 kg
Information on product safety
Report error

Air filter -PIAGGIO- Dellorto SI20/20D, SI24/24E, SI24/24H (long, with hole)

The Dellorto SI reacts sensitively to changes in pressure. This PIAGGIO air filter is the version for Cosa models and has holes above the nozzle block. It is intended as a standard filter for Vespa Cosa and can also be used on PX engines with SI carburettors with adapted jetting.

long SI version with hole above main air jet for SI20 to SI24 Cosa standard filter PX only adaptable with jetting

Advantages at a glance

Stable SI base: defines the pressure ratios above the nozzle block and makes tuning more reproducible.
Cosa version: filter base with holes for improved air flow to the air nozzle.
Can be used for conversions: can be used on PX engines if jetting and air correction are adapted.
Practical: prevents typical misconceptions such as "float chamber empty" if air correction is incorrectly selected.

Why the SI quickly gets bitchy without a filter

The SI is a downdraft carburettor in which the nozzle block is fed by a main air correction nozzle. Changes to the filter, filter base or air box change the negative pressure in the nozzle block. This has a direct effect on the fuel level at the atomizer.

Hole above the nozzle block: often improves pre-foaming, but can significantly reduce the mixture depending on the setup.
Heavily modified engines: Vacuum in the nozzle block can become too low, the fuel level drops below the outlet holes.
Symptom: lean full-load operation up to the point where the mixture supply stalls. This is often wrongly interpreted as an "empty float chamber".
Solution: in many cases this is a tuning issue via the main air correction jet (often select a smaller one) and not necessarily via an ever larger main jet.

Note for PX, Rally and Sprint

This Cosa filter has holes above the nozzle block. On PX, Rally or Sprint, the mixture can be lean compared to a filter without holes. To prevent this from leading to engine damage, the jetting must be adjusted.

Secondary jet: often choose richer.
Main jet: often select larger.
Main air correction jet: depending on the setup, select smaller if necessary to increase the vacuum in the nozzle block.

Differences between the SI filters

Short version: only for small SI carburetors such as 20/15 or 20/17 (e.g. early largeframe engines).
Long version: standard for SI20 to SI26.
Fine filter: double filter fleece, lower flow rate. Jetting often has to be adjusted leaner.
With hole above main air jet: Cosa standard, in addition to the plate filter solution. On PX usually leaner, adjust jetting.
Without hole above main air jet: PX standard, easy to tune. Small hole (e.g. 3 mm) above the air jet is often retrofitted as a starting point.
T5: higher filter, only for T5 specific SI24/24G with air funnel and special screws.

Setup starting point (practice)

Many engines are easiest to tune with an unprocessed filter. The classic PX200 base is often used as a starting point:

HLKD and mixing pipe: 160/BE3
Secondary jet: 55/160 or 52/150
If the filter base is perforated: adjust at least the main jet, depending on the setup also HLKD.

Technical data

productPIAGGIO air filter for Dellorto SI (long version)
Suitable carburetorsSI20/20D, SI24/24E, SI24/24H
Versionwith holes above the nozzle block (Cosa type)
ApplicationAdapt Vespa Cosa, PX according to jetting

Suitable for the following models (excerpt)

Vespa Cosa with Dellorto SI (Cosa standard filter)
Vespa PX80, PX125, PX150, PX200 with Dellorto SI20 to SI24 (only with adapted jetting)

FAQ

In many cases it is difficult because the SI reacts strongly to pressure conditions. A defined filter setup makes tuning much more reproducible.
The holes change the flow of the air correction and thus the vacuum in the nozzle block. This can make the mixture leaner. Adjust the jetting.
Not necessarily. Depending on the setup, the main air correction jet is the lever for increasing the vacuum in the nozzle block. The main jet and secondary jet should still be checked.
This is how we ensure, that reviews come from real consumers.
Markus H. 10/22/2025
Anonym 08/26/2025
Volker V. 08/14/2025
Marc M. 07/28/2025
Eric M. 07/21/2025
Dirk F. 07/17/2025
Volker R. 07/17/2025
Frank G. 07/13/2025
Gerhard M. 06/29/2025
Christian E. 05/12/2025
Anonym 01/21/2018

Ottimo perfetto come da descrizione

Anonym 01/21/2018

Prodotto esattamente come in fotografia, di ottima qualità, non come i soliti prodotti scadenti venduti su altri siti internet con scarsa qualità e scarsi materiali. Lo consiglio al 100%, ricambio eccellente, molto contento è molto soddisfatto.

OEM numbers (for comparison purposes only)
Piaggio:
285352, 990331
Matching
Accessories
Matching
Alternative products
Matching
Customers also bought
Top