Venturi intake funnel - PX SI 24/26 - more air intake area, higher flow rate, for increased performance
for SI 24/26 carburetor
Larger air intake area
Optimized intake characteristics
noticeable increase in performance*
Re-tuning required
PX series
Concept
The SI carburetor is widely used on Vespa PX engines due to its compact design and short intake path.
The Venturi intake funnel increases the air intake area at the carburetor inlet and improves the intake characteristics.
The result is a better breathing capacity of the engine and thus a higher performance potential*, especially with tuned setups.
Tuning and peripherals
Depending on the fill level in the float chamber, a defined fuel level prevails in the SI's nozzle block. Below the atomizer is a fuel column: at the bottom, the main nozzle meters the amount of fuel flowing in, in the middle the mixing tube(e.g. BE3) works half in air, half in fuel, at the top the main air correction nozzle (HLKD) regulates the air flowing in.
The HLKD is usually available from 120 to 190: a 120 (bore approx. 1.2 mm) lets in little air and has a lubricating effect, a 190 lets in a lot of air and has a lean effect. If the air jet is too large (e.g. 190), not enough vacuum can build up - the fuel supply leans dangerously under full load or collapses (often wrongly interpreted as an "empty float chamber"). In this case, even a larger main jet will only help to a limited extent.
Modified air ducts strongly influence the choice of HVACD: In the case of drilled standard filters with too large a hole above the air jet (e.g. 8 mm) or air funnels (Polini, Pinasco, Dexter, Vesptech) that replace the filter, significantly larger main jets and a smaller HVACD are usually required (e.g. 140 instead of 160) in order to stabilize the part-load pattern.
For high-performance setups, we recommend a BGM main jet set up to 162, a fresh float needle and new gaskets. Optionally, a COSA float chamber with an enlarged bore between the chamber and the nozzle block improves the supply.
We do not recommend float chamber spacers: although they increase the fuel level, they disrupt the function of the premix and can lead to overflowing into the engine.
Installation instructions
Carefully align the venturi funnel, secure the fastening screws with screw adhesive and ensure that the surrounding parts can move freely.
After installation, carry out a complete carburetor adjustment (main jet, mixing tube, HLKD, idle speed).
| vehicle |
Note for |
| Vespa PX 125/150/200 | for Dell'Orto/Spaco SI 24 and SI 26 |
| Other large frames with SI 24/26 | Suitable depending on body, check installation space |
Note: T5 uses a different SI layout, check compatibility separately.
Key technical data
| Carburetor | SI 24 / SI 26 (Dell'Orto/Spaco) |
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| Function | Venturi intake funnel to increase the air intake area |
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| HVACD range | 120-190 (120 ≈ 1.2 mm, rich; 190 lean) |
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| Tuning | Re-tuning mandatory; depending on setup, main nozzle range 150+ |
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| Peripherals | Replace float needle and seals; optional COSA float chamber with larger bore |
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| Not recommended | Float chamber spacers (interfere with premixing, risk of run-off) |
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| Assembly | Bolted, use threadlocker |
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Scope of delivery
| Venturi funnel | Intake funnel for SI 24/26 |
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| Small parts | Fastening screws (depending on version) |
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If the HVACD is too large (e.g. 190), too little vacuum builds up. The fuel supply collapses under full load - an even larger main jet then only helps to a limited extent. Remedy: Select a smaller HPDC and tune it properly.
No. They increase the fuel level and disrupt premixing in the nozzle block. This can result in over-greasing and stalling. Better: tuning via HLKD and main jet.
If the hole above the air jet is too large (e.g. 8 mm) or with Polini/Pinasco air funnels, more air is fed in. Larger main jets and a smaller HVACD are usually necessary (e.g. 140 instead of 160) in order to keep part load and full load stable.
* The actual increase in power depends heavily on the overall setup (cylinders, exhaust, timing, compression, airflow).
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