Vespa PX Carburettor Jets Explained – SI 20/20 & SI 24/24 Jetting Guide
If you own a Vespa PX125, PX150 or PX200, understanding how the jets and adjustment screws in the Dell'Orto SI carburettor work can make diagnosing and tuning your scooter much easier.
The SI carburettor may look relatively simple, but several different circuits work together to control the fuel mixture throughout the throttle range.
Understanding what each component does is particularly important if you're servicing a standard PX, replacing a carburettor or setting up an engine with a performance exhaust or cylinder kit.
In this guide we'll explain:
- How the SI 20/20 and SI 24/24 carburettors work
- What each jet does
- How the main jet, atomiser and air corrector work together
- How the idle jet affects low-speed running
- Why the mixture screw is so important
- The difference between the mixture screw and tick-over screw
- The two different PX mixture-screw types
- How to correctly adjust the mixture and tick-over
- Common factory PX jetting
- How modifications can affect jetting
- Symptoms of rich and lean jetting
Vespa PX SI 20/20 & SI 24/24 Carburettors
The Dell'Orto SI carburettor was fitted to many Vespa PX models in different sizes.
The two you'll most commonly encounter are:
SI 20/20
Commonly associated with Vespa PX125 and PX150 engines.
SI 24/24
Most commonly associated with the Vespa PX200, although larger SI carburettors are also used on modified engines.
Although the carburettor sizes differ, the basic jetting system works in the same way.
The important thing to understand is that not every jet controls the whole throttle range.
Different circuits overlap as the throttle opens.
What Does Each Vespa PX Jet Do?
| Component | Main function | Most influential area |
|---|---|---|
| Idle jet | Controls fuel through the low-speed circuit | Idle to small throttle openings |
| Mixture screw | Fine adjustment of low-speed mixture | Idle to small throttle openings |
| Main jet | Controls fuel through the main circuit | Larger throttle openings |
| Atomiser / BE tube | Mixes fuel and air in the main circuit | Mid to high throttle |
| Air corrector | Controls air entering the main circuit | Mid to high throttle |
| Choke jet | Supplies extra fuel for cold starting | Starting |
These aren't completely separate systems. They overlap, which is why carburettor tuning can sometimes require a little trial and error.
Vespa PX Idle Jet
The idle jet, or pilot jet, controls fuel supplied through the low-speed circuit.
You'll often see a PX idle jet marked something like:
48/160
or:
55/160
The two numbers relate to the fuel and air passages within the jet.
The idle circuit is important for:
- Starting
- Idling
- Pulling away
- Low-speed riding
- Small throttle openings
- Transition onto the main circuit
A blocked or incorrectly sized idle jet can cause surprisingly poor running even if the main jet is completely correct.
The Vespa PX Mixture Screw – One of the Most Important Adjustments
The mixture screw is one of the most important adjustments on a Vespa PX SI carburettor, but it's also one of the most commonly misunderstood.
It controls the mixture supplied by the low-speed circuit and is particularly important around idle and small throttle openings.
On the SI carburettor used on PX models:
Turn mixture screw IN → leaner
Turn mixture screw OUT → richer
This is because the screw is positioned on the engine side of the carburettor. Turning it out allows more fuel-rich mixture through the low-speed circuit.
The mixture screw can affect:
- Idle quality
- Starting
- Low-speed running
- Throttle response
- Pulling away
- How quickly the engine returns to idle
A correctly sized idle jet still needs the mixture screw adjusted correctly.
There Are Two Different Vespa PX Mixture Screws
This is particularly important when working on PX carburettors.
There are two main mixture-screw types used during PX production.
Early PX Mixture Screw
The earlier type is the shorter, slotted mixture screw with a coarse thread.
This is commonly found on earlier Dell'Orto SI carburettors.
A typical starting setting is approximately:
1½ turns out from gently seated.
Later PX / Lusso Mixture Screw
Later PX carburettors use a longer mixture screw with a fine thread and hexagonal head.
This is often referred to as the Lusso-type mixture screw.
A commonly used starting setting is approximately:
2 turns out from gently seated.
The important point is that the number of turns isn't directly interchangeable between the two screw types because the thread and screw design are different. The commonly quoted baseline settings are approximately 1½ turns for the early coarse-thread screw and 2 turns for the later fine-thread Lusso screw.
Never force the mixture screw against its seat. Gently turn it in until it reaches its stop before backing it out to the starting position.
Electric-Start Vespa PX Models
The later mixture screw is particularly useful on electric-start PX models because access to the mixture screw can be extremely restricted by the starter motor.
The long hex-headed screw was designed so it can be adjusted despite the restricted access.
This is worth knowing because trying to get a conventional screwdriver onto the mixture screw on an electric-start PX can be extremely awkward.
Some suppliers specifically identify the long hex-headed fine-thread screw as the type suitable for electric-start applications because the starter motor blocks normal screwdriver access.
Take care when adjusting it
If access is restricted:
- Don't force a screwdriver into the screw
- Don't damage the screw head
- Make small adjustments
- Keep track of how far you've turned it
- Make sure you're adjusting the mixture screw, not the tick-over screw
A suitable tool makes the job considerably easier.
How to Set the Vespa PX Mixture Screw
The mixture screw's baseline position is only a starting point.
The final setting should be made with the engine fully warmed up.
1. Start with the correct base setting
For a standard/original-type setup:
Early coarse-thread screw: approximately 1½ turns out
Later fine-thread Lusso screw: approximately 2 turns out
These are starting points rather than guaranteed final settings.
2. Start the engine and warm it up
Don't attempt to establish the final mixture setting on a cold engine.
Allow the engine to reach normal operating temperature.
3. Set the tick-over slightly high
Use the tick-over screw to get the engine running steadily at a slightly elevated idle.
4. Adjust the mixture screw
Make small adjustments, around ¼ turn at a time.
Listen to the engine and look for the point where it runs smoothly and responds cleanly.
5. Find the best running position
You're looking for a smooth, stable idle and clean throttle response.
Don't simply turn the screw until the engine reaches the highest possible revs and leave it there.
The final setting needs to work with the idle jet and the rest of the engine.
6. Reset the tick-over
Once the mixture is correctly adjusted, use the tick-over screw to bring the engine back to its normal idle speed.
The Vespa PX Tick-Over Screw
The tick-over screw is completely different from the mixture screw.
The tick-over screw controls how far the throttle slide remains open when you release the throttle.
Therefore:
Turn tick-over screw IN → slide opens further → idle speed increases
Turn tick-over screw OUT → slide closes → idle speed decreases
It does not set the fuel mixture.
This distinction is worth remembering:
Mixture screw = mixture
Tick-over screw = engine idle speed
What Is the Correct Vespa PX Tick-Over Setting?
There isn't a universal number of turns for the tick-over screw.
Instead, it should be adjusted to give the engine a stable idle at the correct speed.
As a useful guide, around 1,300–1,400 rpm is a sensible target for a standard PX, with approximately 1,350 rpm commonly used as a reference.
Before adjusting it, make sure the throttle cable isn't holding the slide open.
The throttle should close properly when released.
The Correct Order for Setting Your PX Carburettor
For the best results, don't randomly adjust both screws.
Work through the process in this order:
1. Make sure the carburettor is clean
↓
2. Check the correct idle jet is fitted
↓
3. Check the throttle cable has free play
↓
4. Set the mixture screw to its appropriate starting position
↓
5. Start and fully warm the engine
↓
6. Set the tick-over slightly high
↓
7. Adjust the mixture screw
↓
8. Set the final tick-over speed
↓
9. Check throttle response
↓
10. Recheck the idle
This gives you a much more repeatable starting point.
What If the Mixture Screw Needs a Lot of Adjustment?
If the engine only runs correctly with the mixture screw far outside its normal adjustment range, don't simply keep turning it.
It can indicate that the idle jet isn't the correct size.
If the engine requires an unusually large adjustment to achieve good running, investigate:
- Idle jet size
- Air leaks
- Carburettor cleanliness
- Carburettor gasket condition
- Throttle slide
- Fuel supply
If the engine doesn't respond properly to mixture-screw adjustments at all, an incorrect idle jet or another carburettor/engine problem may be responsible.
Vespa PX Main Jet
The main jet controls fuel through the main circuit and becomes increasingly important as the throttle opens.
For example:
96
100
116
The larger the number, the larger the fuel passage and generally the greater the fuel flow.
Therefore:
Larger main jet = richer
Smaller main jet = leaner
The main jet is particularly important at larger throttle openings and under heavy engine load.
This is why selecting the correct main jet is critical when tuning a performance PX engine.
Vespa PX Atomiser / BE Tube
The atomiser, or mixer tube, works with the main jet and air corrector.
Common Dell'Orto SI atomisers include:
- BE1
- BE2
- BE3
- BE4
- BE5
The atomiser has holes that allow air to mix with the fuel as it passes through the main circuit.
This influences how the mixture changes through the mid-range and towards larger throttle openings.
Changing the atomiser can therefore alter the way the engine runs even if the main jet remains unchanged.
Vespa PX Air Corrector Jet
The air corrector controls the amount of air introduced into the main fuel circuit.
Common sizes include:
120, 160, 185
The air corrector works together with the atomiser and main jet, so it shouldn't be tuned in isolation.
As a general principle:
Smaller air corrector → richer
Larger air corrector → leaner
The actual result depends on the complete combination of main jet, atomiser, air corrector and engine specification.
Vespa PX Choke Jet
The choke circuit supplies extra fuel when starting a cold engine.
Pulling the choke lever activates this circuit and provides a richer mixture to help the engine start.
Once the engine is warm, the choke should be returned to its normal position.
If a PX only runs correctly with the choke partially pulled, this should be treated as a symptom of a problem, rather than normal operation.
Possible causes include:
- Incorrect jetting
- Blocked jet
- Air leak
- Fuel supply problem
- Carburettor fault
Common Factory Vespa PX Jetting
Factory specifications changed over the PX production run, so the following should be treated as common reference settings rather than a universal specification for every PX.
| Vespa PX | Carburettor | Main Jet | Atomiser | Air Corrector | Idle Jet |
|---|---|---|---|---|---|
| PX125 | SI 20/20 | 96 | BE5 | 160 | 45/140 |
| PX150 | SI 20/20 | 100 | BE3 | 160 | 48/160 |
| PX200 | SI 24/24 | 116 | BE3 | 160 | 55/160 |
These are useful reference points for standard or near-standard engines, but always check the exact specification for your particular year and market before treating a jetting combination as factory-correct.
Why Factory Jetting May Not Work on a Modified PX
Factory jetting is an excellent starting point for a standard engine.
However, if you've fitted:
- Performance exhaust
- Larger cylinder
- Different air filter
- Increased compression
- Porting
- Performance crankshaft
- Larger carburettor
the engine's airflow and fuel requirements may have changed.
A modified PX may therefore require different jetting.
Never copy another scooter's jetting blindly.
Rich vs Lean Jetting
Too Rich
The engine may:
- Four-stroke
- Sound muffled
- Feel flat
- Struggle to rev cleanly
- Smoke excessively
- Load up
Too Lean
The engine may:
- Run excessively hot
- Hesitate
- Surge
- Lose power
- Sound unusually sharp
- Potentially suffer serious engine damage
A lean mixture can be particularly dangerous on a tuned two-stroke engine.
Don't Use the Main Jet to Fix an Idle Problem
This is one of the most common mistakes when tuning a Vespa PX.
If the problem occurs at:
Idle / very small throttle opening
look at:
- Idle jet
- Mixture screw
- Throttle slide
- Air leaks
- Carburettor condition
If the problem occurs at:
Large throttle openings
the main jet becomes much more important.
Understanding where in the throttle range the problem occurs is one of the most useful skills when tuning an SI carburettor.
Keep Your Vespa PX Jets Clean
Small carburettor jets can become partially blocked surprisingly easily.
When servicing an SI carburettor:
- Remove the jets.
- Clean them with suitable carburettor cleaner.
- Blow through them with clean compressed air if available.
- Make sure the passages are completely clear.
- Never enlarge a jet using a drill or unsuitable wire.
A damaged jet may no longer flow at its marked size.
If a jet is damaged or questionable, replace it with the correct quality replacement.
Simple Vespa PX Jetting Diagram
I'd add a custom diagram to this article later showing:
CLOSED THROTTLE → ¼ → ½ → ¾ → FULL THROTTLE
with the circuits visually overlapping.
The diagram will identify:
- Mixture screw – idle & low throttle
- Idle jet – low-speed circuit
- Atomiser / BE – mid/high throttle
- Air corrector – main circuit air
- Main jet – main fuel supply
- Choke jet – cold starting
- Tick-over screw – controls throttle slide/idle speed
I'd also add a small inset showing:
Early PX mixture screw
Short, slotted, coarse thread
Later PX/Lusso mixture screw
Long, hex-headed, fine thread
That would make the article much easier to understand than trying to explain everything with text alone.
Final Thoughts
Understanding the Dell'Orto SI carburettor fitted to your Vespa PX125, PX150 or PX200 doesn't need to be complicated.
The important thing is understanding what each component does.
Idle jet → low-speed fuel
Mixture screw → fine adjustment of the low-speed mixture
Tick-over screw → idle speed
Atomiser → fuel/air emulsion through the main circuit
Air corrector → air entering the main circuit
Main jet → fuel supply through the main circuit
Choke jet → extra fuel for cold starting
Once you understand these circuits, diagnosing a PX that isn't running correctly becomes much easier.
Remember that factory jetting is a starting point for a standard engine. Once an engine has been modified, the correct jetting needs to be established for that particular combination.
