The DELLORTO SI carburettor for VESPA

Created by André Jueterbock at 12:05 on May 5, 2023

Genius or madness - with no other Vespa component are these two assessments so close to each other ... Those who take a closer look at the SI carburettor usually realise that this is more than just a cheap piece of original equipment: Almost all large-frame Vespas built from 1962 onwards are equipped with it.

Among all carburettors, the DELLORTO SIs are solitaires. Because of their vertical intake path, they are said to be descended from car carburettors, but it is much closer to the fact that they are the high-volume version of DELLORTO's epochal SSI racing carburettors. In combination with horizontal cylinders, they offer the shortest intake path and have a lot of potential in this respect - also in terms of tuning.

But there are many other reasons why SI carburettors are the dream partners for Vespa life, e.g. the easy accessibility for adjustment and service work. The position above the rotary slide cheek of the crankshaft ensures the best lubrication of the connecting rod. On the outside, the carburettor bowl nestles perfectly against the large-frame frame and the encapsulated installation ensures the lowest possible noise level. Spare parts for it are available in every Vespa workshop, which is especially grateful on tours and in case of an unforeseen defect.

It doesn't matter whether you leave the SI in its basic condition or tune it up with accessories such as an airbox and Venturi funnel: Correct adjustment and tuning remain the be-all and end-all.

SI carburettor adjustment

The carburettor should generally be tuned when the engine is at operating temperature.

Idle speed

First of all, adjust the idle speed by turning the high vertical screw that protrudes from the top of the air filter. Target speed: approx. 1,350 rpm.

Idle mixture (LLG) screw

As a basic setting, the mixture adjustment screw is turned all the way in and then unscrewed by 1.5 turns. This is the ideal position for original engines. For tuned engines, the position of the correct setting can be approx. between 1 and 5 turns.

If you turn the mixture adjustment screw in, the engine will lean at idle. If you turn it out, the idle mixture becomes richer.

First, set a slightly higher idle speed and find the point of highest speed via the mixture adjustment screw. From there, turn the LLG screw out half a turn to a full turn so that the engine speed drops audibly (mixture becomes richer). Then reduce the idle speed again until the engine runs smoothly and accepts the throttle well. Then give the throttle a short burst. The engine should rev up spontaneously and then quickly drop back to idle speed.

Note: Make sure that the petrol hose is short and that the petrol tap has sufficient flow!

  • If the engine only runs with choke and has so little power that engaging the clutch to drive away seems impossible, the carburettor is not getting any fuel. Either the secondary jet is much too small, or the fuel tap or filter is clogged, or the fuel hose is too long or has air bubbles in it.

  • If it takes a long time to drop to idle, the idle mixture is too lean (remove LLG screw, increase LP)

  • If the engine only accepts the throttle after a delay, either the mixture is too rich or the idle speed is too low.

  • If the engine revs significantly higher when warm, the idle is too lean. If the warm engine stalls even though it ran smoothly when cold, the idle is too rich.

Main nozzle

Characteristic for the SI carburettor is that ALL channels branch off from the main jet (HP). This means that all elements are dependent on the main jet, and every time the main jet is changed, all other settings also change ..

If you still want to change the main jet, it is best to start with the main jet and with the throttle slide set to full throttle. Starting with a rich one (large number), work your way down to the smaller main jets until the engine revs out nicely.

Note: If the engine revs really high, the tuning is usually already too lean. To be on the safe side, you can go back 2-5 jet numbers from this maximum speed setting (get bigger); this way you stay on the safe, too rich side and have reserves.

In the best case, adjusting the carburettor via the main jet alone is already sufficient.

Mixing tube

Unfortunately, there is no orderly procedure for the mixing tube BE1 to BE6 and the numbering does not follow any logic. The only thing that helps here is trial and error. Thank God there are only six mixing tubes and only two or three of them are common, especially BE2, BE3 and BE4. Common combinations are, for example, "BE3 mixing tube/160 main air correction nozzle/air filter with hole" or "BE4 mixing tube/190 main air correction nozzle/air filter without hole".

Main air correction nozzle

In the mixing tube, the fuel that enters from below through the main nozzle is foamed with air that enters from above through the main air correction nozzle (HLKD). The size of the HLKD determines the amount of air or how much the fuel is foamed before it is released through the brass tube into the intake tract. Foaming is also influenced by the cross holes in the mixing tube. The further down these holes are in the mixing tube, the further (downwards) the air can penetrate into the fuel level.

Caution: This effect can become so strong that the air flowing in from above no longer allows fuel to flow into the mixing tube from below; the mixture then becomes very lean. For a long time, this effect was cited as a reason why SI carburettors are generally not suitable for high-performance engines. In the meantime, however, mixing tubes with so few transverse bores are available that they do not allow leaning even at the highest engine speeds and air volumes.

As a side effect, the HVACD influences the vacuum in the entire duct system of the SI carburettor. It is a kind of bypass, a "leak" with respect to the negative pressure from the intake duct. Small HVACD: high negative pressure in the fuel supply (strong suction). Large HVACD, hardly any negative pressure in the fuel ducts, nozzle block & co.

Throttle slide

Another characteristic of the SI carburettor is that they have no jet needle.

This is defective because the secondary jet lets in fuel up to about half the throttle slide opening and the main jet jumps aside from about one-third throttle. This means, for example, that from one third to half throttle, both jets are in use at the same time and virtually "flood" the engine with fuel.

To avoid this overfattening, there is another bypass for additional air; this is the recess/notch/cutout on the underside of the gas slide. Harald from MMW has done pioneering work here and has been offering the highly recommended "3.0" and "4.0" sliders for years via SIP Scootershop. They are available for vintage (SI27), standard (SI24) as well as tuning carburettors (SI26/28).

The shape of the cutout on the underside of the gas slide (width, depth, beginning, end) thus influences the transition area from approx. one quarter of gas to three quarters of the gas slide opening, in addition to the mixing tube and HLKD. If you don't want to get lost in the multitude of possibilities here, the "3.0" and "4.0" slides from SIP are recommended. In combination with the right mixing tube, these are a safe bet for good and fast tuning.

Side nozzle

Don't be afraid of the double designation of the secondary nozzles! The first number is the size of the secondary nozzle, the second number is the secondary air correction (NLKD). The secondary nozzle works in much the same way as the main nozzle, because it too pre-foams the fuel by air correction.

Finding the right LP is easier than with the main jet. It must not be too lean or the engine will not accept throttle; also, adjusting the LLG screw should produce a response from the engine at least in the 1-5 turn range.

Start with a large nozzle number and you can also start with a large air correction number, e.g. 62:160. If this is too lean, reduce the bypass, i.e. the air correction of the NLKD: 62:140. If you get into a usable range with this NLKD (engine accepts throttle and LLG screw reacts), make the fine adjustment via the nozzle size, here e.g. 58:140 (NLKD remains the same).

Note: The one-piece side jets of the oldie carburettors work in conjunction with an NLKD pressed firmly into the carburettor housing. This is a small brass sleeve with a diameter of 1 mm. It is therefore a 100 secondary air correction. So a 52 bypass in the Oldie carburettor corresponds to e.g. 52:100 in a PX carburettor. This means the other way round: if you close the brass sleeve (grub screw), you can use all the secondary jets of e.g. the PX Vespas in the Oldie carburettors.

All in all, the coarse jetting can also be tested by means of the choke:

  • If you pull the choke while driving (at constant slide position) and the scooter accelerates, the mixture is clearly too lean in this range. If the engine stalls or throttles back, the fuel mixture is already somewhat OK!

  • If the engine hesitates significantly when accelerating and only accelerates when you reduce the throttle a little, the main jet is too small!

An engine that is too lean will be much hotter, sound much brighter and have little power. A setting that is too rich, on the other hand, does not allow the engine to warm up properly and often causes it to stutter and throttle at relatively low revs (so-called "four-stroking"). The spark plug, combustion chamber and exhaust get very sooty.

Because air-cooled engines are also cooled by evaporation of the fuel, they are quite sensitive to lean tuning. However, they are grateful for a slightly too rich setting, because of low temperature and good lubrication.


When the fuel mixture seems to be optimal, the air filter, the carburettor bowl cover (airbox) including all gaskets and the intake rubber are mounted again and everything is tested again. In most cases, this will make the settings slightly richer than before. A little readjustment of the idle mixture screw and idle screw "from the outside" is therefore normal, but then it should fit.

If you want to know exactly about your combustion and the fuel-air mixture, we recommend the SIP kits for connecting the lambda sensor and EGT to the SIP speedometers. This way, you can see on the tachometer whether the engine feels good and is getting a sufficiently rich mixture, or whether it is running too lean and you should correct the carburettor setting.

Scheme SI-Carburettor
Adjustment elements on top of the SI carburettor
Elements on the underside of the SI carburettor
Side jet air correction for oldie carburettor

Intake channel and intake path

In addition to correct adjustment, the following tuning measures have also proven useful and performance-enhancing for the DELLORTO SI carburettor:

Adjusting the intake timing

The rotary slide sealing surface can be extended to the front, to the rear and slightly to the side. Theoretically, a larger channel can allow a larger amount of fresh gas to enter the engine ..

In order to exhaust the possibilities here, it is essential to also consider the crankshaft, especially its width and lateral position. During operation, the crankshaft in a Vespa engine may move sideways by up to 0.5 mm, e.g. when the clutch is operated.

The change in timing should preferably be determined by the geometry of the crank web. Although some older "milling instructions", e.g. even from MALOSSI, suggest widening the inlet in the housing "backwards" against the direction of travel, it is not advisable to allow the overcurrent and inlet timing to overlap by more than 5 to 10 degrees! This is because this is at the expense of the available torque and can considerably impair the easy startability of the motor.

As a rule of thumb, the intake on 200cc engines should close at 65° after TDC at the latest and on 177cc tunings (PINASCO, POLINI) at around 53-58° after TDC.

Inlets that do not fit or are already too wide open can be reliably reduced with JB Weld. JB Weld is the only liquid metal known to us that does not dissolve under the influence of fuel and lubricating oil, even in the long term; provided that the bonding surfaces have been appropriately pretreated.

Holes in the air filter bottom

The bottom of the standard air filter has a heart-shaped recess next to the air duct, which is located above the main and secondary nozzles. With two holes (5 mm hole above the idle nozzle and 8 mm hole above the nozzle block), the supply to the main nozzle block and the idle nozzle can be made independent of the gas flow in the intake duct. This results in a more even supply to the air correction nozzles. The carburettor then behaves more evenly and becomes more precisely adjustable, the overall performance improves considerably. If the holes are not already there, this measure is basic. If a Venturi intake funnel is fitted, this measure takes care of itself.

Venturi intake funnel, carburettor bowl cover, airbox and spacer

In general, it has been shown that the larger the volume of the carburettor bowl, the higher the performance of the engine. For this purpose, PINASCO, POLINI and VESPTEC now offer considerably larger pan covers "airboxes". It is also possible to use the standard pan cover and insert a spacer ring between the pan and the cover (DRT); however, the increase in volume is not as pronounced as with an airbox.

Under an airbox, a so-called venturi intake funnel is usually used instead of the air filter. The combination of airbox and venturi often reliably produces a power increase of up to 1.5 hp compared to PIAGGIO parts; all this without any risks or side effects and is therefore highly recommended.

Intake path

The carburettor sump can be connected to the Vespa frame with a rubber hose, so-called "frame-suctioned". This intake system, which is also provided ex-works, is the quietest in terms of noise. Contrary to many rumours, there is no negative pressure in the frame, even at high speeds and with large engines. Special mention should be made of the plastic cable duct cover under the large-frame tank: if this is not fitted, pressure fluctuations can occur inside the frame at high driving speeds. The engine then surprisingly does not run in fourth gear at high speeds, although all other speed ranges are completely normal.

The air sucked out of the frame is relatively clean. An additional air filter can possibly be dispensed with in favour of a venturi funnel. Alternatively, there are air filters that attach directly to the front of the carburettor bowl. The advantage is that the intake path is shorter and the air can be transported more easily than from the frame of the Vespa. The disadvantage is that at this location near the cylinder the air is usually warm. In addition, this air can contain dirt swirled up by the rear wheel.

In order to seal the inlet correctly, the rotary slide cheek of the crankshaft should still cover the inlet sealing surface laterally by at least approx. 1 mm in every possible position.


Accessories, manual processing and functional errors

In addition to the parts required for tuning the SI carburettor and explained in the chapter "Adjustment", there are a number of accessories that are useful for standard engines and necessary for tuning engines.

Float chamber covers T5, T7 and T9

Standard float chamber caps have a fuel inlet hole about 2.5 mm in diameter. On other carburettors this would be called a "250 float needle valve". In certain cases (tuning) this may be too small. SIP offers in cooperation with DRT float chamber covers with 3 mm inlet and the larger float needle of the PIAGGIO Cosa. In addition, side holes make it easier for the fuel to flow in.

Supply line of the main jet

From the bottom of the float chamber to the main jet assembly, the fuel passes through a diagonal hole of approx. 2 mm diameter. This hole is usually too small in large engines and main nozzles. The fault is quite easy to notice, because the engine does not react to a further enlargement of the main jet above a certain size (e.g. above 140). It never gets "too rich" and even without a main jet it still runs.

Nothing helps here but to do it yourself: The soft material of the carburettor can be easily drilled out with a 3.0 drill bit.

Caution: Do not drill through the channel downwards out of the housing.

Caution: Do not damage the sealing surface of the float chamber cover with the drill chuck.

Side jets

Caution: Some manufacturers deliver them bent as standard.

Fuel hose

Nowadays there are fancy fuel hoses with an integrated metal spiral that can hardly be kinked. Original in connection with the Si carburettor is the very thick plastic hose, which has the property of hardening if it is installed for a longer time; this is also a kind of kink protection.

Correctly installed, the fuel hose is as short as possible and without a "siphon" for any air bubbles. To achieve this, there is only one correct assembly sequence: connect the overlong fuel hose to the tank, install the tank, insert the fuel hose into the carburettor sump while pulling it slightly out of the frame and then cut it to fit.

Fuel tap

SIP has put a lot of work into the development of its own fuel tap. The product is mature and outperforms all other competitors several times over in flow rate. Old petrol taps have to be replaced at the latest when they either leak or the filter network is corroded or dirty, so that there is no longer sufficient permeability. However, you can also make it easy on yourself and bring all parts of the fuel supply line to a good state of repair as a preventive measure. This is a quick way to achieve success, avoid getting grey hairs and losing your nerves unnecessarily.

Fuel pump

Can an SI carburettor be operated with a fuel pump? Yes, it works.

Diaphragm inlet and slot control

Can you use an SI carburettor on a modern engine with an inlet diaphragm? Yes, this is possible. LML diaphragm engines and repair diaphragms for PIAGGIO engines are examples. Special conversions with QUATTRINI M1XL and tuning cylinder PINASCO for Vespa GS160/SS180 also work.

The SI carburettor also works on slot-controlled engines ("piston control"). For example, there is a suitable conversion kit for most slot-controlled Spanish Motovespas. The advantage here is that the SI carburettors have a built-in choke. The Spanish UA and UB carburettors rely on a choke flap in the carburettor sump. If this no longer seals well (wear), there is no choke function. The engine is then very difficult to start.

Conclusion: The SI carburettor is a gem that can be polished into a sparkling diamond with the accessories available today.

Video: SI Carburettor Test VESPA 180 SS

André Jueterbock
André Jueterbock

Known from many videos, born with a spanner in his hand, a passionate racer and has been working as a product designer and manager for SIP Scootershop for years.