Carburettor for Vespa & Lambretta (2-stroke engines)

Created by Andrè Jueterbock at 14:05 on May 15, 2023

The carburettor ensures that fuel (petrol-oil mixture) and air are processed into an ignitable mixture. We hope we can give you some useful tips about the carburettor with our technical guide. If you have any further questions, our telephone customer service will be happy to help you, but you can also visit us directly at our Flagshipstore in Landsberg and take part in one of our events at the performance test bench, for example. There we also have a Lambda sensorwith which you can check your carburettor settings.

Tasks

Too rich a mixture: If the fuel content is too high, the engine loses power and wastes fuel because there is not enough oxygen for efficient combustion. The engine "four-strokes", i.e. it burns only every second, third or fourth revolution. A trained ear can easily recognise this noise.

Too lean a mixture: Engines that run too lean often sound very cleanly tuned, reach high revs and deliver high power. But this impression can be deceptive! Evaporating petrol also helps to cool the engine. If the mixture contains too little fuel, it burns at higher temperatures and at the same time no heat can be dissipated by evaporation. This is why too lean a set-up often leads to piston sticking or a hole in the piston.

The carburettor must provide the right mixture in every situation (depending on engine speed, throttle position, load and environmental influences). Therefore, the "tuning" or "jetting" of the carburettor, i.e. the carburettor setting (i.e. the use of components that exactly match the respective setup) is extremely important. A tuning that works at sea level will be a little too rich in the mountains, where the atmospheric air pressure is lower. This is why, for example, the tuning of racing engines is optimised for each individual track.

How it works

The carburettor makes use of Bernoulli's principle. This states that a fluid flows faster when passing a bottleneck and at the same time has a lower pressure. In the carburettor, this fluid is air, which flows through the tapering carburettor funnel (venturi). While the air flows quickly through the funnel towards the engine, a negative pressure is created at the narrow point. This sucks fuel from the various openings of the carburettor and enriches the passing air with it.

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Several different distribution channels for the fuel are provided in the carburettor:

  • Main jet

  • Needle and mixing tube

  • Secondary nozzle and, if applicable, secondary nozzle mixing tube

  • Choke

The above-mentioned four main channels plus slide geometry (angle cutaway) and idle mixture screw are necessary to always supply the engine with the correct amount of fuel at all slide positions, at all speeds, under partial load as well as under full load and despite different environmental conditions (temperature, air pressure, humidity). Unfortunately, the operating ranges of the individual channels sometimes overlap, so that setting them is not always easy.

We have published a series of videos on carburettor adjustment and put a lot of effort into creating them. Check them out, they can guide you on the way to the correct setting:

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The individual parts

1 Gas slider

The throttle slide regulates the air flow to the engine. Its position is determined by turning the throttle grip while driving. For some carburettor models, there are throttle slides with cutaways of different sizes on the underside. These are largely responsible for the amount of mixture that is made available when the throttle is opened.

  • No or small, flat cutaway (= low air content) = rich

  • Large, steep cutaway (= high air content) = lean

2 Float and float needle valve

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Float needle SI

In order to work evenly, the carburettor needs the fuel level in the float chamber to be as constant as possible. For this reason, it has a float and a needle with a rubber tip. When the fuel level is low, the float sinks and with it the float needle, exposing a hole in the float needle valve through which fuel can flow. If the fuel level rises due to the fuel running in, the float floats up, presses the needle into the valve and thus closes it again; the overrun is stopped. The valve must be large enough to fill the float chamber faster than it is sucked out by the carburettor jets.

A fuel pump can help to compensate for an unfavourable carburettor position or a tank that is too low (not enough gradient) and to cope with greater delivery rates. If the engine power changes noticeably depending on whether the tank is full or nearly empty, a fuel pump is advisable.

  • A larger float needle valve fills the float chamber more quickly, but is susceptible to shocks and tilting of the carburettor, and easily overflows the carburettor.

  • The heavier the float, the higher the level in the float chamber, the higher the petrol level in the mixing tubes of the carburettor, in simple terms: the richer the mixture in different channels of the carburettor.

  • The fuel level, i.e. the float height at which the needle valve closes, can be adjusted on many carburettors by slightly bending the clamp that connects the needle and float. Methods and binding specifications for adjusting the float height are different for each application, often only "try and error" helps, but sometimes valuable hints can be found in the documentation of the respective carburettor manufacturer. E.g. Dell'Orto carburettor tuning manual.

3 Slave jet

The secondary jet is mainly responsible for an even fuel supply at idle and when the slide is slightly open (from idle to about 1/3 or ½ throttle slide stroke). The number of the nozzle corresponds to the size of the hole for the fuel passage. E.g. a 52 nozzle has a hole with a diameter of 0.52 mm.

  • Large = rich

  • Small = lean

4a Mixture adjustment screw

Alternative to the air screw (see 4b). The adjustment screw regulates the amount of fuel in the mixture when the throttle slide is closed and at idle speed. Most Dell'Orto PHB/SHB carburettors have a mixture adjustment screw. Depending on whether the screw regulates the fuel or air content, turning the screw out, for example, results in a richer or leaner mixture. This is where the fuel portion is determined:

  • Turning the screw out (more fuel) = a richer idling mixture

  • Turning the screw in (less fuel) = leaner idling mixture

4b Airscrew

An alternative to the mixture adjustment screw (see 4a). The air screw regulates the amount of air in the idle mixture. If a carburettor (e.g. MIKUNI TMX) has an air screw, it is located BEFORE the throttle slide.

  • Turn the screw in (less air) = grease the idle mixture

  • Unscrew the screw (more air) = lean the idle mixture

5 Choke

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Choke nozzle SHB

The pulled choke provides an extra rich mixture for the cold start of the engine. With some carburettors, the size of the choke nozzle in the carburettor can be changed.

  • Large nozzle = rich mixture when choke is pulled

  • Small nozzle = lean mixture when the choke is pulled

The choke function only has an effect when the choke is pulled. However, it can also be used as a test. An engine that is considerably too lean experiences a noticeable increase in power when the choke is pulled.

6 Main jet

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Main jet PHB 6mm

The main jet should adjust the petrol content in the air-fuel mixture, especially when the slide is fully open. Unfortunately, there are very big differences here depending on the type of carburettor:

  • Dell'Orto carburettors with flat or round slide and needle/mixing tube (PHB, VHB) are made to let the main jet (HD) act exclusively from 3/4th to full throttle slide stroke. Here it is recommended to tune all other elements of the carburettor completely and only add a main jet last. The needle range should be adjusted here exclusively by the shape of the needle and the size of the mixing tube.

  • SI carburettors from Dell'Orto work in exactly the opposite way. Here, all other elements of the carburettor depend on the main jet. That is why you have to start the adjustment work here by finding the right main jet (full throttle).

  • Mikuni and Keihin carburettors (as well as the cheaper replicas, e.g. from Koso and Polini) work differently from Dell'Orto and also regulate the flow into the mixing tube via the size of the main jet. Here, the mixing tube and needle cannot be isolated. Here the main nozzle has an effect from half throttle.

  • Large number = rich

  • Small number = lean

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7 Mixing tube

Before the mixture is discharged into the carburettor funnel, the petrol provided by the main jet is premixed with air in the mixing tube to break it up into smaller particles. The Italian name for mixing tube is "atomiser" and explains the function more clearly.

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PHB mixing tube

On many Dell'Orto carburettors, the diameter of the mixing tube can be selected to allow more or less passage (= richer or leaner) between the needle and the mixing tube inner diameter. Mikuni and Keihin control this passage via the main jet, which is screwed onto the bottom of the mixing tube.

  • Large mixing tube diameter / thin needle = rich

  • Small mixing tube diameter / thick needle = lean

The length and shape of the upper part of the mixing tube and how far it protrudes into the carburettor funnel also have a major influence. If it protrudes far, less vacuum is sucked into the mixing tube, if it protrudes less, more pressure is sucked into the mixing tube. Small-volume engines (e.g. Smallframe 144 with 38 carburettor) therefore use Dell'Orto DP mixing tubes (short), large-volume engines with small carburettor DQ (e.g. BFA 306 with 30 carburettor). Whether or not the right mixing tube is selected is most noticeable in the revs BEFORE the resonance range.

Exception:

The mixing tubes in the SI carburettor of many Vespa engines are a major exception. These have a completely different effect! There is no needle that is related to the mixing tube, instead the various lateral cross bores, their size and number play a role. The further the holes continue downwards (up to four rows of holes, "floors"), the deeper the air can penetrate into the petrol level, the more the fuel is pre-mixed. Caution, this can lead to a leaning effect. The amount of admixed air is determined by the main air correction nozzle on Si carburettors. (For more information on the SI carburettor, see separate blog post)

8 Needle

The mixing tubes of modern carburettors (non-SI) work in conjunction with a cone-shaped needle attached to the throttle slide. This connection regulates the flow of fuel when the slide is partially open. The needle usually has a clip on its thick end, the position of which determines how far it dips into the mixing tube.

The clip positions are usually numbered from top to bottom (lean > bold). The top position is often referred to as "T1". There are needles with different diameters of the cylindrical part (=Ø A), different lengths of the tapering part (=C) and different diameters of the tip (=Ø B). Mikuni specifies the angles of the cone instead of the diameters.

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The cylindrical part of the needle determines the mixing ratio when the throttle slide is approximately ¼ open. The larger Ø A, the leaner the mixture.

Between ¼ and ¾ open slide, the diameters of the tapered part of the needle determine the mixture composition. If A and C have the same value, the larger Ø B, the leaner the mixture.

If both diameters (A and B) remain the same, a change in length C affects at which point (throttle slide stroke) the mixture is enriched. The longer C, the earlier the enrichment phase begins. Also by changing the clip position on the needle, the area C / tapered part of the needle, comes into effect sooner or later.

  • Clip down = cone pulled far out of the mixing tube = large passage = fat

  • Clip on top = thick part of the needle in the mixing tube = small passage = lean

  • Thin needle = fat

  • Thick needle = lean

Overview needles

All major carburettor manufacturers have published directories of available needles with all relevant dimensions.

Example:

If an engine works very well in the needle range at 1/3 throttle slide opening, but four-strokes at 2/3rd throttle slide opening, it is too rich at 2/3rd throttle. At 2/3rds throttle slide lift, the tapered part of the needle is engaged in the mixing tube. Here the flow appears to be too great, the mixture too rich. To reduce the flow, a needle with a thicker tapered part is required for the same mixing tube.

So you have to find a needle with the same diameter A but a larger diameter B from the manufacturer's needle list.

Main air correction nozzle and combined auxiliary nozzles

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Main air correction jet

The Dell'Orto SI carburettors are in many ways different from conventional carburettors. For example, they are equipped with a main air correction nozzle (HLKD) on top of the nozzle block above the mixing tube. Similar nozzles are also known from the small Polini CP carburettors. Such a correction nozzle is like a kind of by-pass, a controlled leakage of the fuel supply channels. The larger the opening of this by-pass, the lower the negative pressure in the fuel supply channels.

  • Large HVAC = lean at low RPM and small slide opening

  • Small HVAC = rich at low revs and small valve opening

The situation is similar with the combined secondary jets of the SI carburettors. In addition to a small hole for the fuel passage, these also have a larger hole in the upper part, which represents a side-nozzle air correction according to the same principle of the HLKD ..

We have prepared a separate blog article for you on the technology of SI carburettors. Please find further information there.

Powerjet nozzle and accelerator pumps

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MIKUNI Powerjet Nozzle

Some carburettors have another circuit that draws petrol directly from the float chamber through a hose into the carburettor funnel. This circuit can be influenced either by the corresponding nozzle in the carburettor funnel (often at the top) or by an adjustment screw. If the nozzle is located above the slide valve, this means that the petrol circuit is only activated when the air flow speed in the carburettor is very high: this is the case when the slide valve is wide open and the engine speed is high. The Powerjet nozzle can be combined with a slightly smaller main nozzle to achieve better mixture formation at low revs and still have enough fuel available at high revs.

  • Large powerjet nozzle = rich at high revs / wide slide opening

  • Small powerjet nozzle = lean at high revs / wide slide opening

accelerator pumps are known from four-stroke carburettors, where they inject fuel into the intake manifold at fast throttle slide strokes. We are not aware of this function being necessary or desired in two-stroke engines.

Which carburettor for which application?

It is difficult to state the ideal carburettor size for a particular engine size or tuning level. Realistically, it depends less on the power of the engine and more on how you want to use it:

  • Standard size: if one wants an economical, durable engine with a decent air filter system (quiet noise), the original carburettor or a suitably tuned carburettor that largely matches the engine in question is a good choice. For example, if you run a 133 cc cylinder on a 50 cc smallframe, you can use the standard (19 or 20 mm) carburettor and intake manifold of the 125 cc smallframes. A standard carburettor will of course not allow the scooter to reach peak performance, but a 210 cc Malossi cylinder and a 24 SI carburettor, for example, will still give a reliable setup that looks absolutely original and can still deliver over 20 hp. A relatively small carburettor on a large-volume engine is usually always quite tunable and therefore rewards with a good running culture.

  • 24-30 mm: For scooters >125 cc, you will definitely find a good compromise between performance, consumption and everyday usability in this range. The larger the carburettor diameter is chosen in relation to the engine's displacement, the smaller the vacuum effect that occurs at low revs. Accordingly, it becomes more difficult to jet the carburettor in such a way that good running quality is nevertheless achieved in all speed ranges and that, for example, no stalling occurs at the fuel supply ducts.

  • > 30 mm: As long as the carburettor is not much larger than the intake manifold on which it sits, a large carburettor will release significantly more power than a small one, especially in the medium and high rev ranges of high-end engines. However, this can be at the expense of a clean mixture in the low rev range on the one hand and is also reflected in consumption on the other. Because where more air flows, more petrol flows ...

Carburettor types

  • Downdraft carburettors: Large-frame carburettors such as the SI series from DELL'ORTO, in which the air flows vertically.

  • Drum carburettors: These carburettors - e.g. PHB and SHB carburettors - are simple in design, very reliable, easily adjustable and inexpensive to manufacture. Very many two-wheelers are equipped with them at the factory.

  • Flat slide carburettors: These carburettors were developed to minimise the cavity under the slide as well as the carburettor length. This avoids turbulence and the short length favours a high resonance speed. They therefore usually perform better than barrel carburettors of the same size, but are sometimes also more precisely tunable because they contain, for example, an additional auxiliary jet mixing tube. e.g. Dell'Orto VHB, Mikuni TMX, Keihin etc.

  • Oval bore: If the venturi of a carburettor is higher than wide, this is often referred to as an oval bore or "smooth-bore". This has the advantage that the venturi area is progressively laid out as the slide opening increases. This combines the advantages of smaller carburettors (clean mixing with the slide only slightly open) with the advantages of large carburettors (greater volume flow). Another advantage of the non-circular opening is that sound waves are refracted, thus partially avoiding a megaphone effect for intake noise. Modern variations also use other funnel shapes (heart, or similar) to further enhance this effect, e.g. Dell'Orto VHSB 34 QD.

Intake manifold

The intake manifold is the connection between the carburettor and the engine. A distinction is made between rotary slide intake manifolds, where the crankshaft determines the opening times, and diaphragm intake manifolds, where the vacuum in the crankcase opens the diaphragm plates. Intake via the crankcase usually means a long intake run, but this is good for high torque and can be a good choice for touring engines because the crankshaft and connecting rod bearings are optimally lubricated.

A diaphragm intake is more than a repair option if the rotary intake on the crankcase is damaged. The TASSINARI diaphragm with eight flaps, for example, offers optimal flow conditions and setup possibilities for high-end tuners. The beauty is that the control times of the diaphragm plates are more or less automatically adjusted to the negative pressure in the crankcase. This results in enormously high-torque engines which, however, also deliver enormous peak power at high engine speeds without the tuner having to worry about control times or timing cross-sections.

A direct intake manifold sits directly on the cylinder, which offers the advantage of a short intake path and very direct response. Direct intake manifolds can be combined with intake diaphragm or piston or slot control. Slot control was standard equipment on Vespa up to the 180 Supersport, but has gone out of fashion because, for example, large fresh gas losses occur (consumption) and the lubrication of the connecting rod bearings could only be guaranteed with large quantities of oil (mixture 1:20).

Another special form is the double intake manifold, which makes sense in some engine layouts and ensures optimal filling.

Air filter

The first task of the air filter is - nomen est omen - to filter the intake air from dirt particles. Large particles can immediately cause serious damage, e.g. in the cylinder, while small particles such as sand dust can cause premature wear of crankshaft and connecting rod bearings. The shape of the air filter or even the intake funnel can influence the flow of the gasoline mixture, and an air filter also contributes significantly to noise reduction.

Petrol & Oil

Petrol is the lifeblood of the engine and what vitamins are to us humans, octane and additives are to the engine. Nevertheless, we recommend 95 octane fuel "Super Petrol" for two-stroke engines. The premium fuel varieties "Super Plus" with 98 and even 100 octane contain fire retardants that help improve the efficiency of four-stroke engines. However, our two-stroke engines burn twice as much fuel as four-stroke engines in the same amount of time and have only about a quarter of a crankshaft revolution to do so, which means that for two-stroke engines it is important to have a very high burn-through speed. This is easier to achieve with Super than with Super Plus fuels.

The new E10 fuel can damage the sensitive shaft seals due to the aggressive ethanol it contains. If you want to be on the safe side, we advise against using it on old scooters. Upon request, the manufacturer PIAGGIO has only approved E10 fuel for vehicles built after 2000.

A particularly important component for two-stroke engines is the oil used. Partially or fully synthetic oils are always preferable to simple mineral oils. At the petrol station you can often find lower quality oils at inflated prices. With our SIP Formula products, we offer very high-quality oils at permanently favourable prices. Our opinion regarding environmental protection and emissions, with which we not only burden ourselves but also our fellow human beings and fellow drivers, is: It is better to use a little of a very good oil than a lot of less good (cheap) oil. In our experience, with today's quality of oils and crankshaft bearings, there is no longer any reason to drive with mixture ratios of over 1:50. With this small active contribution, we hope to maintain our good reputation as scooter riders for as long as possible and not be driven out of the inner cities as stinkers.

Andrè Jueterbock
Andrè 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.

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