Scooter and moped CarburetorThe carburetor is one of the central parts of a scooter or moped. It has the important task of producing the fuel-air mixture that the engine is supposed to burn. In short: Gasoline is atomized in the carburetor and a certain amount of air is supplied. While it was replaced in the automotive industry by complex injection systems, the carburetor has kept its place on the scooter and moped. In the scooter scene people also like to set on Tuning Carburettor with a high speed mixture preparation. The sporty carburettor kits come with adapted air filters and intake manifolds as a matched set, depending on the version.
The mix matters!It quickly becomes clear that incorrect mixture preparation leads to poor combustion in the engine and also causes starting problems. Ideally, the engine of a scooter or moped gets a fuel mixture with 14.7 parts air and one part petrol. This mixture ratio corresponds to a Lambda value of 1.0. If this value is higher, there is more gasoline in the preparation and one speaks of a "rich" mixture. If the value is lower, the proportion of air is correspondingly higher and we speak of a "lean mixture". Both have negative consequences. If the mixture is too rich, often the exhaust gases from the Exhaust still clearly visible as light smoke clouds even with warm engine. The reason for this is unburned fuel. For the scooter, this means poorer performance and increased fuel consumption. This is not only bad for the wallet, but also damages the environment. If the mixture is too lean, however, there is too little fuel. Since most scooter engines are lubricated by fuel at the same time, a too lean mixture also means a lack of lubricant 2-stroke oil, which is added directly to the fuel). This can lead to "piston seizure" and premature engine death. This risk should not be underestimated and, in case of doubt, it is better to visit the specialist workshop of your choice once again.
Design and function of a carburettorThe carburetor is a complex component consisting of many, very precisely manufactured individual parts that have to meet high requirements. Essentially these parts are the nozzle needle, needle nozzle (main nozzle), idle nozzle, slider, throttle valve, float and choke. These components are usually made of brass and aluminium and should be handled with great care during conversion work, cleaning, etc. The components are usually made of brass and aluminium. Basically, it is not advisable to process the parts yourself, e.g. by grinding or filing.
The most important partsMain nozzle
The size of the main nozzle determines the mixture composition in the partial and full load range. Depending on the carburetor model, different nozzle sizes are used. To optimize performance and consumption, the mixture composition can be positively changed by replacing the main nozzle. Since each carburettor is differently worked in and no scooter resembles the other, may apply sometimes: The proof of the pudding is in the eating. With a Naraku main nozzle set for Gurtner or Dellorto carburettors and others you have a suitable set for the optimization at hand. If the suitable nozzle size is already known, you can get the corresponding Main nozzle also individually, for example from the well-known manufacturer 101 Octane.
Idle flow nozzle
Here's the name program. The idle or auxiliary nozzle is responsible for mixture preparation in idle gas, but also acts in the partial load range. For optimum performance of the scooter, it is therefore not only the correct setting at the main nozzle that is decisive, but also that of the idling mixture. If, for example, the scooter is running too lean, it will not only start heavier, but a rotary hole may also occur alternately from the partial load to the full load range.
Since the gate valve is constantly in motion due to air currents in the intake duct and alternating throttling, it is considered a wear part. If grooves have formed which impair the smooth running of the component, the gate valve should be replaced.
The same applies to the nozzle needle. By sitting on the main nozzle, a slight heel or ring can form and the needle can deform. If the carburetor needle is faulty, the mixture preparation is no longer correct. In this case, the needle must be replaced.
Due to the constant negative pressure, air flows continuously through the intake tract. To prevent this and to control the air flow, there is a throttle valve. Depending on how much throttle is applied, it opens further or closes.
The float is a small "balloon" made of metal which floats in the float chamber on the surface of the fuel in the chamber. A float needle is attached to it. When a certain level is reached, the needle closes the fuel inlet from the fuel tank. Fuel can no longer run after and overfilling is prevented. If the level in the chamber drops, the inlet opens again. The float thus has the task of maintaining a certain level in the chamber. If the level is too high, the mixture becomes too rich and the engine can "sink". If the level is too low, the engine will quickly run too lean and can suffer severe damage. Changes to the float should always be avoided.
Behind the English term "choke" lies the answer to what happens during a cold start with the choke pulled. Translated it means "suffocate" or "choke". The air passage is mechanically reduced via the choke and the suction in the air duct increases. This means that more fuel is sucked in and the mixture is enriched, i.e. richer.
Short explanation: This is how the vacuum carburettor works on the scooter and mopedAir is sucked through the intake tract to the engine by negative pressure. On the way there, fuel enters the carburettor through the holes of the idle and main nozzles and is carried along by the vacuum suction. In the carburettor, the so-called slider protrudes into the air duct. It is pushed over the Gas cables which leads to the throttle. The nozzle needle is attached to the underside of the slider. By turning the throttle grip, the slider and needle are lifted and the proverbial "throttle" is activated. Due to its conical shape, the needle gives the nozzle hole depending on the turn at the throttle grip more or less free. The resulting gap increases the amount of incoming fuel according to the lift. If no throttle is applied, the main nozzle is closed by the needle. The engine would die completely without fuel, but finally there is the way via the smaller idle nozzle, which always sucks fuel into the combustion chamber.
Wearing parts, correct setting and carburetor cleaningA well adjusted carburetor not only protects the engine, but also provides more power. Almost every scooter and every moped can do a few km/h more without any tuning, just because all components work properly. In order for the carburettor to do its job in the best possible way, the individual parts must first be in good condition and must not show any major signs of wear. A precise visual inspection often helps further, but some faults cannot be seen by the eye. A run-in main nozzle, which has been hollowed out by the constant flow of petrol, can be seen with a nozzle gauge. At a higher mileage all parts have already worked properly and depending on the condition it can be cheaper to use a carburetor Repair Kit of 101 Octane than to buy more expensive parts one after the other for the carburettors of Naraku, Keihin or Vicma. But not in every case there is a defect.
Particularly after longer periods of inactivity and the winter break, old fuel can become resinified in the carburetor and clog nozzles and ducts. Basically there are two ways of cleaning: First, if the scooter's engine is running badly, you can use a fuel system cleaner such as the Motul Fuel System Clean Moto to use. Experience has shown that the use of such a cleaner is often sufficient to eliminate carburettor problems due to contamination. The clear advantage is that the cleaner is added to the fuel as an additive. A removal is not necessary. If the additive does not help, the carburettor must be removed and cleaned. Although it is quite complex, but who has the technical knowledge, the carburetor should also disassemble into its individual parts. Contamination can be removed with a suitable cleaning agent, for example the carburettor cleaner. P1 Carbu Clean by Motul and a fine brush can be removed carefully. On this occasion the parts can be checked for wear.