A carburettor basically consists of an open pipe, a 'throat' or 'barrel', through which the air passes into the inlet manifold of the engine. The pipe is in the form of a venturi. It narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part.
Below the venturi is a butterfly valve called the throttle valve — a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so that it almost completely blocks the flow of air. This valve controls the flow of air through the carburettor throat and thus the quantity of air/fuel mixture the system will deliver, thereby regulating engine power and speed.
The throttle is connected, usually through a cable or a mechanical linkage of rods to the accelerator pedal.
Fuel is introduced into the air stream through small holes at the narrowest part of the venturi. Fuel flow in response to a particular pressure drop in the venturi is adjusted by means of precisely-calibrated orifices, referred to as jets, in the fuel path.
As the throttle is progressively opened, the manifold vacuum is lessened since there is less restriction on the airflow, reducing the flow through the idle and off-idle circuits. This is where the venturi shape of the carburettor throat comes into play, (i.e., as the velocity increases, pressure falls). The venturi raises the air velocity, and this high speed and thus low pressure sucks fuel into the airstream through a nozzle or nozzles located in the center of the venturi.
This is the same principle as the LPG induction Mixer, which also has a venturi. With a mixer the fuel ports are a series of slots or small holes around the inside of the venturi. As the air rushes through the venturi it causes a depression on these slots, which in turn pulls the diaphram inside the vaporiser/reducer, there by increasing the amount of gas flow.
Twin Carb V8 with LPG plate mixers fitted
This engine is fitted with K&N style air filters, which I would not normally recommend using with LPG. Due to the high air flow of these types of filters it can cause a problem. Namely that the vaporiser cannot keep up with the increased air flow and the engine will run lean. However in this case I fitted a lambda control system which allowed me to monitor the mixture. By doing this I could see that the lpg system was coping.
The risk of running an engine to lean is a real one. Lean fuel mixtures cause excessive heat which can damage the engine, particularly the valves, valve seats and heads over time.
The carburettor engine is the most straightforward to convert to LPG/autogas. There are very few wires to connect and even less components! It's a good engine to convert if you have not had much mechanical or LPG experience, but it is not without its pitfalls. The system still needs to be installed correctly. Pay attention to the mixer/s, make sure it/they are fitted the correct way round and that there are no air leaks where they connect to the carb/s.
Any air entering the engine that has not been through the mixer will cause running problems and a lean mixture. Most kits supplied for carb engines will be open loop, which is ok, but you will never have a good mixture throughout the hole rev range. Consider fitting a Lambda control system with a retro fit probe in the exhaust. This is what I do with all the older engines that I convert. It gives you a good mixture throughout the rev range, better mpg and a more responsive drive.