Before electronic fuel injection made its appearance, engines relied on mechanical carburettors to mix the air and fuel required for combustion. Weber carburettors were manufactured in Bologna, Italy by Magneti Marelli from 1920. If your engine is fitted with a Weber carburettor you can easily ensure your engine is set up to run at optimum performance. A Weber's simple design makes it easy to ensure the engine is tuned correctly and anyone, with a basic understanding of automobiles, can adjust them.
Start and run the engine until it is at normal operating temperature. Open the bonnet to gain access to the carburettor . You may have to remove the air filter, depending on the model of vehicle you own, to make access to the carburettor easier.
Unscrew the idle speed screw until the idle stop screw disengages with the throttle stop lever. The idle speed screw is located on the left side of the carburettor. The throttle stop lever has a flat paddle that, when depressed with the thumb, increases the engine speed.
Turn the idle mixture screw, located to the right of the carburettor, clockwise with a flat head screwdriver. The screw is located towards the bottom of the carburettor and controls the fuel air mixture when the accelerator is not being activated. The engine will begin to sound like it is about to stop as the ratio of air to fuel rises.
Rotate the idle mixture screw counterclockwise slowly until the engine begins to run smoothly. Turn the screw an additional 1/4 turn counterclockwise then turn it clockwise. The aim is to get the mixture as lean -- the lowest mixture of petrol to air -- as possible with the engine running smoothly.
Adjust the idle speed screw above and to the left of the idle mixture screw with the flat blade screw driver. Rotate the screw clockwise to advance or speed up the engine and counterclockwise to retard or slow it down. Only make 1/4 turns at a time and aim to get the engine idling at between 900 and 1000 RPM.
Re-adjust the idle mixture by adjusting the idle mixture screw with the flat head screw driver. Try to make sure the mixture is as lean as possible with the engine still running smoothly between the 900 and 1000 RPM range.
Switch on the heater blower or air conditioning to its maximum power to cause a slight strain on the engine. The revolutions per minute should keep between the 900 and 1000 RPM range and the engine should still run smoothly. If it does not, adjust the idle mixture screw by 1/4 of a turn counterclockwise.