Before trying to repair or replace a carburettor, it's important to analyse the symptoms to determine the cause of the problem. A carburettor is not very complicated. Its main purpose is to deliver the correct blend of fuel and air to the open throttle. Over time, normal wear and tear can cause the carburettor to malfunction.
Carburettor problems are usually caused by a blend that's too lean, a blend that's too rich or an incorrect adjustment. The symptoms are generally quite easy to diagnose.
A Blend That's Too Lean
When the blend of fuel and air is too lean, the carburettor is delivering too much air and not enough petrol. Symptoms include backfiring, jumping when accelerating and a whitish colour on the tail pipes. Discolouration can also occur on the spark plugs and exhaust pipes.
Lean mixtures are usually caused when the owner installs after-market parts or accessories like an air filtering system, exhaust system or a replacement carburettor that are the wrong type or size. A lean mixture can also occur when the fuel level is set too low in the float chamber, not allowing a sufficient amount of fuel to be drawn into the jets.
In some carburettors there is a slow speed fuel adjustment screw to regulate the air and fuel mixture at lower rotations per minute. This screw can be adjusted by simply turning it clockwise. This will allow less air to enter the carburettor, thereby enriching the fuel. When in doubt, refer to the manual for the proper setting.
A Blend That's Too Rich
When the blend of air and fuel is too rich, the carburettor is delivering too much petrol and not enough air. Symptoms include poor fuel economy, a strong smell of petrol when idling and extra sooty spark plugs and tail pipes. The driver will generally notice the vehicle not responding as it should and may stall when stopped at a red light.
This problem is usually caused by clogged or dirty air filters. In some cases, it is a result of the owner improperly replacing all or parts of the exhaust or carburettor system. A rich mixture will occur if the fuel level in the float chamber is set too high. This problem can be fixed by correcting the float level and replacing the air filters.
If there haven't been any changes made to the car, and it was running fine before, a lean mixture might be caused by a leak in the exhaust or a leaky inlet manifold. The leak is often located where the cylinder head and header pipe meet.
There is a screw on the carburettor to control the flow of fuel and air. An incorrect adjustment of this screw can produce the same symptoms as a rich or lean mixture. On a multi-cylinder motor, an adjustment problem can cause poor performance in general, a rattling in the clutch and frequent stalling. The motor may also misfire and not accelerate properly.
An incorrect adjustment on the carburettor generally occurs when it isn't maintained properly. The engine vibrates when it runs. This vibration causes certain parts in the engine to rotate. Adjusting screws are the most common parts to vibrate out of position. Balancing screws on cylinders and running jets should be checked periodically to ensure that they are all still in the correct position.
Carburettors can usually be fixed, but if none of these solutions work, than the carburettor may have to be replaced. Since it is a simple device, replacement is as simple as removing the old one and installing the new one. All of the parts you need can be found through online retailers.