Carburetor Basics

The Basics of a Carburetor

A little more than a decade ago, cars were not so complex as they are now. In those days carburetors were popular and the few fuel injected engines were also pretty simple, those systems could be repaired by anyone with a little common sense.

Today however the scenario is much different. The ten years when CNG became an average Pakistani’s fuel of choice caused the art of carburetor tuning to be lost. Now with CNG becoming scarce and expensive (not to mention what an ordeal it is to actually get it) people are reverting back to petrol.

Some people are buying the new fuel efficient electronic fuel injection vehicles, while others are trying their luck with repairing their carburetors.

Finding  a good carburetor mechanic is like finding a needle in a haystack. having spent three months in trying to “buy” a solution to my car’s carburetor problems proved futile. In the end the carburetor was replaced with a “kabli” unit and installed by a 3S dealer. Surprisingly they could’t even get the vacuum pipe connections correct and so the car was still running pathetic with the new carburetor.

In the end the carburetor was fixed by the help of some well experienced PakWheelers. The old one is now undergoing surgery to fix it. Surprisingly these devices are not as difficult to fix as it seems, mechanics just exaggerate it. All it takes is a screw driver set, a set of spanners, some WD-40 and a good work space.

Carburetor Diagram

How It Works

The Fuel Injection System

The fuel inlet is opened or closed by a needle valve which is controlled by a float (which sometimes develops leaks and causes the air fuel ratio to go haywire). When the preset fuel level has been obtained in the float chamber, the needle valve closes.

From there the fuel goes to fuel emulsification circuits (usually in the air horn) and the two jets (one for low speed and one for high speed). At idle speed only the fuel emulsification circuits work (so even blocked jets can’t stop your car from idling, unless the circuits are also blocked).

Contrary to what most people think, the jets do not do the job of making a spray of fuel, like an injector of an efi engine. The jets are screwed in at the base of the venturi. The venturi does the spraying.

The fuel from both, jets and the fuel emulsification circuits goes through the venturis.

Though for that moment when you suddenly floor the gas, there is the accelerator pump, which relies on vacuum to suck petrol directly from the float bowl and inject it into the intake. (the hiccough before acceleration is usually caused when this goes bad).

Finally there is the power-valve which enriches the air fuel ratio (mixture) at higher RPM.


The Vacuum System

The carburetor contains small holes in the barrels which provide vacuum for various parts. The vacuum is used to move the power piston in the air horn, it is also used to advance the distributor timing depending on the rpm and it is used to by the dashpot to make sure that the RPMs don’t drop too quickly when you let go of the gas pedal.

There are also vacuum ports on the intake manifold which provide vacuum for the vacuum pods that increase RPM when you switch on the headlights or the air conditioner.

In a nutshell, that is all there is to it. The weird names make it seem scary but it is not difficult at all, you just need patience to fix one of these.