Valvoline.com

Removing and Replacing a Carburetor

The devil's in the details

Back in the days of carbureted and plentiful used Plymouth Satellites, B-52's frontman Fred sang of the devil in his car. Beehive-sporting singer Kate did Fred one better by wailing that she had the devil in her CAR-buretor! This is, of course, not at all a good thing.

Before modern multipoint electronic fuel injection became commonplace, the carburetor was the king of supplying fuel and air in the automotive world. The correct mixture of fuel and air is what gives an engine the power it needs to turn the wheels across all manner of speeds and situations. From a sleepy idle to a tire-scorching burnout, the mixture of fuel and air is the key to all the action. The carburetor mixes up fuel and air by employing what is known as Bernoulli's principle by way of a venturi.

The automobile internal combustion engine is essentially a big air pump. The mixture of fuel and air entering the engine is compressed by the pistons of the engine and lit aflame to make power. The carburetor takes advantage of the air moving into the engine to mix the liquid fuel into the air and create a vapor of fuel and air mixed together. It does this using a venturi, and the miracle of Bernoulli's principle, which states in a nutshell that air becomes less dense the faster it moves. As the air moves through the throat of the carburetor, the vacuum created draws fuel into the air to, in theory, produce the ideal fuel-air mixture based on engine demand. The carburetor venturi amplifies this effect by squeezing the incoming air into a tighter space, increasing velocity and thus vacuum. Simple right? In theory maybe, but in practice, no.

Mixed Nuts

The reason things get complicated is that the carburetor has to, at some points, be as calm as a meditating Tibetan monk; yet, at other points it has to perform with the frenetic energy of an Olympic sprinter. Sometimes the engine demands very little, whereas at other points it demands everything. On top of mixing the fuel and air in the right measures, the carburetor has to do this according to engine demand. In a sense, the carburetor is a liquid analog computer. While multipoint fuel injection mixes the ideal amount of air and fuel together by way of various electronic sensors and a digital computer, the carburetor achieves this same task mechanically.

Through a series of precisely engineered air passages and channels, needles and seats, check balls, and float levers, the carburetor feeds the correct amount of fuel into the incoming air regardless if your foot is off the throttle at a red light, or attempting to put it through the floorboards when that same light turns green. The devil is of course in the details. Anything with that many moving parts is bound to be a bit of a cantankerous device.

As the end of the carburetor-era approached, this complexity reached almost mythical proportions. Removing the air cleaner of a mid-Eighties carbureted vehicle is the mechanical equivalent of looking Medusa right in the eyes. The sheer number of snakelike vacuum tubes can paralyze even the seasoned mechanic. Adding to the operating complexity of a carburetor is that it is bolted down to the top or the side of a perpetually shaking engine that goes from stone cold to two hundred plus degrees and back again all the time.

Gummed Up

This torturous environment wreaks havoc on the carburetor. Fuel left over when the vehicle is turned off combined with engine heat conspires to leave a thin film behind. Over time this coating can become thick and crusty enough to create blockage in the tiny passages that transfer air and fuel through the carburetor. Check balls can get stuck. Vacuum pots can rupture. Problems can start to manifest themselves in the form of poor idle, loss of power, and coughing or hesitation. For these reasons, when it comes time to service a carburetor, the best advice is often an outright R&R, or remove and replace. If ships in a bottle, three dimensional labyrinths, and Chinese finger puzzles are your idea of a smashing good time then carburetor rebuilding may be the right choice.

A remanufactured or new carburetor from carburetor professionals like Holley may be a better bet if bringing back peak engine performance in a hurry is more your speed. The most important tool in carburetor replacement is a service manual for your vehicle. While a malfunctioning carburetor can cause the aforementioned symptoms, it is of vital importance to troubleshoot all other possible causes before deciding to bolt on a replacement. There is also the matter of adjustment. Even though Holley has built countless numbers of new and remanufactured carburetors going all the way back to the 1890s, all carburetors leave the factory adjusted with baseline settings. This is true of any remanufactured or new carburetor.

Adjustment of idle speed and fine-tuning of the choke or other systems may be required after installation. A service manual will clearly outline these post-installation procedures. That said, there are universal carburetor installation tips. Safety is the first! Gasoline is flammable. Always disconnect the battery and have an approved fire extinguisher nearby before beginning any work involving gasoline or other flammable fuels.

Resource

http://www.holley.com