Ode to the Carburetor

Once upon a time, there was a simple mechanical device that fed fuel to your car’s engine. It had no electrical hook-ups, was not dependent on sensors nor controlled by a computer. It was held in place by four bolts and could be physically removed from the engine in less than five minutes with basic hand tools. Its workings were visible — and so, comprehensible.

This miracle device is the carburetor.

You can still find them, too — just not under the hood of any car built since the late 1980s. Lawnmower engines, weed whackers and other outdoor power equipment still use them. Probably not for long, though. And here we come to the why carburetors are no longer found under the hoods of new cars:

They are mechanical devices and cannot adjust themselves to deliver precisely the right amount of fuel (and air) to the engine under constantly varying conditions. This is a problem from both an emissions control and a fuel efficiency point-of-view. The carburetor is jetted to meter so much fuel to the engine. The jetting can be rich (more gas) or lean (less gas) but it cannot be both at the same time.

Nor in between.

The jets — small orifices through which the liquid gasoline flows — are a given size (diameter of the orifice) and that’s that. In some of the more sophisticated carburetors — like GM’s famous Rochester Quadrajet — the flow through these jets can be modulated by tapered metering rods that move up and down within the orifice, increasing (and decreasing) the amount of fuel that can flow through the jets. But the range of metering adjustment is mechanically limited; the extremely fine control possible with fuel injection is not possible with a carburetor.

And there is another problem.

In a carburetor, liquid gasoline is pumped (via a mechanical fuel pump, usually) into a small holding tank called a fuel bowl inside the carburetor. Some of this fuel inevitably leaks into the engine, where it dilutes the oil — reducing its ability to lubricate internal parts and also shortening the useful life of the oil (necessitating more frequent oil and filter changes to avoid premature/excessive wear and tear).

Some of the fuel also leaks outside the engine — evaporating into the air.

These are evaporative emissions — and while the smell of gasoline in the morning is a fine thing indeed, it’s a big no-no these days. And it’s not a fixable issue with carburetors, because their design requires they be vented — open to the atmosphere.

Fuel sitting in the bowl is sucked through the carburetor’s jets by the vacuum of the running engine. Air is drawn through the carburetor and mixes with fuel; the resultant air-fuel charge is “inhaled” by the engine through an intake manifold (the thing the carburetor sits on) into the individual cylinders.

In a fuel injected system, the fuel is injected under positive pressure into the engine. Modern systems have injectors positioned directly ahead of each cylinder’s intake valve(s). Air is still “inhaled” by vacuum but it is not mixed with fuel until it reaches the cylinders.

The gas side of an FI system is effectively sealed; no raw (liquid) gas is present and so evaporative emissions are much lower. This was among the main reasons for the demise of the carburetor. Even if jetted perfectly, gas fumes are still going to escape into the air.

The Dude could abide this — but Uncle not so much.

Which is a shame. Because all cars now sound pretty much the same.

Intake-wise, at least.

There is still the hiss of vacuum. But the signature moan of a Quadrajet’s massive secondaries opening up when you floored the accelerator pedal is as much a thing of the past as the steam whistle. Fuel injection (especially port-fuel injection, which is what’s common these days) is all pretty much the same. Each cylinder has its injector, which sprays the fuel. There may be differences in capacity/pressure, but none of these are aesthetically meaningful. Whether you have injectors that flow 45 pounds of fuel per hour or 33 pounds of fuel per hour, you could never tell the difference by sound or feel.

Much less look.

Carburetors came as two and four barrels — and sometimes, you got more than one. Imagine three two barrel carburetors. Or two four barrels. Maybe a set of 45 pound per hour injectors flow more fuel. A dual-quad set-up still looks a lot better.

Sounds better, too.

But — most of all — a carburetor is involving. Making fine adjustments and tweaks; learning how the thing works — and learning how to work on it. An emotional bond is created; the satisfaction of knowing you fixed it. FI is mostly plug and play.

Which is probably why so few people play with cars anymore.



Not an NMA Member yet?

Join today and get these great benefits!

Comments are closed.