While the webster 1913 entry is probably very correct, it most certainly doesn't fit the more modern definition. In common usage, a carburetor (or carb) is part of an internal combustion engine .

The carb sits between your fuel pump and your engine (or your gas tank and engine if your setup uses a gravity feed, like many older motorcycles). It's job is to atomize fuel and mix it with air. A pretty simple job, in concept - however it is anything but a simple device.

Modern cars, since the advent of fuel injection don't use carburetors, as the fuel injection system takes care of atomizing the fuel and mixing it correctly. However, they are still very common on motorcycles (in fact, I have never heard of a bike using fuel injection).

The carb consists of several "jets". Each jet is a hole where fuel and air can flow through. The air/fuel mixture is determined by the size of the jets, and the adjustment of the overall system. It is quite an involved process. It must be adjusted so that the engine can idle. Too rich of a mixture (too much fuel, for the amount of air) and it can foul the spark plugs and really hurt fuel economy (I had a bike that was getting 10 miles per gallon on the highway until the carb was fixed - now it gets 50 mpg - amazingly its city performance didn't change - has always been about 30 mpg)

Adjusting the carb requires special tools like a portable tachometer (the built in tachometer on cars and bikes is not good enough. My bike manual calls for one that can detect a difference of 25 rpms between 0-1,000 rpms - and this is a cruiser not a high performance sport bike).

An improperly adjusted carb (or one that needs adjustment) can cause several different problems. Any of the following could be caused by a carb problem:

Of course all of these can be caused by other things too. However if several of these symptoms apear - then there is a good chance that it is a carb problem.

As a side note: a carb is really anything that mixes fuel with air to aid burning (as the webster 1913 definition pretty well states). It is interesting to note that while the hole in many bongs and pipes is called a "carb". It is a matter of debate over whether this is, indeed, a carburetor. On one hand it is backwards - opening it up causes less air to be drawn through the bowl and thus hurts combustion - its use is just to clear out smoke. However - this change in air flow does effect the burning - so it could still be seen as regulating the burn, even though that is not its primary function. I am inclined to say that, in this case, the term "carb" is a misnomer.