In a carbureted engine, the carb-to-manifold gasket is probably the second biggest potential air leak next to the manifold gasket itself. The carb gasket is responsible for providing that delicate boundary between where air is and where it shouldn't be; a failure here can ruin your motor's whole day.
The Basic Problem
A carburetor works by using engine vacuum to suck fuel through the carburetor's metering jets, and introducing it in the highest point of vacuum and airflow to mix it in a precise quantity with the air. A leaking carburetor gasket won't let fuel out, but it will let air seep in right where the vacuum signal is most important.
A very slight leak may cause a bit of engine roughness at idle, when the throttle plates are closed and vacuum is supposed to be at its highest. It is at idle where the engine is most sensitive to minor changes in airflow, because the throttle plates are almost all the way shut. A very slight leak might not be noticeable as more than a minor engine vibration at idle and a millisecond hesitation before acceleration.
A larger leak will cause noticeable engine vibration and probable misfire at idle, and severe hesitation under acceleration. One of the vacuum leak's signature symptoms is that it seems to go away under full throttle conditions, particularly at high rpm. A medium-sized leak will often result in a "hunting" or rising-and-falling idle. This occurs when the distributor's vacuum advance realizes that it's not getting the vacuum signal it should be and and pulls or adds timing advance.
You can usually judge the severity of a vacuum leak by how high its disturbance reaches up into the rpm range. The more you open the throttle, the less vacuum and more air the engine "expects" to see; thus, the less effect a vacuum leak will have on performance. But, that additional air will also have a nasty side-effect in the form of a lean air/fuel mixture. Lean mixtures are better for fuel economy, to a point, but they also cause your engine to run much hotter.
Diagnosis and Further Symptoms
If you've got a vacuum-advance distributor and you experience idle hunting, the simplest way to test for a vacuum leak is to disconnect the advance tube at the distributor and plug the line. If the hunting stops or the engine stalls, you've got a vacuum leak. Vacuum leaks will also tend to cause a "lean backfire" through the exhaust. Lean backfires happen when the mixture won't sustain a flame in the cylinder and, instead, burns its fuel in the exhaust pipe.
- Auto Fundamentals; Martin Stockel