Below  is a simple explanation of how a basic system works as well as a few tips for fuel systems repair.

Basic fuel system concepts

Gas engines get their power by burning fuel. In order for a fuel such as gasoline to burn, it must be vaporized. Liquid gasoline doesn't burn, only the vapors do. The process of burning is combining the fuel, in this case gasoline, with oxygen - the oxygen in the air we breath. In short, we must supply the engine with fuel in the form of gasoline vapor, as well as oxygen in the form of air. Since we've said that liquid gasoline won't burn - we must somehow turn it to vapor. This is the job of the carburetor.

Here's how it happens: If you were to set a glass of water on the sidewalk, it may take hours, or longer, for it to evaporate or turn to vapor. But pour that water out onto the sidewalk, and it will evaporate in minutes. This is becausethere is more surface area for evaporation to occur. The same is true for gasoline - if we put large drops of gas into the engine, it couldn't vaporize quickly enough to properly burn, so we must somehow break it down into tiny particles - atomize it, so it may evaporate quickly enough to be vapor by the time is gets to the cylinder.
The drawing at the right shows the basic operation of a "venturi".  As the piston moves down on intake stroke, it creates low pressure. Outside pressure being higher tries to push into this low pressure area. (what we call "suction" is actually high pressure trying to equalize by PUSHING into an area of lower pressure, just as a high pressure area in the atmosphere creates wind by pushing air into the low pressure area in an attempt to equalize pressures). As the air molecules pass through the venturi, (restricted area) they must speed up. Since we are trying to push the same number of molecules through a tiny area as we are through the larger area, they must move faster in the smaller area to maintain the same rate of flow. As they pick up speed, through the restriction, they leave "gaps" or space behind them. This means fewer molecules in that area to push and shove against each other, or anything else - causing lower pressure.

"Nature abhors a vacuum". In an attempt to equalize, or fill in that low pressure area created by the "venturi effect", air pushes up the tube, pushing up on the liquid that is in its way. The example above shows how the pressure in the various points in our venturi differs - the lowest pressure being at the most restricted portion of the venturi.

We now have the fuel pushing up through a tiny tube, and turbulent, fast moving air above the end of the tube.  This set of conditions causes the fuel to be pulled apart into tiny pieces - atomized, so that it may be easily vaporized, as it enters the stream of air entering the engine.  We no longer have liquid fuel, but a mix of air and fuel vapor being pushed into the cylinder by the higher pressure outside the cylinder. This is a carburetor in its most simple form.

To control the mixture of fuel and air, or "ratio", a tapered needle shaped screw is used to restrict the opening in the tube leading into the venturi.  Turn the screw in, the tube is closed off, resulting in less fuel in the mixture - this is called a "lean" mixture.  Turn the screw out, and you once again open up the tube opening, allowing more fuel to enter the air above, this is a "rich" mixture.
In the drawing on the left, the red indicates a needle valve which may be turned in to restrict the fuel being pushed into the airstream.

In a throttle governed engine, (see governor page) the amount of air-fuel mix entering the engine is controlled by a "throttle" disk (represented in blue at left) which is rotated on a shaft to close off the 'throat' of the carburetor. This has the effect of reducing engine power, thus also it's speed. The throttle may rotated to open the carburetor throat to allow more fuel into the engine, producing more power, thus increasing speed.

One final item of importance on most antique gas engines is the check valve.  This valve is between the carburetor or mixer, and the fuel tank - often at the bottom end of the pickup tube. The purpose of the check valve is to keep fuel up in the pickup tube and prevent it from draining back into the fuel tank between intake strokes. Should the fuel be allowed to flow out of the tube, it would take a longer portion of the intake stroke to "pull" fuel up the tube and into the venturi area.

Many antique engine fuel systems are as simple as pictured here.  Troubleshooting a fuel system is very easy. The airway - the carburetor throat, and venturi must be free of debris such as mouse or wasp nests!

Next, the fuel must have a clean, open path from the fuel storage area to the venturi. This means the mixture, or needle valve must be open a bit, and the fuel pickup tube also free of dirt and debris. If your engine pulls fuel directly from the fuel tank, and not from a fuel bowl on the carburetor, it most certainly will have a check valve, as pictured on the right.   It may LOOK OK, but if you've ever seen gasoline leak from a tank seam, you know it will find its way through the smallest of openings.  This means that the check valve must be in very good condition to seal off the fuel's escape route back into the tank. Make sure the check valve body and ball are both clean, and that the ball is free to move.

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