FUEL. The vast majority of motorcycle problems are caused by old fuel. This is how it works:
Most old bikes that I come across have had some amount of gas left in their tanks for years and years. This causes a lot of maintenance to be done on the bike before it will fire up, but it is all manageable work, and no "damage" that old gas does to a bike is irreversible.
As gas gets older than 6 weeks it begins to lose its volatility. The most explosive parts of the gas evaporate out, and the gas gets slightly thicker and darker in color. The longer it sits, the darker it gets. Ethanol exacerbates this problem as it evaporates into a gloppy mess. However, this takes years. My XT550 still fired up on gas that was over 2 years old, but since an old project bike might have been sitting for a decade or even longer, any gas that was left in it is now thicker than honey.
Gas is filled into your tank. From there it passes through the petcock, into fuel lines. Some bikes have 1, some have a couple. From there they enter your carburetor. They pass around the float needle which raises and falls as the floats allow more gas to come into the float bowls. The bowls are like tiny little gas tanks on the bottom of the carbs, and they collect gas so that it might be drawn up through the jets and enter the middle of the carbs, mix with air molecules, and get sucked into your engine to be burned.
This process requires a lot of passing through tiny spaces. Gas must pass through the petcock which has passages smaller than a drinking straw. It flows around the needle which is also small, and shaped like a small square peg in a round hole, so it flows around the 4 sides of the square peg. Finally your jets may be as small as a thread. If the gas gets too thick it will clog these passageways. If it clogs the petcock you won't get any gas to your carbs or your engine. If the float needle gets clogged, it might stick open or closed, therefore either flooding or starving that cylinder. And finally obviously the jets are the opening to the engine, if they are clogged no gas can get through.
So, even though there are only a couple of components to the fuel system, each one needs to be meticulously assessed and serviced if you believe that you may have a fuel delivery problem. The following pages will discuss each of these components, some common symptoms related to their dysfunction, and how to repair them.