OK so we are checking your air. Like I said previously, its important to have everything all tight in the intake system. Basically, the theory goes like this. One of the great things about the airbox is it collects and organizes the air that your bike needs, and sends it shooting into your carbs in a nice, smooth stream. If there are vacuum leaks, or the bike has pod filters, or parts of the intake are all disconnected and damaged, vortexes will form. These vortexes are chaotic and are not efficient at picking up gas particles in the carbs. Therefore, you will get pockets of lean and rich air as it enters the engine, which does not produce an efficient explosion.
Consider this as well: if you are drinking through a straw, would you get more water if you drank out of a bowl of water, or a stream of water coming out of a faucet? That is similar to how the airbox collects air and makes it available for the engine. Vacuum leaks or disconnected parts of the intake will create the vortexes which make it more difficult to take in a solid stream of air.
Checking for vacuum leaks is very important. Old bikes are notorious for getting vacuum leaks. These usually occur around the carb boots, both on the airbox and the motor side. The rubber gets old and they crack, the clamps get rusty, or the rubber shrinks and no longer makes a good seal with the airbox. Here is how to check for vacuum leaks:
Additionally, your bike might not be getting enough air. The cause of this is an old dirty air filter, intake components which are not assembled correctly, or perhaps carburetor slides which are stuck closed. See the "fuel" section for more information on carburetor cleaning. Don't discount the possibility of some junk in the airbox, I've pulled animal nests, pieces of plastic, old rags, tools, leaves, and all kinds of junk out of airboxes. So take a look in there and see what you can find.
Each bike has a specific air:fuel ratio that it needs to run, and this changes as the bike warms up, and requires different ratios depending on whether it is accelerating, cruising, decelerating, etc. The carburetor adjusts this ratio along with the choke; on startup, the ratio can be as low as 8:1 particles of air to gas, when cruising your bike might need 40:1. If the ratio gets too low or high the mixture is no longer combustible and therefore the bike won't run. Even though the carburetor maintains these ratios as needed by the bike, it need access to a solid flow of air at all times in order to do its job.