Old fuel is the single biggest cause that makes a bike not start. Modern gasoline begins to break down 30-45 days after the day it is refined. To make matters worse, today's gasoline is blended with ethanol and it wrecks havoc with your fuel system when exposed to water or condensation. Start by making sure you have enough gas in the tank. It sounds obvious, but I have fixed a lot of bikes by simply filling the tank or switching the petcock to reserve. Now, take a good whiff inside your fuel tank. If it smells like varnish it's time to drain the tank and the carburetor. After you have drained the tank, remove your petcock. Remove debris from the tank and check the condition of your fuel filter. Fill the tank with fresh gas and check to see that it flows freely out of the fuel line with the petcock open. Next, reinstall the fuel line and open the carburetor drain screw to see that gas flows from the overflow. If it doesn't, you have a clogged float valve and you will need to replace or clean the float valve and seat. On aluminum framed motorcycles you will often need to remove the carb from the bike to do this. Finally, remove and clean your pilot jet. If your bike is still running rough a thorough carb clean will make it run smoothly.