So you have a bike that is not running. Maybe it is a bike that you have had for years and it has just been sitting or it is a bike your looking at for sale but the owner states "ran when parked" meaning, it doesn't run now. As a matter of fact the latter is how you can generally get a great deal on a bike and with this article and video you can take advantage of that non running bikes low price.
Typically the reason a bike was running and now isn't is usually fairly simple. Most problems can be traced to the fuel system, probably causes is fuel was left in the system creating a varnish like substance when the lighter elements of the fuel flashed off. This leaves the small passageways clogged or plugged resulting in inconsistent or no fuel flow. There also could be minor ignition issues but usually that is rare, more on that later. Lets get back to basics.
Click on the Getting a non running bike running video to see an in depth video of the procedure outlined below:
Evaluate the bike, is it missing crucial parts for it to run? If not start the dis-assembly for cleaning and inspection this includes the fuel tank itself. If it is missing parts, locate or source the missing parts before going further as this will minimize delays further in the process.
Fully disassemble the carburetor and fuel shut off. These are the most common places that the problems will originate as well as where most of the debris preventing proper fuel flow and metering will be found.
Clean, clean, clean, and more cleaning. Carburetor dip is a good idea for the carb body and other related parts. Just remember to remove any rubber or fiber parts as carburetor dip may "eat" or dissolve them.
While your carburetor parts are soaking check out the ignition components. Replace the spark plug, it is cheap, and will rule out a potentially faulty plug of unknown condition, refer to a service manual for specific testing if you have a "no spark" condition. Most "no spark" conditions can be traced to a kill switch wire grounding out at the handlebar or somewhere along the harness.
Reassemble the carburetor returning all the settings like the float height, jetting specs, and air/fuel screw (depending on which it has) to the factory specs found in the manual. Re-install the tank and fuel shut off. Attach the fuel lines use clamps when necessary. Fill the tank with a little bit of fuel open the tap and check for leaks and/or a flooding carburetor.
If the previous steps were done properly attempt to start the bike. Be sure your in neutral and have your hand on the throttle and kill switch for safety so you can shut down the bike should something happen.