Make sure the key and kill switch are both in the "on" position. Ensure that the proper starting procedures for your bike are followed. Is it in neutral? Clutch pulled in? Gas in the tank? Then try to start your bike. Does it turn over? If not, check to see that the battery is properly connected and the terminals are not corroded. If they're loose or dirty, clean and tighten them. Then, using a voltmeter that measures ac/dc and ohms, check to see that your battery has enough charge to crank the engine. If not, replace or charge your battery and try again. If it still doesn't turn over, there may be a loose connection between your battery and starter; a bad ignition or starter switchl or a bad safety relay. Check a repair manual for proper testing procedures for your bike, as each motorcycle differs.
If your bike turns over but doesn't catch, check to see that it's getting fuel. If the bike has a fuel petcock, make sure it is in the "on" (or, on certain bikes, "prime") position. Then remove the main fuel hose and check to see that fuel is flowing freely. If fuel isn't getting to the carburetor or injection system, your bike won't run. If that's the case, your problem is likely something in the fuel system. If fuel is flowing freely, reattach the lines. If it's not, check to see if the fuel filter is clogged, if a line is pinched or if the petcock is working properly. One way to determine if the problem is in your fuel system is to put a few drops of fresh gas into each spark-plug hole, replace the plugs and turn the bike over. If it starts and then quits, the problem is likely in the fuel system.
If you're getting fuel and the bike turns over but still doesn't catch or start, check the spark plug or plugs. Start by pulling off a spark plug wire, then removing a plug using the spark plug socket supplied in your bike's toolkit. Now inspect the plug. It should not be wet (usually caused by fuel, when the plug is not firing) or coated in carbon/burned oil deposits. Now check to see if the bike is getting spark. Although you can get a special, insulated set of pliers to hold the plug, there's a "quick and dirty" method for this: After reattaching the plug wire, lay the threaded part of the plug against the engine (not over the plug hole, as the spark could ignite any fuel that is blown out when you try to start it). Now, making sure you're not in contact with the engine or plug, hit the starter. You should see a nice blue spark. If you don't, make sure the threaded portion of the plug is touching the engine (but the electrode is not) and try again. If you still don't see a spark, you either have a bad plug or a problem with the electrical system. Check to see that all the ignition wires are properly connected and that you can't see any cracks in the wires. If the wires are cracked, they should be replaced. If you're still not getting spark, it's time to consult a repair manual or call a mechanic.
If you've got fuel and spark, ascertain that your bike is getting enough air. Start by pulling off the air filter. If it's too dirty, you won't get the proper mixture of air and fuel in the carburetor or injection system. If it's clean, check to see that the air box is properly connected-- a loose hose or air leak can feed too much air into the system. If your bike is equipped with a choke, ensure that it's able to move freely and is not stuck in the "on" or "off" position.
If you've followed these steps and still can't get your bike to run, call in an expert. If you think you've narrowed down the source of the problem, describe the steps you've taken to point the mechanic in the right direction.
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