There are three basic elements involved when trying to start any internal combustion engine, and motorcycles are no exception. First is fuel, second is fire -- or rather, electricity -- and last is air. If the engine isn't getting enough of any of these elements, it won't start. In most situations, if you run through a short checklist of potential causes, you will find your problem -- and usually be able to fix it without spending a cent.
Check the choke lever. If it is disengaged, pull the lever down until you feel resistance. Try to start the bike.
Check the fuel petcock. Make sure that it is turned to the "On" position. With the choke still engaged, try to start the bike again.
Check how much fuel you have. Add fuel to the tank, and try again.
Check the spark plug cables. Make sure they are properly seated. Try to start the bike again.
Remove the battery cover. Use a voltmeter to check the charge of the bike. It should be around 12 volts.
If the charge is low, try to jump the bike off of another battery. Any car or truck will do. Alternatively, you could use a plug-in jump starter instead.
If the bike starts while being jumped, disconnect the jumper cables. If the bike stays running, you should replace your battery. If it dies you may need to replace the alternator. If the bike doesn't start, go to the next step.
Check the spark plugs. Remove them, and check for signs of wear or damage. If they are damaged replace them. Remember, even though a Honda Shadow only has two cylinders, you still need to replace all four spark plugs. If they appear operable, go to the next step.
Connect the spark plug to the spark plug cable. Touch the spark plug to the frame or engine block. Push the starter button, while watching the spark plug. If you see a spark, the plugs are working, and your problem lies elsewhere. If there is no spark, renew your spark plug cables and coils. If that still does not fix the problem, you need to take the bike to a shop.
Make sure that your gas tank is full.
Disconnect the fuel cable from the fuel petcock. Turn the petcock to the "On" position and then quickly back to "Off." If fuel comes out, your petcock is in working order. If not, replace it.
Remove one spark plug from each cylinder, and wait for approximately one hour.
Replace the spark plugs and attempt to start the bike again. This will clear any flooding that may have occurred in the process of trying to start the bike.
Remove the seat from the bike.
Remove the air filter cover.
Check the air filter. If it is gray or dark brown, or if there is debris caught inside of it, replace it. If it is clean, put it back in place. Attempt to start the bike again.
If the bike still won't start after checking everything, you will need to take it to a mechanic.
