Engine Fails to start

Perform the following Spark plug test to determine if the ignition system is operating properly.

Remove one spark plug
Connect the spark plug wire and the connector to the spark plug and touch the spark plug base to a good ground like the engine cylinder head. Position the spark plug so you can see the electrode.

Warning
During the next step, do not hold the spark plug, wire or connecter or a serious electrical shock may result. If necessary, use a pair of insulated plug wires to hold the spark plug or wire. The voltage generated by the ignition system could produce serious or fatal shock.
Crank the engine over with the started. A fat blue spark should be evident across the spark plug electrodes.
If the spark is good, Check for one or more of the following.

a. Obstructed fuel line or fuel filter
b. Leaking head gasket.
c. Low compression
If the plug is not good, Check for one or more of the following.

a. Loose electrical connections.
b. Dirty electrical connections
c. Lose or broken ignition coil ground wire
d. Broken or shorted high tension lead to the spark plug
e. Dead battery
f. Disconnected or damaged battery connection
g. Oxidized breaker points (1959-1978).



Source: http://ironheadsportster.com/_fails_to_start.html