Harley-Davidson motorcycles require three conditions to be met in order to run: a precisely metered air-fuel charge, sufficient compression and a timed ignition. Component failure or maladjustment in any of these systems can lead to a non-running condition. Troubleshoot the systems one component at a time, starting with the easiest-to-access areas first; eliminate the components one at a time until the problem is rectified or the bad component or adjustment is isolated.
Check the fuel level in the tank either by checking the fuel gauge with the ignition key in the "On" position or by unscrewing the gas cap and looking inside the tank. Replace the fuel with fresh gasoline if the gas appears old or stale. Old gas smells like varnish and appears discolored. Stale fuel can cause the engine to not run.
Ensure that the fuel petcock is in the "On" or "Reserve" position.
Remove the fuel-line retainer from the fuel-inlet barb on the carburetor with pliers. Pull the fuel line off of the carburetor by hand.
Place the loose end of the fuel line into a fuel-safe container. Put the bike in "Neutral" and turn the engine over with the ignition switch. Observe the fuel line for fuel flow. Check the vacuum line connecting the carburetor to the petcock for leaks or damage if the fuel fails to flow. A bad vacuum line leaks air and prevents the vacuum-operated petcock from opening. Replace the vacuum line and try to start the engine.
Inspect the fuel that was discharged from the fuel line for signs of water, foreign matter and age. Water or stale gas in the fuel system will prevent the engine from running. Change the fuel in the tank for fresh gasoline if these conditions are present in the gas discharged from the fuel line.
Remove the petcock and inspect the screen if no fuel comes from the fuel line during the flow test or if the fuel has trash or rust flakes in it. Petcock filters clog and prevent fuel flow and have been known to vibrate off of the petcock, allowing unfiltered fuel into the system. Rebuild the carburetors if the filter is found to have fallen off of the petcock with trash or rust flakes present. Install a new fuel petcock filter and try to start the engine.
Remove the spark plugs with a spark plug socket and ratchet. Inspect the spark plugs for fouling or damage that may prevent firing. Replace the spark plugs if they are fouled or damaged. Try to start the engine.
Connect a timing light's power leads to the bike's battery. Connect the positive lead first, then the negative lead.
Activate the timing light and watch for the strobe. Check the spark-plug wire's connections and the coil wire's connections if the timing light indicates that there is no spark. Test for spark again.
Remove ignition-coil bolts with a wrench and have the coil tested by a service center if there is still no spark. A bad ignition pickup or ignition module is indicated if the coil passes the tests and there is no spark being produced.
Remove both spark plugs with a spark plug socket and ratchet. Install a compression gauge into the front cylinder spark plug hole.
Open the throttle all the way with the throttle control on the handlebar. Turn the engine over with the ignition switch for about six seconds.
Turn the ignition switch off and read the gauge. Compression should be between 165 to 225 psi. Compression readings lower than 165 psi indicate worn cylinders and rings. Engines with worn cylinders and rings will not develop sufficient compression to fire and must be rebuilt.
