How to Troubleshoot an Oil Leak in the Air Cleaner of a Harley Davidson


The presence of oil leaking from your Harley-Davidson's air cleaner is the first symptom of a condition called "blowby," which allows the pressure generated within the engine's combustion chamber to blow past the pistons. This pressure forces oil out of the engine crankcase and into the air cleaner. Blowby affects the engine's ability to compress air and fuel to create the combustive force needed to operate. However, a similar oil leak can occur if the motorcycle's oil tank is overfilled. Though the second cause is not as harmful, it can cause blowby if it is not quickly dealt with.

Things You'll Need

  • Hand pump
  • 5/8-inch spark plug socket
  • Socket wrench
  • Compression gauge
  • SAE 20W-50 motorcycle-grade engine oil
  • Torque wrench

Oil Level Check

  • Start the engine and let it idle in place for three minutes to allow the engine oil to warm in the oil tank. Stop the engine, then let the oil settle for five minutes.

  • Check the oil level using the dipstick attached to the oil tank. If the oil level is above the upper mark on the dipstick, the oil tank is overfilled. The tank has a vent tube that leads into the engine crankcase, where it will eventually be blown into the air cleaner through a separate breather tube.

  • Drain out the excess oil, using a hand pump, until the oil level is between the upper and lower marks on the dipstick.

Compression Testing

  • Remove the spark plugs from both engine cylinders using a 5/8-inch spark plug socket and a socket wrench. Push the spark plugs into the spark plug cables, then ground the spark plug electrodes against the engine cylinders to avoid damaging the ignition coil.

  • Screw a compression gauge into the front cylinder spark plug hole. Twist the throttle grip into a completely open position.

  • Turn the ignition on and press the starter button to build up pressure in the front cylinder. Release the starter button once the compression gauge needle no longer moves. Ideally, the gauge should indicate a minimum compression of 125 psi.

  • Remove the spark plug if the compression is less than 125 psi. Pour a half-ounce of SAE 20W-50 motorcycle-grade engine oil into the cylinder, then retest the cylinder's compression. If the compression reading increases, the piston rings and the cylinder must be overhauled because the piston can no longer maintain a seal against the cylinder wall.

  • Unscrew the compression gauge and place it in the rear engine cylinder. Check the cylinder's compression using the same method as for the front cylinder.

  • Screw the spark plugs into the engine cylinders and tighten them to 18 foot-pounds using a torque wrench.

Tips & Warnings

  • A loss of compression in one or both engine cylinders will show other symptoms as well, including a loss of power, burning oil and fouled spark plugs.
  • Do not operate the motorcycle's starter without grounding the spark plugs against the engine or the motorcycle's frame. Turning the engine over without grounding the spark plugs will damage the motorcycle's ignition system.
  • Keep the oil level below the dipstick's upper mark. Oil blown into the air cleaner through overfilling will eventually be pulled into the engine, where it will be burned and turn into carbon deposits on the cylinder wall. The deposits will cause excessive wear on the piston rings, creating the possibility of blowby.

Related Searches


  • The Professional Motorcycle Repair Program: Lubrication and Cooling Systems, Volume 5; Professional Career Development Institute
  • The Professional Motorcycle Repair Program: Four-Stroke Internal-Combustion Engines, Volume 5; Professional Career Development Institute
  • The Professional Motorcycle Repair Program: Harley-Davidson Maintenance, Volume 23; Professional Career Development Institute
  • The Professional Motorcycle Repair Program: Four-Stroke Engine Top-End Inspection, Volume 14; Professional Career Development Institute
  • Harley-Davidson Service Manual: Touring Models; Harley-Davidson Motor Co.
  • Photo Credit Jupiterimages/ Images
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