Oil carryover, also known as slobbering oil, is a common problem in Harley-Davidson motorcycles. It is caused by the engine ejecting air and oil while operating.
Motorcycle engines need to “breathe.” Engines operate by pistons moving up and down in cylinders. As they do so, air is displaced and has to go somewhere. This output of air is called breathing. When the air is pushed out of the cylinder, it carries with it a certain amount of oil particles; this is why slobbering oil problems exist in the first place.
Crankcase breathers were included on older Harley-Davidson models and originally ejected this oily air straight into the atmosphere. As society turned to a more environmentalist bent, the EPA declared that this was unacceptable. Such models are now required to be modified so that the oil is not ejected into the air. To accomplish this, the air is run through a filter or the oil run into a catch can. The breathing line is usually the source of an oil carryover problem because it is worn out, broken or improperly fitted, but a faulty filter or oil catch can also be the problem.
Harley-Davidson began to favor head breathers during the early 1990s. Head filters have two large, distinctive bolts that attach the breather to the engine. Instead of being released into the atmosphere, air from the pistons is rerouted into an air filter. Slobbering oil is generally caused by improperly fitted bolts or tubes. Off-brand air cleaners can also be a problem, since they may not be designed to fit correctly.
Once the problem is diagnosed, the solution will likely seem obvious: a replaced filter, catch can or breathing line. However, it is important to look at the machine as a whole. A damaged line should be replaced with a model that will fit both the engine and the catch can/filter. While some off-brand models may do the job as well as their official counterparts, it is also important to do research to be sure this is the case.
These are merely guidelines for diagnosing the oil slobbering problem. The exact cause can vary from machine to machine, and inexperienced motorcyclists should consult a Harley-Davidson expert before attempting any repairs themselves. Moreover, oil slobbering problems are very dangerous, and not just because of environmental concerns: having oil on the tire reduces traction, which can lead to a wreck, and oil on the street can cause problems for other drivers later.