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Oil Carryover In Air Cleaner


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Just a quick explanation from J&P Cycles...

How many times have you cleaned your bike till its spick-n-span only to go for a ride and return with oil/gas coming from the bottom of the air cleaner, making a mess of the right side of your bike? Annoying isn't it? What we are going to discuss is what causes this, and how to fix it.

The condition we are describing is known as "oil carryover" and has been around for many years as a condition inherent to our beloved V-twinâ„¢. The reason this happens is we have a 4 stoke engine with a lot of the oiling characteristics of a 2 stroke. Sounds confusing doesn't it? To get a grasp of this I'll describe our basic oiling system 1936 to 1999.

First off, we have an engine that is a dry sump system. This means the oil is stored in a tank remote from the engine. The oil is pumped out of the engine and back to the tank. Oil is gravity fed to the pump where it is pressurized and sent to three locations in the engine. The Crankshaft, the rocker shafts and if the engine is so equipped, the hydraulic lifters. These are the only places to receive pressurized oil. Every thing else is lubricated by the "Splash & Drip" method. (For this reason the Harley® engine does not require a lot of pressure to operate, it requires volume.) "Splash & Drip" is where oil is slung or allowed to drain to give the lubrication required. As the oil is pumped to the three locations, it gets released into the air inside the engine. This oil laden air is moved around by pressures created by the up and down movement of the pistons (This is the 2 stroke part). As the air moves it releases most of its oil to take care of the various other lube requirements.

What all does this have to do with oil getting all over the side of your bike? Well... now that we have established these principles we can better understand what happens next. We all brag about the displacement of our engines, but have you ever realized for every cubic Oil Carryover By Scott Holton, J&P Tech inch displacement above the pistons we have a compressor with the same displacement under the pistons. As the pistons come down pressure builds unless we have a path for it to escape. These escape hatches are known as "breathers". All Big Twins® prior to 1992 had a rotating device timed to open as the pistons came down, pushing a slug of that oil laden air over the camshaft. As the piston began going up a vacuum was created sucking the oil from the rocker box area. These engines had the breather off the rear of the crankcase for Big Twins or under the generator on Sportsters®. By now most of the oil in the air had fallen out and done its work, but any left was removed as the air was diverted through a series of baffles and out the breather. Any oil left suspended would be carried out into the atmosphere (hence the name "carryover").

In the 70's the Environmental Protection Agency had a law passed requiring any emissions from the breather be routed to the backside of the air cleaner so it could be burned returning through the carb. Initially H-D® ran a hose from the existing breather points, but this didn't satisfy the EPA (to easy to modify). So starting in 1993 the crankcase ventilation was moved to the cylinder heads where it remains today. We still use the same principles to lubricate our engine, but now we use umbrella valves instead of the timed breather to allow the air out. Remember our oil-laden air? This system is not quite as effective separating it. This is part our difficulty. There are some rocker boxes that do not allow the umbrella valves to fit flat, compounding the problem. A little work with a Dremel can solve this.

Another possible cause of oil’s early exit has to do with where the vent originates in the rocker box. As oil falls out of suspension, it can puddle in the bottom of the rocker box. The vent entry is part of the rocker box floor, so oil can dribble out. Here at J&P® we sell a little device called "The Cure". The Cure raises the vent height to stop this condition and generally solves the problems in stock air cleaner applications. Note: When changing to a different backing plate it's a wise idea to use one of the Spyke Krank Vent or the Doherty Power vents to keep unwanted air from returning into the heads. These devices are very effective in assisting the breathing system.

For you Twin Cam™ owners... don't feel left out. There is a new device out from S&S® to help carryover issues with your engine. This slick little unit consists of 4 reed valves (there's that 2 stroke again) to control pressures inside the cases. This gives us much more control of our oil-laden air. At this time this device is only available for TC "A" engines. When installed on every engine they tested that had carryover problems, oil out the breather stopped! Pretty impressive if you ask me.