Understanding Evo breathers and oil return
Article from HOT BIKE Magazine, February 1999

Text & Photos by Jimmy Johnson
Reprinted with permission

A stock Harley breather system is perfectly fine for a stock Harley. But when you start modifying things and building that monster motor you have always wanted, the stock system becomes less than adequate. You end up with oiling problems like the breather flinging oil everywhere and leaking base gaskets.

The best way to cure your problems is to first understand the workings of your oiling and breather systems and second, anticipate the parts that may fail. Let's start by explaining the system.

THE SYSTEMS
All 1970 and later big twins (except the Twin Cam 88) breather systems do basically the same things: They must control the internal pressure of the crankcase, separate the air/oil mixture and aid in draining return oil inside the motor. They use a 21-tooth rotary breather valve (located in the cam chest) that is timed with and turns in the same direction and speed as the motor. The breather valve opens to relieve high crankcase pressure during the piston down stroke as well as returning the air/oil mist from the crankcase into the low pressure area of the cam chest. The breather window closes when the pistons are on the up stroke to create a low pressure area in the lower end.

The oil return system is based mostly on gravity with some help from the breather to get oil back to the return side of the pump. Oil drains from the upper end to the lower end where it is picked up by the flywheels and slung into the breather valve inside the breather bore. The breather then blows this oil into the cam chest on the piston down stroke. The oil pump return gears pick up the oil and force it out into the oil filter and then back to the oil tank.

Now that you understand the system, let's take a more in-depth look at the early and late systems.

1970 - 1992
The early system uses a sealed breather trap area located in the lower left-hand corner of the cam chest.	After leaving the breather window the crankcase air/oil mixture works its way through a steel baffle in the cam cover.
By the time the mixture makes its way to this point the oil is supposed to separate from the air by gravity. The air then makes its way into the breather trap through a pipe in the cam cover.	Inside the breather trap another pipe expels the air outside the motor ...
... that comes out here, the lowest point on the case, just below the oil pump. A 3/8-inch hose is connected to the fitting and runs to the air filter or through an aftermarket filter breather. You should not run an open breather hose because not only does the breather blow air out, but it also sucks air back in.	The oil left in the sealed breather trap area is sucked back into the breather valve through this hole during the piston's upstroke (negative case pressure).

1993 and later

The newer breather is used to expel air only. It is an internal system that completely eliminates the outside breather hose from the crankcase to the carb. Its design brought many changes to the motor in the cases, heads, rocker boxes and cam cover. The motor breathes through the pushrod tubes and one-way umbrella valves in the middle of the rocker boxes. The one-way valves are opened by positive pressure from the crankcase, allowing an air/oil mixture into the rocker box.

Oil mist carried by the crankcase air is supposed to separate and return to the valve pockets through a small drain hole in the middle rocker box piece. The cylinder heads are drilled to allow only air to go through the two hollow bolts that mount the air cleaner backing plates.

Now on to the variables ...
One of the biggest problems encountered in a breather system is oil return blockage. If the return is blocked, excess oil builds up in the cases, causing oil temperature to go up, loss of viscosity and horsepower-robbing build up around the flywheels. Then the oil build up starts to leak out of every weak spot in the system. The ultimate cure for oil return system problems is the Zipper's Pro-flow oil pump for big twins or Sportsters.	A mixture of seven different breather valves (plastic and metal) have been used on Harley big twins since 1970. S&S makes just two metal breather part numbers -- one for pre-'77 models and one for post-'77 models. While you might think the plastic H-D gear would be better for aluminum cases than the steel, that is not really correct. Pieces of metal and carbon can become embedded in the plastic and damage the breather bore. Also the plastic can twist or distort and throw off the breather timing.
Your motor must be sealed to create a proper vacuum. A leak in the internal sealing of the motor can cause an increase in crankcase pressure--a bad thing. Make sure your oil tank cap seals OK, your pushrod tubes don't leak and all your gaskets (especially base gaskets) are sealing properly.	Worn piston rings can cause blowby into the crankcase creating extra crankcase pressure. A leak-down test will help you find out if your rings are sealing properly.
Oil burping (sumping) on start up is not a breather problem, it is an oil pump check ball problem. That means that oil is leaking past the oil pump and into the cases when the bike is parked. Either fix the check ball seat or keep a can under the bike for cleaner start-ups.	Imagine doing 50 push-ups a second in 400 degree oil for any length of time. Well that's exactly what your umbrella valves do. Over time they get hard and stop sealing. That's when you really start seeing oil blow out the breather. Stock valves can be replaced with higher silicon content James Gasket valves that last longer in the heat.
Blown base gaskets are a common big twin problem. Their failure is almost always attributed to excessive crankcase pressure. The Spyke KVS valve was designed just for this purpose. As an added bonus, by correcting crankcase pressure your pistons have less fight on the down stroke.	The Cure from Creative Cycle Products is for 1993 and later big twins and 1991 and up Sportsters. It helps to fix leaks in the rocker boxes to the heads.
You will have to remove your rocker boxes and locate the vents that are closest to the carb when installed. Use the supplied 1/8-inch drill to open the drain hole in the middle rocker box spacer ...	... then use the 13/16-inch drill to make the opening for the new vents.
With the rocker box section on a flat surface, insert the vent extension. You may need to gently tap the extension into place but be sure not to damage the gasket surface. Keep in mind that sometimes you will have to clearance the top rocker box for the vents, so check before you bolt things up.	The KVS valve is built from the same material used to control the flow of liquids in nuclear reactors, so it is guaranteed for life. But if you feel a need to tinker with it you can clean it with shop solvent and compressed air.
If you have a pre-'93 big twin, connect the KVS to the breather line that comes out of the case. You will notice an almost immediate change in your bike after the installation.	'93 and later big twins and '91 and later XLs have breathers that vent back into the air cleaner. You will need an aftermarket air cleaner and breather manifold kit like the Screamin' Eagle shown here.
1990 and earlier Sportsters breathe through a fitting on the cam cover. Disconnect the line from the back of the air cleaner, install the KVS and mount it wherever you like.	A Harley breather separator upgrade (p/n 25329-91) was first used in April of 1991. It is a spiral, plastic air/oil baffle that is installed inside the cam gear cover steel breather tube. It is a retrofit part on 1992 and earlier bikes and is recommended any time the cam cover is off. Use it in conjunction with the OEM breather gear (p/n 25310-82a) or the S&S gear (p/n 33-4236) on any motor subjected to high rpm use.