Crank Case Venting


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A few words from Donnie Peterson about venting.

Before we look at a brief history of Harley "marking it’s spot" I will state a few rules that the thinking mechanic always follows. Please forgive my rambling, but the preaching must be preached.

Rule 1: Always examine the last thing touched on your bike if a problem develops. For example; a short has developed in your bike’s wiring system. Last thing touched was a replaced battery. Remove the battery and see if a wire was inadvertently pinched during installation.

Rule 2: Always examine when a problem developed and try to determine why it started then. Like why did it start at a certain time or mileage and not at another time or mileage. As an example, your beloved Shovelhead has started to smoke oil through the exhaust with only 5000 miles service. The engine was broken in correctly and the oil has been changed religiously. Last thing touched was a pesky leaking rocker box. No correlation with this repair and a smoking problem….or is there? Lots of non-dissolving silicone seal was used to seal the gasket to prevent further leakage. Only so much will fit on the gasket surface and the rest is squeezed off to the outside and to the inside of the engine. A glob of this resilient and effective sealer has blocked the oil drain from the top end. Oil fills the top end and has nowhere to go but down the valve guides into the combustion chamber to be burn

Rule 3: Fix the cause not the symptom. If a problem develops at a certain mileage…is that normal? If not…look for the cause. If the cause, instead of the symptom, is not discovered and remedied, then the same problem will only develop again. If an engine gradually begins to smoke at 40 or 50,000 miles it is logical to assume the internal parts are wearing normally. Using the example in Rule 2, where smoking is not indicated by wear, what would be accomplished by replacing the valve guides and rings when they aren’t worn? It will only have to be taken apart again to find the real cause for the smoking which is a blocked return oil passage.

Beware of fixing symptoms and not causes. Symptoms lead to the cause, which leads to a permanent solution.

There are certain axioms that the thinking wrench learns over time. There are many simple solutions to seemingly complicated problems. A mechanic who is worth his or her salt will effectively dial in on common situations that frustrate those who hesitate to think and charge blindly ahead in the wrong direction with expensive, ineffective fixes…that do not work.

If there are oil deposits coming from the venting system, oil caps exploding off the oil tank or oil coming through seals that are thought to be sealing properly….always, always check that a vent is not obstructed in any way. Remember that there is more than one vent. The engine is also vented to the oil tank and sometimes to the primary system.

This brings up another axiom to always follow in order to avoid frustration and extra-unneeded expense.

Always go from cheap and easy fixes to progressively more complicated and expensive solutions.

One of many defining characteristics of the Harley riders’ psyche is to invariably assume that an oftentimes simple problem requires the most complicated and expensive fix. As an example; I can’t tell you how many thousands of Harley motors have been rebuilt because the owner heard a knock from the spring loaded compensating sprocket in the primary case and assumed it was a worn bottom end in the engine.

Now let’s go back to generalities to further understand your problem.

The pistons, in their separate cylinders go up and down at the same time although they are on different strokes of the 4 cycles that constitutes one 720 degree revolution of the engine. Yeah, I know a revolution is one circle, which is 360 degrees, but a 4 cycle engine like Harley needs 720 degrees to finish the job.

On the upstrokes, one piston will be on the exhaust stroke while the other will be on the compression stroke. Conversely while going down, one will be on the power stroke while the other is sucking in the new virgin air/gas on it’s intake portion of the 4 cycles.

The pistons are big….fully 3 ½" in diameter….and they are moving fast. The rings sealed by a film of oil separate all that is below from all that is above.

As the pistons descend in tandem, there must be somewhere for the gases and air to go. It begins to compress giving it more force. It has to go somewhere. And it does. It goes through a vent. If there were no vent, the displaced gases would be forced through areas of least resistance such as seals, which wouldn’t be effective at sealing anymore.

On the upstrokes, a different problem occurs as a vacuum begins to happen since there is not enough air to fill the ever-increasing void. Air must be sucked through a vent from the outside world to fill the vacuum created by the sealed pistons moving up their respective cylinders.

Now, if it was only air we were worried about, there wouldn’t be much to concern us since it would move gracefully with the pressures created by the moving pistons as long as there was a clear path or vent to outside the engine.

The problem is compounded by oil mixing with the air in a gaseous and semi-gaseous state. The churning flywheels, the whirring engaging gears and the forcefully sliding pistons all combine to mix the two free flowing elements…air and oil.

When the engine is venting, there has to be a mechanism to separate the oily residue from the air, at least in an emissions free world.

This is why the symptoms you describe, Bob, dictate that a properly operating vent system is the first thing to check since it is logical, cheap and inexpensive.

Furthermore it is the one of the last things touched on your bike. Another recent change also points to the possibility of a venting problem. The Screaming Eagle air cleaner requires a vent rerouting process.

If the venting problem corresponded to the installation of the air cleaner kit, the, I think you have isolated the problem area to delve into. It is true that the umbrella valve, (part of the oil/air separation process), replacements were a result of the venting system not working properly but since we are looking in that area it doesn’t hurt to make sure that all the parts were installed properly. There is an oil separator pan in each head that the umbrella valve is part of. This valve is made from flexible rubber and looks like an umbrella. These, one in each head, operate as one way valves. It is important to physically examine these rubber valves for wear and correct installation.

The Venting Process

Let me describe the whole venting process and a trick to help your Harley vent more effectively.

The breather valve, located inside the timing cover in evos, turns inside a cavity. It is timed to open on the down strokes of the two pistons, by interfacing a rectangular window in the gear extension with a similar one in the cavity. The buildup pressure of oil and air is forced through this opening and into another compartment where the majority of the oil separates via gravity and is returned via the oil pump to the oil tank.

The gaseous air is forced up the 4 pushrod tubes and into the 2 heads. The umbrella valves are near the intake pushrod tube openings and are located in the middle wafers, (rocker box covers). These are the two closest to the carburetor. The exhaust pushrod tubes are the furthest forward one on the front head and the rear one on the rear head closest to the exhaust pipes. There are no umbrella valves on the rear portions since they deadend although the gases are free to be directed over through the working separator unit. They dead end because the goal is to channel the gases back into the carburetor for reburning and not out of the exhausts. The carburetor is closest to the intakes.

Whew! Bear with me as we are almost done.

The oily mist goes up through the one way umbrella valve into a depressed separation area where gravity once again assists in further separation of the oil and the air. Theoretically the air goes out through a raised hole down into the carburetor breather. This vent is seen externally as the two big bolts the screw into the heads, right and left, near the top of the inside breather plate and into the air box.

The bolts are hollow to allow the gases through.

The Trick!

The separated oil in the depressed middle wafer area flows via gravity through a small drain hole.

In a perfect world, tricks are not required. However, we’re talking Harley here so we need every trick available.

While double-checking the umbrella valves, drill out the drain hole to drill number size 42 which is 93 thousandths of an inch. This is to make sure there is no buildup of oil waiting to drain that might overflow into the breather hole and down into the air box and out all over your engine.

Let me tell you what just happened to Heavy Duty in our service department. We built a bike from scratch and it had your problem from day one. First we checked the vent to the oil tank, (as you should with your bike), to see if it was blocked or kinked or obstructed in any way. Then we checked the timing gears in the nosecone, (timing gear case), to make sure they were timed correctly. There was little or no chance that the breather gear was worn since we always install metal ones made by S&S not late style Harley ones that albeit are a fraction the price but are made from plastic. Plastic gears are not known for their longevity especially when interacting with metal as is the case in your bike. Furthermore, since Harley started replacing their own old style steel breather gears with the plastic ones, the Factory has bounced around using different ones with differing size breather openings for the problem you are having Bob.

Since the venting on your bike didn’t start acting up until 9000 miles we can assume that the right breathing gear was installed in you machine at the Factory.

This of course doesn’t alleviate the concern that the plastic breather gear didn’t wear excessively.

Now it isn’t that common at the mileage you report but if foreign matter were introduced or there was initial improper fitment that caused premature wear then it is another matter entirely.

However, because it is more complicated and more expensive to do this repair we will leave it to near the last on our list unless you have symptomatic evidence to suggest moving this fix up in priority. While a worn breather gear can exacerbate your problem, Bob, it is not the first area to look at

Guess what we eventually found? A partial blockage in the return line to the oil tank. Oil couldn’t easily return so more was left in the engine and the venting system couldn’t cope with the extra work generated by oil that wasn’t supposed to be there in the first place.

I only tell this story to show what happens to experts that have to be occasionally re-taught to follow their own advice.

A history lesson

All this vent talk reminds me of a '38 Flathead 80 I used to bomb around on which had a total loss oil system. I'd be riding along and the motor would start to get progressively louder, indicating a need for a squirt or two of oil. There was a plunger on the oil tank, which I would manually pump until the noise subsided to its normal clatter. If the beast started smoking, it meant there was too much oil in the bottom of the cases.

Government emissions control laws have forced Harley to modify their venting system over recent years. Knuckles, Pans, early Shovels, and Iron XL’s simply had a hose or fitting running to the outside atmosphere. If some oil came out with the piston-displaced air or the vapor was an oily mist….so what! No big deal, according to the thinking of the day. Well, it's a big deal now.

Late-model Shovels, Ironheads. and early Evos, (up to 1991), had their vent hose directed up to the carb breather where theoretically the vapors are drawn into the combustion chamber and burned innocuously. The use of gravity to help separate the heavier oil from the lighter air also assisted unless for whatever reason, the bottom end had too much oil residing in it’s bowels. With this scenario, some of the oil will go up to the breather and either flow out all over the motor or be drawn into the engine, causing a temporary smoking problem as the oil burns.

In 1992. the Factory mercifully relocated the vent to two positions, one in each head, where the umbrella valve/separation chambers allow displaced air/vapor to escape into the top of the breather. These vapors are then sucked into the combustion chamber for more complete combustion in the best attempt yet at controlling harmful emissions and annoying oil deposits.

The only problem is, as you have discovered Bob, that when all the emissions and oil are not adequately separated, Harley once again makes it’s mark…all over the side of your bike.

Gravity, again, is supposed to assist in separating the heavier oil from the expelled vapors before making its way all the way up to the head as opposed to just dropping out onto the ground as was the case with earlier models.

The new Twin Cam 88 "Fathead" has attempted to address this ongoing problem with a 3 stage oil/air separation unit. It is located inside the wafers on top of the heads between the rocker arms. After separation, the air is vented along a "tortuous" path into the air breather and reintroduced into the combustion chambers for burning those nasty emissions. "Tortuous" is an engineering term for a convoluted pathway to discourage any oil from following the air.

We have gone back to the last areas worked on to see if improper parts or installation has caused or exacerbated your problem.

The Screaming Eagle kit is worth looking at, as is the last umbrella valve repair…not to mention drilling out the drain hole.

The carb kit and the exhaust do not relate to the excessive venting.

Since the problem developed gradually for no other apparent reason, then, it is logical to assume a worn part. For this reason, the plastic breather gear is worth examining and possibly replacing with a steel one.

So far, we have attempted to fix the cause and not the symptom.

If all else fails, there is an exception to the rules stated at the beginning of this article..

The Exception: sometimes we have to subvert the symptom because it might not be possible to fix the cause without engineering and design changes.

The fix is quite effective from a perspective of years ago when emissions were not considered important. Running the vent hose to the ground has subverted the symptom so it is less annoying. If your bike used a rear chain instead of a belt it could even be a constructive and productive subversion. The vent line could be run over top of the chain to oil it.

Here is something else you can try.

Place the vent hose even higher with an inexpensive vent filter on the end to allow gravity a second chance…like under the gas tank near the front of the bike.

Yeah, I know. You shouldn’t have to do this but we’re dealing with a Harley here.

They can be real persnickity just because they weren’t totally designed right in the first place. Don’t get me wrong.

Harley Davidson is moving in the right direction. The Evo was a huge improvement over the Shovel and the Fathead will be a huge improvement over the Evo once the production bugs get worked out.