DK Custom Products

Phone: 662.252.8828
Text: 662.420.4891
Email: [email protected]

Why is there Oil down the Side of My Bike?!?!

There are so many questions each week about:

Why is my bike dripping oil from the air cleaner?

Why do I have so much blow-by (or as HD calls it, oil carryover)?

Why does the side of my bike get covered with oil mist?

Here's an FAQ Video. 


To start with, it is important to understand that all internal combustion engines have a certain amount of blow-by. This is because, by design, they build crankcase pressure, and that that pressure has to have a place to vent.

On modern Harley's, this happens at the head breather bolts.

Prior to the hot oily air exiting via the head breather bolts, HD has what is called an umbrella valve. Its' job is to filter out the majority of the oil and have mostly hot air vent out the breather bolts. They do a pretty good job, and when everything is right, it is mostly air and a small amount of oil that vents out the head breather bolts.

Umbrella valves can go bad, however, the majority of time that there is excess oil venting out the breather bolts it is NOT from bad umbrella valves, it is usually something else.

Below is a list of the most common reasons for excessive blow-by, and what can be done about it.

1. The single most common reason is incorrect oil level. Every owners manual has instructions on what the correct level is. 

However, here is the bottom line, straight from the HD owners manual-

"The correct oil level should register midway between the FULL and ADD marks on the dipstick.

If oil level is at or below the ADD mark, add only enough oil to bring the level to the middle of the two marks on the dipstick."

Of course, this is confusing because HD refers to the top mark as "Full" when in reality, they say in the same sentence that midway between the two marks is full. Typical. lol

2. Old oil is losing it's viscosity, or Incorrect viscosity for temperatures you are riding in.

In the HD Owners Manual the following chart appears-


If you are riding in temps of 60° or higher, the best viscosity, which will result in the least amount of blow-by, is straight 50 weight. 20-50 may have just as good of lubrication qualities as straight 50 weight, but IT WILL produce more blow-by at temps of 60 degrees and higher.

If you are riding in temps of 80* or higher, the best viscosity is straight 60 weight. IT WILL produce LESS blow-by.

3. The bike is running too hot. The hotter the oil, the more blow-by. Modern synthetic oil starts breaking down around 300 degrees, and dino oil around 250 degrees. However, LONG before reaching those temps it gets thinner and by-passes the umbrella valves and results in more blow-by. Most will agree that the ideal oil temp is between 180-210 degrees.

4. It may not be the blow-by that is dripping oil or putting it across the side of your bike. If you have a re-useable filter, it may be over-oiled. Sometimes they come from the MFG. over-oiled, and sometimes they are over-oiled after they are cleaned.

5. Excessive lugging of the engine will produce more blow-by.

The above are all things you can address fairly easily.


Below are more serious reasons, that can require more effort to fix-

1. Engine worn creating excessive blow-by. (rings, valve seals, etc.)

2. Umbrella valves not functioning correctly.

3. Crank out of round (creating excessive crankcase pressure).




How can I manage This Mess?

 All DK Custom Outlaw Air Cleaner Systems have an EBS Included!




What is an External Breather System?

EPA mandates have Harley-Davidson, & almost every aftermarket air cleaner company, pushing the blow-by into the air cleaner.  The blow-by that comes out of the breathers is primarily exhaust gasses and oil, it is also hot.   This is counter-productive to performance & engine longevity.  Harley engines perform better with cool dry air & gasoline, not hot, oily air and gas. 

An EBS routes this hot, oily, oxygen depleted air to the atmosphere, instead of into the combustion chambers via the air cleaner.


How does an EBS Increase Engine Longevity?

Simple!  It only allows clean air into the engine!


Most air cleaners route the oily crankcase air into the air cleaner, which then goes throught the carburetor or throttle body, and then into the combustion chambers.  

Since the engine only burns gasoline and oxygen, that oil ends up as crusty carbon deposits on the pistons and valves.

Periodically, little pieces of this carbon break off and scrapes the cylinder walls or gets stuck between the valves and the valve seats, both of which prematurely wear the engine. 


 An EBS equipped Air Cleaner does not allow any oily air into the carburetor, throttle body or combustion chambers!


Photo of the inside of an engine with a stock air cleaner or a typical "Stage I" air cleaner that does NOT have an EBS.


Photo of the inside of an engine with an EBS, that allows only clean dry air into the intake.



  Photo of a valve that has very high build up from oil coming through the breathers.



Photos of the glop that was drained from an EBS Catch-Can.  This oil-water mix would normally go into the throttle body/carb and then into the combustion chambers.

With an External Breather System, it keeps this mess out of your engine!





How does an EBS Increase Horsepower & Torque?


Gasoline engines make power by burning a combination of oxygen and gasoline.  

With an EBS equipped Air Cleaner, only gasoline and oxygen rich air enter into the combustion chamber.  

With an EBS equipped Air Cleaner, the air is also cooler.  Cooler air is more dense. More dense air contains more oxygen, equalling more power on each combustion stroke.

You may have noticed that your Harley runs stronger/more crisp in cold weather. That is because the colder the air, the more power.  Search "cold air intakes", you will see that folks spend hundreds, sometimes thousands, to get cooler air into their combustion chambers in order to generate more power.


With a non-EBS equipped air cleaner (all stock & most aftermarket, like the SE, Big Sucker, etc.) there is oxygen depleted HOT air and oil going into the combustion chambers.  Not only will this not burn, it also takes up space that would otherwise be occupied by oxygen rich air & gasoline, thus reducing Horsepower & torque.


 Dyno Charts showing the Increase in Power achieved by venting externally, via an External Breather System (EBS).




How Does an EBS help keep your bike in Tune?  

Tune Stabilization.


We hear it everyday!  "My bike was running great 6.000 or 8,000 miles ago.  I have not done anything to it, but it just is not running as good as it used to.  I took it to my mechanic and he says it needs to be dyno tuned, again."  

That is not because the tune changed, it is, more than likely, because either the compression ratio changed, the carburetor got gummed up, or the throttle body and sensor got gummed up with the oily air passing through...usually Both!

When there is carbon build-up on the pistons, this increases the compression ratio.  If your bike was running great before the carbon build-up, then after the carbon build-up, that tune that made your bike run great is no longer matched to the increased compression ratio, resulting in a poorer running bike.

If the carburetor or throttle body/sensors are gummed up, they cannot function properly, resulting in a poor running motor.


With an EBS equipped air cleaner there is no oil being fed where it does not belong. There is no change in compression ratio, there is no gumming up of the intake. There is no need to constantly re-tune the bike just to keep it running well.


Photo of a gummed up intake, due to oil the breathers being routed into the air cleaner.



All Stage I Air Cleaners are NOT the same!  Most "Stage I" air cleaners route performance robbing hot, oily air back into the engine.  

Our Outlaw Air Cleaners, Velocity Stacks, and Optimized Air Cleaners are true Stage I Performance Air Cleaners that allow only clean cool air into the intake.  You can see them HERE.

If you already have a free flowing air cleaner that you are really attached to, we likely have an External Breather System that will work with it.  That way you can keep it AND get better performance and longer engine life.  Click HERE to see our selection of EBS kits to optimize your existing air cleaner.  



Types of External Breather Systems:

Breather Bolts that have a filter built into them.  These are good for most Sportsters, Big Twin Evo's and Twin Cam 88 & 96.  Some Twin Cam 103's do very well with them also.

If your engine does not have excessive oil coming out of the stock breathers, then the small filters in the Breather Bolts will work well.  

Hose & Filter/Catch Can style EBS.  These work well with all Harley Engines.  There are many variations (seen in the link above), and they all keep the hot, oily, oxygen depleted air out of the engine.  The filter is designed to absorb it and prevent it from dripping on the ground or spraying on the bike.  If a Catch Can is used instead of a filter, the oil collects in the Catch Can for future dumping/inspection.


What EBS will fit your Air Cleaner?


Twin Cam & Milwaukee-Eight





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