Harley Breather Bypass: Should You and How? (2024)

Harley Davidson motorcycles are known for their unique design and impressive performance. However, one aspect of these renowned bikes that often sparks debate among enthusiasts is the breather system, particularly, the idea of a Harley breather bypass mod. In this comprehensive guide, we delve into the specifics of the Harley breather system, why you might consider bypassing it, and how to do so.

Table of Contents

What Does a Breather Do On Harley Davidson Motorcycles?

The breather system forms a critical part of Harley Davidson engines. It’s a closed-loop system designed to vent positive air pressure, blowby, and oil mist from the crankcase back into the air box. The primary purpose of this system is to minimize harmful emissions by burning the blowby rather than releasing it directly into the environment, which was a typical practice in older engines.

Here's an infographic diagram showing how the oil breather system works on Harley Davidson motorcycles. — Here, you can see how the oil is (typically) recirculated through the system.

The breather system is in line with current EPA regulations, which prohibit venting crankcase gases into the atmosphere. The venting process is facilitated through the crankcase breathers located on the right side of the engine, which then direct it towards the intake, just before the throttle body – when using a stock air box. Ultimately, the blow-by is routed to oil catch cans near the air box.

While this design is environmentally friendly, it’s not without its drawbacks. When the engine produces excessive amounts of oil vapor, the closed loop breather system can struggle, leading to complications that affect performance and engine health. Enter the breather bypass mod, keep reading below.

Why Would You Bypass It?

The idea of a Harley breather bypass mod arises from the challenges faced by the breather system when dealing with large amounts of oil vapor. With an overabundance of oil vapor, the breather system can cause oil to foul the air cleaner, throttle plate, and lead to substantial carbon buildup on the pistons, and inside of the air box. If using anything other than a stock air cleaner, this procedure likely doesn’t apply to you.

This carbon buildup can disrupt airflow and increase the compression ratio over time, leading to poor performance and a rough idle. In severe cases, this could potentially shorten the engine’s lifespan.

A Harley breather bypass offers a solution to this issue. It reroutes the blowby to a catch can or the atmosphere, significantly reducing carbon buildup within the combustion chamber. This simple modification can extend the life of your engine and optimize its performance.

Related: Harley Oil Breather Problems? I Have The Answer!

Here’s an example of a breather bypass kit for Harley Davidsons:

example of a breather bypass kit for harley davidson motorcycles

Pros and Cons to A Harley Breather Bypass Mod

Like any modification, a breather bypass has its benefits and drawbacks.

Pros

Extended Engine Life: By reducing carbon buildup on the pistons, rings, and valves, a breather bypass can reduce wear and tear on your engine, leading to a longer engine life.
Improved Performance: A bypass can enhance your bike’s performance. By eliminating hot, oxygen-depleted blowby vapors from the intake, your bike can access more fresh oxygen, leading to slightly increased power.
Reduced Oil Consumption: If your bike has been burning oil through the breathers, a breather bypass can help maintain your oil level, reducing your engine oil consumption.
Cleaner Air Cleaner: With a breather bypass, you won’t have oil inside your air cleaner anymore, saving you cleaning time and potentially extending your air cleaner’s lifespan.

Cons

With the benefits come a few drawbacks:

Additional Maintenance: A breather bypass system can add another maintenance task to your list. If you use a catch can, you’ll need to empty it regularly to prevent overflow.
Potential Legal Issues: Venting crankcase gases into the atmosphere may violate environmental regulations in some regions. Always check local laws before making modifications.
Installation Complexity: Installing a breather bypass can be complex and may require a decent mechanical skill level or professional help.

Keep reading:
> Who Makes Harley Davidson Oil Filters
> Who Manufacturers Harley Davidsons Spark Plugs

Why Do Motorcycles Have Breathers In The First Place?

Breathers are crucial in modern engines, including motorcycles, as they help manage internal pressure and emissions. As the engine operates, a certain amount of gas and mist escapes past the piston rings into the crankcase. This “blowby” can build up and create pressure within the crankcase, potentially leading to leaks or other issues.

This is an infographic detailing how blow-by occurs inside an engine. This also illustrate why breathers are used in a recirculation system. — Here, you can see an example of blow-by in action.

Breathers help vent this blowby, relieving pressure in the crankcase. In modern engines, this is usually done in a way that minimizes environmental impact, often by rerouting the blowby back into the engine to be burned off.

Related: Harley Oil Pressure Problems? Read This! (Help Is Here)

Can You Bypass The Breather Yourself?

Yes, it is possible to bypass the breather yourself, but it requires a certain level of mechanical skill. The exact process can vary depending on your specific air cleaner and its design, but the general steps involve installing new bolts or barbs, connecting hoses, and running the hose to an exit point or catch can.

close up picture showing a Harley after a Breather Bypass

There are kits available online that can make the job easier, but they can be more expensive than a DIY approach. If you’re not comfortable making these changes yourself, consider hiring a professional to ensure the job is done correctly.

How To Bypass A Breather On A Harley

Here’s a general guide on how to bypass a breather on a Harley:

Remove the air cleaner and filter: Start by removing the air cleaner cover and the air filter from the assembly. Depending on your model and whether you’ve upgraded from the OEM, you may also need to remove the back plate.
Install new bolts or barbs: You’ll need new hose barbs or bolts to connect hoses and redirect the blowby. These are usually included in a breather kit, or you can repurpose the factory breather bolts and reroute the hose.
Connect hoses: Connect your hoses and secure them to your bolts or barbs. Depending on your setup, you might use a T-connector or run a hose from one bolt to the other, then down to a catch can or breather.
Run your hose to an exit point: Finally, run your hose to an exit point on your bike. You could use the end of the exhaust pipe or install a catch can in an inconspicuous location under the bike. Remember, if you use a catch can, it can’t be fully sealed. You’ll need a breather or vent in it, usually at the top.

FAQs

In this section, we’ll answer some common questions about the Harley breather bypass.

Why is my Harley leaking oil from the air filter?

If your Harley is leaking oil from the air filter, it’s likely due to the breather system. When the engine produces excessive oil vapor, the breather system can struggle to handle it. This can lead to oil fouling the air cleaner and eventually leaking out.

Will a breather bypass void my warranty?

Modifications like a breather bypass could potentially void your warranty, depending on the terms of your specific warranty agreement. Always check with your dealer or read your warranty terms carefully before making any modifications.

Can a breather bypass improve performance?

Yes, a breather bypass can improve performance. By eliminating hot, oxygen-depleted blowby vapors from the intake, your bike can access more fresh oxygen, which may lead to slightly increased power. It can also reduce carbon buildup in the combustion chamber, leading to smoother operation over the long term.

Can I install a breather bypass on any Harley model?

While this guide focuses on the Milwaukee-8 engine, a breather bypass can be installed on many different Harley models. The exact process may vary depending on the specific engine and air cleaner design.

Wrapping Up

In conclusion, a breather bypass can be a beneficial modification for your Harley, potentially extending engine life and improving performance. However, it’s not without its drawbacks, including additional maintenance and potential legal issues. As always, do your research, weigh the pros and cons, and consider seeking professional advice before making modifications to your bike. Happy riding!