Why Is There Oil In My Motorcycle Air Filter Box?

I see this question asked in forum posts, and I wish I had found those posts before I started noticing my dirt bike was leaking oil. I noticed my TaoTao TBR7 Dual-sport Motorcycle was leaking oil when I was parked eating a sandwich, and smoke was coming from my motorcycle’s carburetor area. About to cover why is there oil in my motorcycle air filter box and wonder if you have seen the same?

I traced it back to the air box, and at that moment, I was stumped. Why was there oil in my motorcycle air box? After some oil line tracing, I figured out the most likely cause of the oil leak.

Oil was collecting in my air box, and I found the most likely source was the crankcase ventilation system. There was positive pressure in the crankcase, which pushed oil into the vent line, which carried it up the breather hose into the air box. I believe this was made worse by using too much oil.

Why Is There Oil In My Motorcycle Air Filter Box?

I believe when I was changing the oil in my motorcycle, I was using the manual’s listing of the oil sump capacity. It seems the listed 1.1 litters oil capacity in the TBR7’s manual was the amount needed to fill a dry sump.

Some oil is left behind in the engine when you do an oil change, so the follow-up oil changes only need to replace the oil drained. There was no need to replace the manual’s listed total oil sump capacity. I use the oil dipstick to measure my sump oil’s level, but in hindsight, I try to keep the oil level right at the top mark, even a little over.

Why Is Crankcase Pressure Pushing The Oil Into The Air Box?

The crack case is sealed; therefore, I was initially baffled by the positive pressure in the crankcase. I know worn piston rings will allow some exhaust to blow by, allowing pressure to build in the crankcase, but my motorcycle was new.  

I often questioned the quality of TaoTao when they made the TBR7, but my motorcycle was too new to wear that soon.

This guess is the answer that I surmised. As the motorcycle engine piston moves up and down, it forces air downward and upward in the crankcase. That is the purpose of the breather hose. The breather hose vents pressure from the crankcase, and the underside of the piston, so the piston can actually move.

If the crankcase weren’t vented, with small amounts of exhaust leakage pass the piston rings, the downward movement of the engine’s piston would be met by an upward pressure that accumulated on the underside of the piston in the crankcase. The crankcase would be pressurized.

Duh, Now I Know Why There Is A Crankcase Breather Hose.

As I surmised, the motorcycle engine crankcase breather hose is necessary to maintain a differential pressure across the piston and compensate for small amounts of piston ring exhaust leakage.

Oil splashing around in motorcycle air filter box.
Mess from leaking oil splashing out of the air filter box.

Was There An Oversight With Oil In Breather Hose?

Not really; I found what looks like a black ‘acorn-thingie’ in the crankcase breather hose. It helps the breather hose make a 90 degrees turn. The hose from the crankcase comes up, hits the ‘acorn-thingie’, and then turns 90 degrees rearward into a tube that goes to the air box.

This Acorn-looking thing??

I look inside, and it’s a foam filter. My guess was this is an oil trap. Catches oil mist, collecting it, so gravity returns the oil back to the engine crankcase. 

If There Was An Oil Trap, Why Is There Oil In My Air Filter Box?

Two causes for the oil collection, I believe:

  1. Operator Error. Yes, I’m at fault.
  2. Design Error.

Too Much Oil In Crankcase = Too Much Oil In Air Filter Box. (My Error).

Yes, I believe I overfilled my sump on more than one occasion.

I used one quart bottles of motorcycle oil and topped off additional oil to get the 1.1 liters of oil listed in the manual as the TBR7’s oil sump capacity. I used the tip stick but still believe the oil was more on the high side than the low side of the full mark.  

I have learned only to put in about a quart of oil with each oil change. I over this in my oil change post: TBR7 Motorcycle Oil Change Steps.

Crankcase Breather Hose Traps Oil In The Air Box.

After the crankcase breather hose oil trap, the outlet hose leads to the air box and this hose slopes toward the air box.  

Outlet of Oil trap(black 'acorn' with numbers) slopes down toward the air box.
Outlet of Oil trap(black ‘acorn’ with numbers) slopes down toward the air box.

Get it? Any oil that gets by the oil trap runs downhill into the air box. Once the oil is in the air box, it’s there to stay. This slope is why there is oil in my motorcycle air filter; it can’t get back into the crankcase.

Now I know these, I will be working on steps to stop oil from accumulating in my TBR7’s air filter box. Oil belongs in the engine, not the motorcycle air box.

Steps To Prevent Oil Getting Into The Motorcycle Air Box.

A simple list I put together to try to prevent oil collection in my air box immediately:

  1. Good care of your motorcycle engine.

    Good motorcycle oil and proper oil change cycles will minimize piston ring wear and tear. This step will limit piston ring wear and, later, the amount of piston ring leakage of exhaust gases and reduce the driving force for crankcase pressure, which means fewer escaping crankcase gases up the breather tube, carrying over oil into the air box.
  2. Minimizing excessive oil in the crankcase.

    The dipstick on the TBR7 is small, and the range between high and low is tiny. Since this motorcycle’s oil sump is very limited, I felt I had to maximize the amount of oil in the engine to extend the life of the oil.

    Not necessary since I do regular, frequent oil changes, which helps with #1 on my list. So I keep my motorcycle oil level between the high and low marks on the dipstick, avoiding keeping my oil level at the high level.

    This practice keeps less of the splashed oil from becoming a carryover in the crankcase vent.

My Changes Prevent More Oil In The Motorcycle’s Air Filter Box?


No. 1 and 2 of the above steps did not prevent all oil from collecting in my air box, but I did see a real slow down in the oil’s accumulation.  Why is there still oil in my air filter box?

Oil collecting slowed down enough with regular checks; I am not surprised by smoking oil dripping on my TaoTao TBR7’s engine from the carburetor air intake hose.

Less Oil Smoke Is A Good Sign.

I plan to eliminate the oil collection in my air box with some modifications and post it here later. Please check back.

In the meantime, Ride Safe, Ride Fun!

Click To See My Recommended
TBR7 Upgrades

Blog Update: Rather than opening the air filter box to wipe out the oil, I found an easier more routine way of removing the oil. Covered it with this post: Draining The Oil In TBR7 Air Filter Box.

FYI, found a product that seems to cater to crankcase breather tubes sending oil to the motorcycle air box, check it out: Motorcycle Oil Catch Cans. What I learned.

Picture of me, as a New Motorcyclist.
Just Me…Newly Licensed.

Hi I’m Tom, A New Motorcycle Rider and Blog Author.

I am a new rider(Pa Learners Permit at the end of 2020, and I received a Pa Motorcycle License in 2021 after passing a Motorcycle Safety Course).

I bought my first motorcycle, a TaoTao TBR7, at the beginning of 2021 and have been doing upgrades on that motorcycle since.

I added to my motorcycle collection by buying a Boom Vader Gen 2 in 2022, and that Grom-Clone motorcycle has been upgraded by me as well.

I continue to ride my Boom Vader Gen 2 motorcycle as well as my TaoTao TBR7 dual-sport bike.

Read more on my About Me page.

Fun Fact: I’ve only been on one group ride.

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