Oil Leaks on 2-stroke Engine Could Lead to Engine Power Reduction

Even an old motorcycle that looks like a beautifully finished, fully restored motorcycle can have “engine malfunction” problems behind the simple beauty of the assembly. While inspecting and cleaning the points, I removed the flywheel and also removed the point base, and found a buzzing leak from the crankshaft oil seal.

If it were a 4-stroke engine, a normal oil leak would not cause a significant change in engine performance. However, a 2-stroke engine is different. This is because in a 2-stroke engine, the “primary compression process” takes place in the crank chamber as the piston descends.

2 stroke dirtbimeleaksoil out of case I illuminated the inside center of the point base with an LED lamp and found the oil seal lip glistening and the nasty oil flowing downward! Did you know that if you have a 2-stroke engine instead of a 4-stroke engine, this oil leak can directly lead to a reduction in engine power?

Flywheel tightening nut is thumping on impact!

2 stroke dirtbimeleaksoil out of case
If you have a flywheel holder (special tool) on hand that clamps the flywheel circumference and holds it from rotating, you can also remove the lock nut with a socket wrench while holding the flywheel. If you don’t have a special tool, use an impact wrench to zap it and the fixing nut can easily be loosened by loosening it all at once.

2 stroke dirtbimeleaksoil out of case
Even if you can loosen the locknut, it is not easy to pull out the flywheel which is fixed to the taper of the crankshaft end. In this case, I used a cross-shaped flywheel puller. Even if it’s not the manufacturer’s original special tool, there are some original products from each tool shop or constructor that match the screw size.

The Phillips “flat head screw” is easy to snag!

2 stroke dirtbimeleaksoil out of case
The pointed base, where the generator and ignition coils are mounted together, is fixed in many models with a flat head screw in the Phillips screwdriver slot. It is natural to use a screwdriver that matches the shape and size of the Phillips slot, but there are many instances of this slot being nudged, so work should be done with caution. Using an electric screwdriver is recommended, but the most important thing is the match of the Phillips bit.

The previously mentioned “homemade tools” appear to shine through!

2 stroke dirtbimeleaksoil out of case
This is a home-made tool that allows you to pull out the oil seal pressurized into the engine by combining the tip of the Yamaha original special tool (sliding hammer) with a tip you made yourself (some oil seals cannot be pulled out, naturally). You can find out more information on the previous article, which was updated on September 27, 2020.

2 stroke dirtbimeleaksoil out of case
The trick is to pull the sliding weight in the direction of the crankshaft with force.

2 stroke dirtbimeleaksoil out of case
The oil seals are OEM Yamaha parts. This model was the FS-1, which was introduced in 1970, but had the same type engine as the YB-1, which was released around 2000 (and of course may have been used in other models as well), so the oil seals were in stock from the manufacturer.

Proper use of liquid gas and rubber grease

2 stroke dirtbimeleaksoil out of case
Apply a thin layer of fuel-resistant liquid gasket to the perimeter of the oil seal and an appropriate amount of rubber gasket to the oil seal lip. Apply carefully to avoid mixing the two together.

2 stroke dirtbimeleaksoil out of case
After cleaning the oil seal holder on the crankcase side, insert the oil seal into the crankshaft, being careful not to damage the oil seal lip. The trick is to push the seal in before the liquid gasket that coated the seal perimeter dries. If it’s not dry, the gasket will slide around and it will bite into the crankshaft after it dries. If you can’t get a good grip on it, use a pipe or deep socket to push it in. The key is to press the oil seal into the crankcase so that it matches the flange of the crankcase.

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This is the maintenance to keep your motorcycle in good condition. The most important part of such maintenance is “incidental” maintenance. By applying a thin layer of grease to the rear axle shaft after removing the rear wheel, it will allow for smoother removal of the rear axle the next time you take it out.

The grease will also play a big role as a rust inhibitor when it is dry and not greasy. You can also inspect the driven sprocket damper by removing the rear wheel. If the damper function is working smoothly, it’s good, but if the damper rubber is crusty or the damper rubber is thin and wobbly, the drive chain is prone to stretching. If you just replaced it, “Isn’t the chain stretching a little fast? If you feel like this, check the condition of the driven sprocket dampers before questioning the chain performance.

Since the wheel paint was finished with paint, the sliding parts of the damper unit became tight and the damper sometimes failed to function. Also, the damper rubber was sometimes so thin and worn out that it was not functioning properly. In any of these cases, the drive chain is prone to stretching, so I recommend inspecting it when you disassemble it. This is a good way to find out if there are any problems with your drive chain.

During the maintenance of this Yamaha Sport FS-1, I were able to find the root cause of the power loss by doing some “incidental” maintenance. In this article, I will report on one such example.

The owner informed me that the engine was sluggish and couldn’t get up to speed, so I checked the starting performance and also checked the ignition timing and the intermittent operation of the point base. At that time, I casually used an LED lamp to illuminate the “crankshaft seal” at the back of the point base. What I saw beyond that was a bad crankshaft seal lip. The engine oil mixed with the air mixture was flowing out of the crankshaft end, over the seal lip.

According to the motorcycle’s owner, even though he tried his best to keep the throttle at full throttle, he was only able to get a top speed of around 50km/h at best. Why is that after a full restoration? They were in distress with these thoughts. According to the manufacturer’s data at the time of its release, the top speed was 95km/h, despite its mere 50cc displacement! There is no way such a speed can be achieved with such an engine!

The oil seal was found to be defective. If the oil seal fails, the pressure of the primary compression will escape from the crank chamber and reduce the momentum of the compression mixture from the scavenging port to the combustion chamber. As a result, a good explosion could not be obtained and the engine power was probably insufficient.

The first thing I did was to replace the leaking oil seal with a new part. I used the DIY oil seal remover tool as described in our previous maintenance article. Thanks to this tool, the oil seal replacement went smoothly.

— Update: 11-02-2023 — cohaitungchi.com found an additional article Why Is My Dirt Bike Leaking Oil? from the website motocrosshideout.com for the keyword 2 stroke dirtbimeleaksoil out of case.

It’s happened already. You found that spot of oil on the ground, and it came from your dirt bike. What does that mean? 

2 stroke dirtbimeleaksoil out of case
Dirt Bike Oil Leak

A dirt bike leaking oil can come from many different areas. Some are problematic, but some are of little consequence. First, you have to find out where the leak is coming from. Once you see the source, the next two questions are why is it leaking and how do I fix it?

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This article will answer all of those questions.

The most common areas that leak oil on a dirt bike are:

  • Shifter seal
  • Drain bolt gasket/o-ring
  • Breather hose
  • Crankcase gasket
  • Cylinder head
  • Forks
  • Exhaust (2 stroke)
  • Cracked Case

How Long Can You Ride With An Oil Leak?

It Really depends on how much oil is leaking and where. Does the engine case have a hole in it from hitting a rock? If oil is quickly draining out, you’re not going to get very far. This is why it’s smart to have quick steel in your trail pack. 

A little drip here and there of oil is not going to hurt as long as there is still enough oil in the engine. Always check your oil before going on a ride so you know what the level is.

If you know it’s a little low and you don’t notice a leak until halfway through your ride, you can be confident that it’s not using much oil by checking the level again and comparing it to when you started. 

Oil Leaking From Shifter Seal

The shifter lever is connected to the shift shift that comes out from the engine case. Most dirt bikes have a rubber oil seal around the shaft to keep the oil in the engine. 

2 stroke dirtbimeleaksoil out of case
Shift shaft case bore is cracked and leaking oil.

This rubber will get hard over time and lose some of its seal, causing a leak. Excessive wear from a lot of use or dirty oil can also ruin the shift shaft seal. 

Simply replacing the seal with a new one will fix your oil leak as long as the engine cases is not damaged. 

How To Fix Oil Dripping From Drain Bolt

The oil drain bolt gets used a lot, or at least it should because you change the engine oil often enough. The bolt should have a crush washer on it that seals the bolt to the engine case. 

An old or cracked washer will cause a slow leak. Replacing it with a new one will usually stop the oil from dripping down onto your floor. 

Another issue that’s more serious is that the engine case is cracked. Clean the cases if they are dirty and inspect for cracks or damage. 

A cracked case where the drain bolt is would either be caused by hitting something or over tightening the drain bolt. 

Why Is Oil Dripping From Breather Hose?

The crankcase breather hose is there to let out the excess of air that the engine pumps up. Too high of an oil level can cause oil to leak out the breather hose on your dirt bike. 

If you see oil coming out of this house or tube, simply check the oil level. If it’s too high, you can try draining some oil out to correct it.

Oil Leaking From Crankcase Gasket

This one may suck to realize. A little weepage is actually not that big of a deal, however. Paper gaskets get hard with age and may lose some of the seal.

About the only time that is a concern for a leak from the crankcase gasket is when there is physical damage to the case, or the gasket/case was incorrectly installed.

Cylinder Head Leaking Oil

This is a similar case to the crankcase gasket. A cylinder head leaking oil is usually from old and hard sealer for the cam tower. This may look ugly and dirty oil may cover a large portion of your cylinder over time.

As long as the oil level isn’t noticeably going down, there’s not much to worry about.

You can wipe off the excess oil, or leave it for “character”.

Forks Leaking Oil? Try This First!

Leaking forks is another frustrating problem. Maybe you just had them serviced. All it takes is some dirt packed into the seal or a small nick in the fork tube for a leak to start.

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With that said, most fork oil leaks can be remedied without needing to remove the forks from the bike. All you need to do is clean the fork seals.

Here are the steps to cleaning your fork seals with a Seal Mate:

  • Remove the fork guard if it has one.
  • Use a pick or small screwdriver to pull the dust seal off (careful not to scratch the fork tube)
  • Take the Seal Mate and stick the “hook” end in the fork seal. 
  • Rotate it around the fork tube while pulling any dirt “out” from in between the seal and fork tube. 
  • Do this until it is clean, then re-install the fork parts and ride again. 
  • You may have to add a little bit of fork oil if a significant amount leaked out. 

If cleaning the fork seals didn’t fix the leak, then the seal is most likely torn and needs to be replaced. Clean and fix any Nick’s in the fork tube that might have caused this before putting everything back together. 

Bad Connection Causing Exhaust Oil Leak

An exhaust leak is caused by a poor mounting connection or a bad seal.

First, make sure all of the connections and mounting hardware is tight. 

Is It Bent?

If everything is tight, take a closer look to make sure the exhaust system is straight. All it takes is a bent mount from a tipover to tweak the exhaust out of alignment. 

Most exhaust systems can be straightened by a professional. This can be considerably cheaper than buying a new one, but you may lose some of the coating if it needs to be heated up. 

Leaking Seal/O-ring?

A bad seal will cause an exhaust leak. This can be caused by an old seal that got hard or installing a new or used seal incorrectly. 

Replacing the gasket or o-ring with a new part and properly installing it on a straight exhaust should give a good seal that won’t leak oil. 

Cracked My Case! Can It Be Repaired?

A cracked engine case is like a sucker punch to the gut. Putting a hole in your case while out in the middle of nowhere will ruin your day… unless you go prepared!

While it’s not a good permanent fix, using some quick steel to cover the hole should get you back to camp or home. 

Make sure to check the oil level before taking off again, because if it’s too low, you may end up seizing the engine due to lack of lubrication. 

How To Fix A Cracked Case On The Trail

Preparing the surface on and around the hole is the most important part. If it’s not clean and dry, the new material will not stick. 

In a perfect world, you would have sandpaper and rubbing alcohol wipes to rough up and clean the surface, but those items aren’t in everyone’s trail pack. 

Get creative and wipe it down as best you can. Stealing some gas from your buddy’s tank will do a good job of cleaning the area to prepare it. 

Whether you use JB weld or quick steel, just remember to follow the instructions for mixing it properly. 

Taking the time to do it right will significantly improve your chances of making it back. Otherwise you’ll end up doing it again or be stuck and have to tow your bike out.

The Bottom Line

If your dirt bike is leaking oil, check to see where it’s coming from and how much. Always check the oil level before riding. If the oil level is low every time because of the leak, then it’s time to repair it. 

How to ride with confidence and control off-road

Learning to ride a dirt bike is easy, but riding it safely with control off-road is a completely different story. It takes many hours of riding to become a skilled rider, but none of that matters if you have poor riding technique.

I want to show you the proper riding techniques to quickly give you more control so that you stay safer riding off-road. Click here to learn these techniques.


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About the Author: Tung Chi