Troubleshooting An Air Leak

A crankcase air leak is a problem many pro saw engines have at some point in their life. Symptoms include: over reving, overheating, stalling on acceleration, failure to idle, and worst of all, an engine seizure if the problem is neglected.

When an air leak occurs, it often puzzles users and even some saw shop technicians because the symptoms of an air leak are varied. Misdiagnosis often causes unnecessary service costs and part replacement. It can also cause down-time and loss of productivity. It can even cause catastrophic engine failure.

How a Two-Cycle Engine Works
Two Stroke Animation

To understand crankcase air leaks, one must have a basic understanding of how a two-cycle engine works, because they are a problem unique to two-cycles.

The animation (right) shows how air flows through a two-cycle engine. Starting at the carburetor, the "tube" under the muffler, you will notice the blue cloud starts here. As you watch the animation, what you see is the piston traveling up the cylinder, and as it passes by the tube (intake port), air mixed with fuel (the blue cloud) enters the main case.  It does this because the piston traveling up the cylinder expands the volume of the main case area. This causes a drop in pressure (vacuum), which is what draws in the blue cloud.

Next, the piston travels back down the cylinder. As it goes down, it will close the intake port and at the same time pressurize the blue cloud in the main case. When this occurs, the blue cloud will seek an exit, which is the transfer port. In the animation, this is located to the right of the cylinder. As the piston reaches the bottom of its stroke, a portion of the blue cloud has found its way into the combustion chamber. As the piston starts to travel back up, it closes the transfer port and starts to compress the blue cloud in the combustion chamber. As the piston nears the top of the cylinder, the spark plug ignites the blue cloud. The heat of the burning air and fuel pushes down on the piston, which rotates the crank shaft. As the piston travels past the exhaust port, the burned air/fuel mixture is discharged

This is a very brief explanation of how a two cycle engine works, but hopefully you will understand that if you have any air leaks in the crankcase area, this engine will not work efficiently. It is easy to see that when there is a vacuum in the main case, if air leaks in through a bad seal or leaking gasket instead of going through the carburetor, the air/fuel charge is diluted.

Where Leaks Occur

Where can these leaks occur? The main bearing seals are common culprits. Bad base gaskets on the cylinder and crankcase gaskets are also frequent offenders. Even the small impulse (vacuum/pressure) hose, which runs from the crankcase to the carburetor, can cause a big leak.

Today's saws have rubber mounts isolating the fuel tank/carburetor from the engine, so there's a lot of movement between these two components. Problem is, one component, the crankcase, supplies impulse to run the carburetor fuel pump in the other component. A rubber hose connects the two, but deterioration or a tear in it often leads to an air leak.

(Text continues after the photos)

Blocked exhaust port

When preparing an engine for testing, the exhaust port can be blocked off with an old muffler flange and a piece of rubber from a tire's inner-tube.


Hand operatied air pump

This pump can produce both pressure and vacuum for air leak testing. Even small leaks will cause the needle on the gauge to fall.


Clutch side crank seal

Crankshaft seals, like the one located behind this oil pump, are a place to look for an air-leak.


Ignition side crank seal

The ignition side crankshaft seal is another component that can leak air.


Cylinder base seal

This pencil points to the base of the cylinder, a common place for a leak to occur. The main crankcase gasket, visible just below the base, is another place to look for a leak.

How does one go about determining if a saw has an air leak? The best way is to pressurize the main case area and test. This is done by removing the carburetor and muffler, and then blocking off these ports - both intake and exhaust. Once the ports are sealed, we apply pressure 6 - 8 lbs. of pressure. Watching the needle on the pressure gauge, if it drops more than one pound of pressure a minute, you've got a leak to fix.

How to Find a Leak

Now it's find the leak time. Sometimes a leak is so bad you can hear it "hiss" as the pressure leaks out and instantly identify the problem. Sometimes it is harder to find. When this happens, pressurize the case as described above and then spray a little WD40 or soapy water around the engine at suspected components. You will probably see bubbles coming from somewhere. If this doesn't work, as a last effort, you can submerge the engine you are testing in a tub of water, but this is rarely necessary. 

Once you've found a leak, fix it and retest. If it passes a pressure test, apply a vacuum. When it comes to leaks, vacuum is just as important as pressure. Some engines will hold 6 - 8 lbs pressure, but not be able to hold a vacuum.  If you find this problem, suspect the main bearing seals.  

If you find one leak, don't assume a saw is fixed. Repair the leak and retest. Before reassembling the saw, the crankcase should hold both pressure and vacuum.

Our Advice

Suspect an air leak if your saw has recently been hit hard or smashed and now doesn't run well. It is easy to miss a cracked heat block or some other damage when you are fixing damage from a hard hit. Also suspect an air leak if your saw's carburetor goes out of adjustment frequently. Some air leaks vary and cause the saw to run rich and then lean. When this occurs, the operator often thinks he's got carburetor problems. Also suspect an air leak if your saw "races." This is a term we use to describe a saw that tends to run lean at a high RPM.

Sometimes small air leaks can be masked with a carburetor adjustment. If you suspect an air leak, but can adjust it out, don't run your saw this way. Even if the saw runs OK, it is injesting unfiltered air.

Some air leaks are easy to fix, while others hard for pro users to repair by themselves. Understanding these operating principles should help you diagnose your problem and fix it yourself or help you to decide to take it into a saw shop with a qualified technician.

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