Troubleshooting An Air Leak
A problem all pro saw users will eventually have with their saw engine is an air leak. This is puzzling problem to many because it is a problem unique to two-cycle engines. When this occurs, saw engines will over rev, overheat, stall on acceleration, fail to idle, and worst of all, seize up if neglected.
To understand air leaks, one must have a basic understanding of how a two-cycle engine works. 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 charge is diluted. Where can these leaks occur? The main bearing seals are common culprits. Bad base gaskets on the cylinder and crankcase gaskets are also frequent contributors.
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 of the rubber often leads an air leak.
How does one go about figuring out where all these leaks are? The best way is to remove the carburetor and muffler and block 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, something is leaking too much.
Now it's find the leak time. Sometimes a little WD40 or soapy water sprayed around the engine will show it. Sometimes it's so bad you can hear it. In the worst case. you can submerge the engine in a tank and it will immediately become visible.
Once you've fixed the leak, 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, change the main bearing seals. Before reassembling the saw, the crankcase should hold both pressure and vacuum.
Understanding these operating principles should help you to diagnose your problem or help you to decide to take it to a qualified technician.
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