To run, your chainsaw needs air, fuel, spark, and compression in the cylinder. If any of those components are missing, your chainsaw won’t work – or at least won’t run and cut efficiently. In earlier columns we’ve discussed air, fuel, and spark. Here, we’ll delve in to compression testing. Compression allows the fuel/air mixture to explode, which creates power.
The Hang Test
If you suspect your chainsaw engine has low compression, as indicated by low power, stalling, or rough running, you can check it using the “hang test.” With the saw turned off, hold it by the recoil handle (attached to the recoil rope). If the saw has high compression, the recoil rope (pull cord) won’t move or will move slowly under the weight of the saw. In other words, the saw “hangs.” A saw with low compression won’t “hang” on its own, indicating that the engine has low compression. PHOTO 1: The saw on the left is almost new, with high compression. The saw on the right has high hours and low compression, as indicated by the fully extended recoil rope.
Measuring Compression
If your saw fails the hang test, it’s time to check the compression using a compression tester (available online). Most two-cycle engines have a 10-millimeter spark plug hole, and you’ll want to make sure that you have a compression tester that’s properly sized for your saw. Additionally, look for a compression tester with a Schrader valve in the end; these are much more accurate for small engines.
Step one: Remove the cylinder cover, spark plug wire, and spark plug. Screw the end of the compression tester into the spark plug hole, making sure that it’s finger tight to seal the O-ring in the cylinder.
Step two: Hold the throttle wide-open and pull the recoil. As you’re doing this, watch the gauge. Continue pulling the recoil until the gauge doesn’t go any higher. In most cases it takes four to six pulls.
Step three: Note the saw’s compression on the gauge. A saw in good condition will have 120-plus PSI. If the saw tests below 90 PSI, the saw will – at a minimum – need a top-end (piston and rings) rebuild. This particular saw tested at 80 PSI, below our threshold for a running saw. Saws that fall between 90 PSI and 120 PSI will run, but will have less power than a new or freshly rebuilt saw.
In the next installment we’ll investigate why this saw has low compression through visual inspection and the use of a pressure tester. From there, we’ll rebuild the saw and return it to service in the woodlot.