Run Check

By Tim Ard, Forest Applications Training, Inc.


How should a saw perform once it is started? What do you look for to make sure it is going to run and do the Job you need it to?


Five areas to check out to make sure the saw is performing as it is designed:


1. Clean Air Filter - you must make sure the chainsaw nose (filter) is breathing properly.    If the filter is restricted the airflow for proper adjustment will be low and cause smoking, low power and poor running results.

2. High Speed Levels to Flutter - let the saw warm to running temperature before attempting to determine if high-speed settings are correct. This will take a couple minutes or so after starting to accomplish. Then hold the saw at wide-open throttle for a few seconds. The rpm should come up to a full throttle position and then level off. It sounds kind of like a flutter or a slight blubbering tone. The flutter sound must be in any two cycle high speed run to make sure you have sufficient lubrication and fuel for the engine. A digital tachometer can be used to check if the run is set to design parameters suggested by the manufacturer. A tachometer will only tell you when an engine is in adjustment - it won't however tell you when it's out of adjustment. An engine can show within the design a maximum RPM setting that’s within specification but can have an air leak or other engine problem and can still be running in a lean state. That's where the operator must understand a two-cycle flutter and make sure the saw at top end is fluttering. If not major damage to the engine can and most likely will take place. The high-speed setting is done with the H screw on the carburetor.

3. Chain Stops at Idle - now bring the engine back to idle for the next observation. We want to make sure that the engine is at idle for a couple reasons. The first is   safety because you do not want to be walking around with a chainsaw where the chain doesn't stop turning at idle. This also indicates that the engine is idling low enough, if the saw chain stops, to disengage the clutch mechanism and let the low speed fuel circuits in the carburetor take over. When the chain turns at idle you adjust the T or LA screw to raise or lower the RPM to disengage the clutch.

4. Idles In All Positions - now that the chain is stopped and the saw is at idle, the saw should idle in all positions until it runs out of fuel. If the saw is getting too much fuel at idle it will puddle up in the crankcase area and as soon as you roll it over it will flood the port and the engine will stall. The L screw on the carburetor adjusts this scenario. In this case you would close the screw slightly clockwise to reduce the fuel flow.

5. Accelerates Without Hesitation - next the acceleration should be checked. Open the throttle quickly and the RPM should come up without hesitation. If the engine hesitates before quickly rising to wide-open throttle, the L screw on the carburetor should be opened counter-clockwise to allow more fuel flow. It takes fuel to create the power to rotate the engine.


You should remember that you could damage the saw engine quickly if you do not run it properly adjusted. I hear often that supervisors and shops do not want the operator to have access to a screwdriver, nor to adjust the screws. They state that they don't want the saw blown-up from someone who doesn't know how to properly adjust the screws. I agree, but I have said for years that I think I see as many saws and trimmers blown up from lack of adjustment as I have adjustment. If the operator doesn't know when it is out of adjustment they just run it. You do not have to have a screwdriver yourself however to adjust the saw. If you know when it is out of adjustment you can simply take it to someone who does.


Think about - anytime you turn a carburetor screw to the right, clockwise, you take a chance of causing engine damage. You remove fuel and lubrication with a clockwise turn of the screw.


Operators should all be aware of how to check the run of any two-cycle piece of equipment. If they are going to work safely and productively with the machine it must be in tune...


More information on carburetor adjustment can be found in our ForestApps eBook available from and from the eStore at


Check out the articles on carburetion and fuel under the info/articles link on the homepage.    Good Sawing...