For chainsaw carburetors, balance is critical. The carburetor controls the ratio of fuel to air coursing through the engine. When there's too little fuel, the engine will run "lean" and won't have enough power, so using it as intended can cause engine damage. If too much fuel mixes with air, it runs "rich," and could choke up and bog down, produce excess smoke, or waste gas. Changing the idle on a chainsaw carburetor isn't complicated, but getting the right mix can take practice. A new chainsaw will be properly tuned, but over time the idle will need adjusting. After you strike the perfect idle, your chainsaw will cut efficiently and smoothly.
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Troubleshooting: Does the Idle Need Adjusting?
As the saying goes, if it isn't broken, don't fix it. If you start the chainsaw easily, and it runs smoothly, the idle's doing just fine and there's no need to tinker with it.
So when should you adjust the idle? If the chainsaw's being stubborn and is hard to start, that's an indicator that the fuel and air mix are off. If it roars to life, but then falters and shuts off, that's a problem often caused by the idle. Finally, if the chainsaw stalls out and dies when it's working under a load, that's usually an idle problem as well.
Clean the Air Filter
A clogged air filter will complicate any attempt to adjust your idle. With the dust, dirt and oil chainsaws are routinely exposed to, experts recommend cleaning filters after five hours of run-time.
To clean your filter, if it's not obvious where the air filter is, check your owner's manual. The filter is usually protected by a raised, round cover that you can take off after removing some screws. Then, the filter is usually easy to lift off its seat. Different filters may have different care suggestions, but many will be easily cleaned with warm, sudsy water once you've tapped off any excess dirt and debris. After some gentle scrubbing and a rinse with clean water, let it dry, and the filter should be as good as new.
Getting Ready to Tweak the Carburetor Idle
Adjusting the idle on a chainsaw all comes down to the turn of a T-head screw. On right-handed chainsaws, the idle screw tends to be on the left. It's best to check the owner's manual, or do a web search for your make and model, to ensure you've found the proper screw or screws to adjust. You'll either tweak a single screw or three separate ones, but the premise is the same in both instances.
One-Screw Idle Adjustments
To find out your starting point, fire up your chainsaw and get a flat-head screwdriver ready. These T-head timing screws are extremely sensitive to turning, so work at most with quarter-turn increments with your flat-head screwdriver on the T-screw, and make mental notes as you go.
When turning the screw clockwise or counterclockwise, watch for when the engine's behavior changes both while you're throttling it but also when idling. If the chainsaw won't stop spinning when idling, or it's smoking when it idles, it's getting too much fuel and running rich, so you'll need to back it down a bit. If it's accelerating too slowly, or sputters and jumps erratically when idling, it's got too little fuel and needs more.
Take note of where the screw is when it runs lean or rich. Clockwise is generally the direction you turn to make the mixture leaner, and counterclockwise usually makes it richer. The sweet spot for the perfect idle is usually halfway between these two points.
If your carb has three screws to tweak, you'll see labels. Three-screw carbs are trickier to tune, so be careful when adjusting the high-speed performance screw. First, locate the "LA" screw. This controls the general engine idle-speed, and tweaking its performance is much like the single-screw carburetor adjustment above. Start by adjusting this screw as you would for the single-screw carbs.
The "L" screw is for low-speed adjustment. This controls the air/fuel mixture when it's idling, or resting at lower speeds. Adjust this screw second.
The "H" screw is for high-speed adjustment. This is the control for air/fuel mixture when you're running the saw at high-speeds or during cutting. This is screw is critical to be careful with, because setting it too lean or rich can cause serious engine damage. Some manufacturers recommend never adjusting this screw more than ¼ turn either way.
With all three of these screws, you'll follow the same process as for the single-screw timing adjustment. Turn it so you know where the lean and rich points are, and then find the medium between them.
It's safe to say that you should never have to completely open or close a timing screw to its maximum or minimum. If this seems like the right setting, it's probably wise to take the chainsaw into the shop for some maintenance.
Last Things to Look For
Before you call it a day on adjusting your carburetor idle, test its performance while throttling and idling. Make sure there's not too much smoke, that the engine purrs smoothly. Ensure the chain comes to a stop when the engine idles.
If you're not confident about your adjustments, or you notice performance issues with cutting or idling after the fact, it can be dangerous to use the chainsaw. In that case, go to a chainsaw shop or hardware store to have a pro check things out. If you do so, ask for tips and watch how it's done, so next time you're sure to strike the ideal balance on your carburetor idle.