By: Author John Cunningham. Published: 2019/05/26 at 1:12 pm
It’s not fun trying to handle timber with a chainsaw that won’t idle. Using a chainsaw is dangerous enough without having to balance the throttle to stop it from stalling.
So why won’t your chainsaw idle? The two most common reasons a chainsaw won’t idle are a blocked air filter and an idle screw out of adjustment. Other possible causes include:
- Dirty Air Filter
- Idle Screw out of Adjustment
- Bad Gas
- Bad Plug
- Vacuum Leak
- Carburetor Faulty
- Muffler / Spark Arrestor plugged
Just before we look at the most likely reason your saw won’t idle, we’ll need to check some of the basics, and I’ll fill you in on why they’re important, or if you want to skip all this, go ahead and slide on down to the “Adjusting the idle screw” below.
- AFR – What is it?
- Check basics
- Check air filter
- Locating idle screw
- Adjusting idle screw
- Saw surging
What’s Air Fuel Ratio?
Chainsaws are two-stroke engines, and they’re fitted with a small, very finely balanced carburetor. As you know, the engine runs on a precise mix of air to fuel, known as AFR (Air Fuel Ratio), 14.7 parts air to one part gas.
Any change to this ratio will cause poor performance a blockage in either the fuel or air, and it just won’t run right, especially at idle. Needless to say, the gas should be fresh and mixed to the correct ratio for your saw.
Old gas will cause all kinds of poor performance problems. Too much oil in the gas can cause the plug to misfire, and too little runs the risk of damaging the engine. The spark plug should be clean and gaped correctly. I like to have a new plug handy. It makes troubleshooting a saw a lot easier.
The spark arrester can block, causing poor performance and problem idling. The arrester may be removed for inspection and cleaning. I’ve covered that previously, and you can check that out here.
Balance – The carburetor is designed to mix gas and air to a set ratio of 14.7:1.
Any change to the ratio will cause performance issues. A blocked air filter or too much/little gas will change the ratio, as will a ton of other possible issues.
Check out How 2-stroke carburetor works for more detail and illustrations on the subject.
Check The Basics
Before adjusting anything, go ahead and check the basics – the airway is clear, the gas is fresh and mixed correctly, and finally, the spark plug is in great shape. You’ll need a few tools for some of these checks.
Tools – A fine flat screwdriver does the job. A Scrench would also be handy for removing the plug to clean or replace.
The Basics – Plug should be clean and gaped to spec. The gas should be fresh and mixed to the correct ratio for your saw. Typical plug gap .020″ – .025″.
Too much oil in the gas mix will cause the plug to oil and misfire, and as you know, too little will damage the engine.
The following links may be helpful:
Check The Air Filter
Cleaning the air filter is the obvious place to start. They get pretty dirty. Chainsaws, as you know, throw out a lot of dust and debris, some saws run a turbo-type system where the air is directed toward the carburetor, and it’s up to the filter to catch all this crap.
All modern chainsaws have tool-less air filter covers, which makes accessing them easy. Air filters differ from saw to saw. Some have a fine mesh screen, and others have a fabric or foam filter.
Compressed air is the best way to clean them, but turn the pressure down. No compressed air, no problem; make do with tapping it on a flat, solid surface to loosen up the debris. A clean rag or bristled brush will clean it out in no time.
Typically, filters last a long time as they are designed to be durable. However, if your filter has a hole or you see a lot of debris in the saw intake, you’ll need a new filter.
Cleaning – Pull the choke before removing the filter. It stops dirt from dropping into the carburetor intake. Filters can be reused unless damaged.
You may find Air filter cleaning helpful.
Locating Idle Screw
All gas chainsaws will have three adjusting screws on the carburetor. These adjusters can be accessed without removing any covers. The adjuster’s identity will be marked on the side cover of the saw where the adjustment occurs. You won’t see the adjusters. You identify the correct access port and insert the correct adjusting tool.
The first adjuster is the low throttle speed adjuster, marked on the body cover and carburetor with the letter (L). The second is the high throttle speed adjuster, marked with the letter (H). These two adjusters are positioned as a pair, side by side.
H & L adjuster screws often require a special adjusting tool, and they are fitted with limiter caps preventing over-adjustment. Adjusting takes a little finesse, and you can check out how to do that right here – 2-stroke carburetor adjustment.
The 3rd adjuster is the Idle screw adjuster. This is usually positioned just above or below the paired low and high adjusters.
This is the adjuster we are going to adjust. It will be marked with the letter I or T, or LA. Usually, you’ll only need a flat screwdriver, which will fit through the hole in the side cover. You may be tempted to remove the air filter to gain access, but adjusting needs to happen with the air filter fitted. That’s the point of the access ports.
Ports – The adjusting ports may be on the left or right-hand side of the saw. On some saws, you’ll need to push the pull handle to the side to access the ports.
The idle adjuster screw will be marked with the letters I, T, or LA. And is located just above or below the H & L portholes.
Adjusting Idle Screw
The saw should be warm before any adjustment takes place. The air filter should be clean, fitted to the saw, and the chain brake should be off.
1 – When the saw is warm and not running, turn the adjusting screw in (clockwise) a half turn, start the saw, and repeat this process until the saw idles.
2 – The saw now idles, great! But it should idle without the chain moving.
If the chain is moving while the saw is idling, then back the idle screw out a 1/4 turn until the chain stops; this fine-tuning can now be done with the saw idling.
If you find the saw keeps falling out of adjustment, like after you use it for an hour or so. Try a drop of super glue or thread lock on the threads of the adjusting screw. The constant vibration of the saw can often loosen the screws.
You may find this page useful; I’ve listed the tools I use to clean and adjust carburetors “Chainsaw tools & parts.”
Still Won’t Idle?
If the saw still won’t idle, you have cleaned the air filter, got fresh gas mixed correctly; the plug is clean & gapped correctly; the spark arrester is clean, and adjusting the idle screw hasn’t worked.
The next likely problem is a low-speed adjustment followed by a carburetor fault. First, let’s eliminate a low-speed adjustment issue.
Adjusting the low-speed adjuster is straightforward; you can check that out right here.
If low-speed adjustment didn’t help the idling issue, then it sounds like we have a carburetor issue. A carburetor cleaning and gasket replacement kit is the usual repair. But you can check out carburetor diagnosis to see how we diagnose carburetor issues.
Repair kits are available, but buying a whole carburetor is often a better option; it depends on your preference.
I’ve covered the carburetor removal, cleaning, gasket fitting, and adjusting in detail with pictures and illustrations, and you can check that out here:
- Carburetor removal
- Carburetor stripping
- Carburetor cleaning
- Carburetor gasket fitting
- Carburetor adjustment
Fitting the carburetor is pretty easy on most models.
If you choose to replace the carburetor, be sure to check and replace the gasket or seal between the carburetor and manifold and also replace the fuel filter and clean the gas tank.
Replace – Carburetors wear out, and when that happens, they are close to impossible to keep tuned. Replacing the carburetor is the way to go.
Is Your Saw Surging?
The saw is revving up and down by itself, and you can’t adjust it, so it runs smoothly. This is quite common, and it means you have a vacuum leak. Finding the leak can be difficult without the right tools.
A vacuum leak is air that enters the saw without passing through the carburetor, which means it’s un-metered. This causes the saw to run lean (not enough gas), which causes the saw to run hot and erratic.
Special gauges are needed to diagnose successfully. Otherwise, you are just throwing parts at it, which is never a good idea.
Vacuum Leak – Components that cause vacuum leaks include Carburetor; Manifolds, Gaskets; Crankcase seals; Cracks in the body casing.
Surging – Try spraying brake cleaner around the likely areas such as manifold gaskets, crank seals, and cylinder gaskets. A change in engine note or a stall indicates a vacuum leak.
Carburetor wear, crankshaft seal leaks, leaking head gaskets, and saw body cracks will all have you tearing your hair out.
You may also find “Chainsaw troubleshooting” section helpful.
Why does my chainsaw start and then stop? The carburetor is most likely blocked. Other possible causes include:
- Bad gas
- Blocked gas filter
- Idle Adjustment off
- Carburetor too lean
- Gas lines cracked and leaking
- Vacuum Leak
- Carburetor Faulty
- Muffler / Spark Arrestor plugged
- About the Author
- Latest Posts
John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s fluff-free How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.