Chainsaws on a bench

Chainsaw Won't Idle

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 is 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
Chainsaws on bench

What's Air Fuel Ratio?

Chainsaws are 2 stroke engines and they are fitted with a small very finely balanced carburetor. The engine as you know runs on a precise mix of gas to air, known as AFR (Air Fuel Ratio) typically 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, makes troubleshooting a saw a lot easier.

The spark arrester (muffler) can block causing poor performance and problem idling. The muffler can be removed for inspection and cleaning.


Chainsaw carburetor


The carburetor is designed to mix gas and air to a set ratio 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.

Chainsaw air filter
Chainsaw spark plug


Plug should be clean and gaped to spec. The gas should be fresh and mixed to the correct ratio for your saw.

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.

Chainsaw gas oil mix

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 air is directed towards 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 really easy. Air filters differ from saw to saw some have a fine mesh screen and others will have a fabric or foam filter.


Compressed air is the best way to clean them but not everyone has a compressor so make do with tapping it on a flat solid surface to loosen up the debris. A clean rag or better, a 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 in it or you see a lot of debris in the intake of the saw, you'll need a new filter. 


Chainsaw air filter


Most modern saws will have tool-less air filter covers, so you can clean regularly in the field.

Pull the choke on the saw before removing the filter, it stops dirt from dropping into the carburetor.

Filters can be reused unless damaged.

Chainsaw air filter

Locating Idle Screw

All gas chainsaws will have 3 adjusting screws on the carburetor. These adjusters can be accessed without removing any covers. The adjusters identity will be marked on the side cover of the saw, where the adjusting takes place.

You won't actually see the adjusters, you simply identify the correct access port and insert the correct adjusting tool. I can't think of any jokes right now!

The first adjuster is the low throttle adjuster, marked on the body cover and carburetor with the letter (L). The second, is the high throttle adjuster and it's marked with the letter (H). These 2 adjusters are positioned as a pair, side by side.

Often these H & L adjusters will require a special tool to adjust and are set from factory. They can be adjusted but it takes real finesse to adjust them correctly. I will link a separate guide for that here in the future.


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, something that 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.


Chainsaw adjusting


The adjusting ports are usually on the left 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 letter I, T or LA. And they are usually located just above or below the H & L port holes.

Chainsaw cover

Adjusting Idle Screw

The saw should be warmed up before any adjusting takes place. The air filter should be clean and fitted to the saw and the chain brake should be off.

When the saw is warm and not running, turn the screw in a half turn, start the saw, repeat this process until the saw idles.

The saw now idles, great, but it should idle without the chain moving. If the chain is spinning while the saws 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 going out of adjustment, 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 parts & tools". If you need parts, check out the MTD parts link below.



A fine flat screwdriver does the job. A Scrench would also be handy for removing the plug to clean or replace.
Chainsaw Idle screw
Chainsaw Idle screw

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 & gaped and adjusting the idle screw hasn't worked.


The next likely problem is a faulty carburetor. Repair kits are available, but buying a whole carburetor is a much better option. Fitting the carburetor is pretty easy on most models.


If you choose to replace the carburetor, be sure to replace the gasket or seal between carburetor and manifold and also replace the fuel filter in the gas tank.


Mower air filter cover


Carburetors wear out, and when that happens, they are close to impossible to keep tuned.

Replacing the carburetor is the way to go.

You will find most saws run a Zama or Walboro carburetor, and the model will be clearly marked.

Fitting is usually a simple job.

Mower gaskets

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 is the problem, they can be difficult to find without the right tools.


A vacuum leak is air that enters the saw without passing through the carburetor, that 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 your just throwing parts at it, never a good idea.


Components that fail, causing a vacuum leak: Carburetors; Manifolds; Gaskets; Crankcase seals; Cracks in the body casing. 


Chainsaw body


Tricky to solve without the right tools.

Worn or damaged carburetor to manifold gaskets and cracked manifolds are a common cause of surging.

Chainsaw engine
Chainsaw cylinder head

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 trouble-shooting" section helpful.

Related Questions

Why does my chainsaw start 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


Auto Technician and Writer at Lawnmowerfixed | Website

John Cunningham is an Automotive Technician and writer on I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.