By: Author John Cunningham. Published: 2023/07/19 at 7:08 am
Oh, come on… you piece of S**t, Rev up already. I know the feeling, and for most of you guys, this is a super easy fix, and we’ll get you back cutting shortly. Let’s dive in!
The top three common causes of a chainsaw that won’t rev up include:
In this post, we’ll cover the top three reasons chainsaws fail to rev up; we’ll cover diagnosis and the fix.
Check the Basics
I’ve been a mechanic for over twenty years and learned not to make assumptions; when I do, it often costs me a ton of time. And so, I like to understand the customer’s complaint before making the next move.
So first, let’s make sure I understand your complaint correctly.
If your saw starts and idles OK but won’t take a rev and makes a baw… baw… sound when you give it throttle, then you can jump ahead to that repair right here.
If, however – your saw starts and idles OK but won’t take a rev and doesn’t really make any reaction to throttle application, then you are in the right place, and you can read on.
Now, let’s get the basics out of the way before moving on to some proper diagnosis.
The basics include the following:
- Gas – check if the gas in the tank is fresh; old contaminated gas will cause a ton of issues with various symptoms.
- Air filter – check the air filter is clean, and the airway is clear.
- Plug wire – check the plug wire is secure and free from chafing or terminal corrosion.
- Spark plug – check the spark plug is the correct type for your saw; check it’s clean and the gap is good.
- General inspection – checking for obvious signs of issues, from loose components to leaks, etc.
You may find the following links helpful:
With the basics out of the way, we’re ready to move on.
1 Clogged Spark Arrester Screen
For most, a clogged spark arrester is why your saw won’t take a rev, and the best news is that the solution is simple. – We need to remove the arrester and clean it.
What’s a spark arrester? A spark arrester is a mesh screen designed to catch hot carbon sparks that leave our mufflers when they run at full tilt. The hot embers could start forest fires and are a safety risk.
Why does the spark arrester get clogged? Clogging is a normal condition, and the arrestor should be cleaned every fifty hours as part of normal maintenance. That said, using excessive oil in the gas mix or using unapproved 2-stroke oil can cause them to clog more easily.
Where is the spark arrester fitted? The spark arrester is fitted on the muffler in front of the exit pipe and just behind the muffler defuser.
How to diagnose a clogged spark arrester? You may notice your saw is quite than normal on idle, and that’s a subtle sign your arrester is blocked up. But a surefire way is to go ahead and remove and inspect the screen.
How to remove and clean the spark arrester? As spark arrester cleaning is routine, most manufacturers make it easy. Typically remove two fasteners, pry off the defuser, and use a pick to remove the screen.
Cleaning is easy too; use a wire brush and some brake cleaner or a burner is excellent at removing oily deposits.
That’s it. Reassemble, and you are good to go.
2 Faulty Ignition Coil
If the spark arrester checked out OK, then a failed ignition coil is our next likely issue.
What is an ignition system coil? A coil is a component with copper windings designed to transform low voltage into high voltage and deliver it to the spark plug with every engine revolution.
Why do ignition coils fail? Ignition coils are under a ton of stress. They carry thousands of volts and are exposed to heat and debris, and like all components, they fail with age and use. That said, using the wrong type of spark plug, a plug with a large gap, or pull-starting your saw with the plug wire removed and not grounded will shorten its life.
Where is the coil located? The coil is located behind the pull assembly adjacent to the flywheel.
How to diagnose failed coil? Ordinarily, testing for a failed coil is easy, pull the spark plug, and if you have no spark with a known good spark plug, then subject to finding a chafing coil control wire, you can bet your coil is faulty.
But that test won’t work here because that’s not how this coil has failed. See, chainsaw coils employ an integrated component that adjusts the ignition timing to compensate for engine speed (RPM). At low RPM, the ignition coil retards the ignition for easier starting.
But as RPM increases, the spark has less time to reach the combustion chamber, so to help it out, ignition timing is advanced.
You may find the following post helpful – How ignition systems work?
However, if the coil fails to adjust the ignition (spark), then the spark arrives at the party late and prevents the revs from picking up.
You can bet your coil has failed if your engine fails to pick up the revs when you hit the throttle; however, if your engine makes a baw.. baw.. sound as you hit the throttle, then you’ll need to read the carburetor issue section before doing anything further with the coil.
How to fix failed coil? Coils can’t be fixed; they are sealed units and must be replaced. Replacing a coil is simple, but we must take care when adjusting the coil air gap.
The coil replacement process is as follows:
- Remove top cover
- Remove the plug wire
- Remove the pull assembly (4 screws usually)
- Remove the coil fasteners
- Remove the control wire
- Remove the coil
When refitting the new coil, we’ll need to pay attention to the air gap; somewhere between .008″ and .012″ is a typical measurement but best to check with your maker. Use a feeler gauge or a business card (which is close to about .010″)
Adjusting the coil air gap is as follows:
- Fit new coil and fasteners but don’t tighten yet
- Don’t forget the ground wire (if applicable).
Coil Air Gap Adjustment
- Fit feeler gauge or business card (rotating the flywheel magnet away from the coil makes it a little easier).
- Now rotate the flywheel so the magnet faces the coil.
- Now push the coil against the flywheel while tightening the fasteners.
- Remove the gauge and reattach the control wire.
Now rebuild in reverse order. Job done!
3 Carburetor Issue
Baw.. Baw… Baw…. to my ear, it sounds like a saw that needs more gas. Adjusting and adding some gas may be enough to fix the problem, but if your saw was running fine and then suddenly developed this bog, it is likely this is a more significant issue.
That said, we will try adjusting the carburetor first, which we’ll cover next.
Tools we’ll need:
- We’ll need a selection of screwdrivers, and some saws will require special adjusting screwdrivers.
- Tachometer to read high-speed RPM (it is possible to adjust some carburetors without tacho, and I cover that below; that said, tachos aren’t expensive and are less prone to mistakes)
(Tachos won’t tune a carburetor correctly if there’s an underlying issue, such as a vacuum leak)
You’ll find all my tools on the small engine tools page.
Why do we need to adjust carburetors, and what are we adjusting anyway? These are common questions my customers ask me.
And I say – Chainsaws don’t run on just 2-stroke oil and gas. They need air (oxygen) also. And when we tune a saw, we adjust the ratio of air to fuel. It is commonly referred to as AFR (Air Fuel Ratio).
Most engines like a ratio of 14.7 parts air to one part gas.
When the ratio of air to fuel increases, it’s known as a lean mix. (running Lean or Skinny)
When the ratio of air to fuel decreases, it’s known as a rich mix. (running Rich or Fat)
A saw can go out of tune for many reasons; ambient temperature, elevation, humidity, fuel type, octane rating, and fuel quality can all affect the fuel ratio.
And so I tell my customers a saw may need a tweak occasionally. But I tell them to adjust cautiously and always err on the rich side. Going too lean can cause engine damage.
Most household hobby saws are manually adjustable, but there are limitations with bigger saws. Depending on your saw make, you may need a special adjuster tool.
And all saws are now fitted with limiter caps on the adjuster screws to prevent us from over-adjusting, damaging the saw or the environment.
The saws, in theory, come set from the factory for optimum performance and low emissions. That said, some saws may be harder to adjust with the limiter caps on, especially as they age. The plastic limiter caps may be removed, but generally, I don’t recommend doing so.
Your saw should respond to adjustments within the adjustment range of the limiter caps; if your saw doesn’t, it’s likely a sign your carb may need a clean or rebuild.
Some modern saws have rev limiters, which makes adjusting the High speed impossible by ear; for those saws, we need to use a tachometer to adjust the H-speed RPM. You’ll need to check if your saw has a rev limiter fitted.
Latest top-end saws have automatic tune – Autotune and M-Tronic; these saws employ a master control module, an electronic fuel valve, and an ignition module to make fuel adjustments while the saw is in use. These saws don’t require manual H-speed adjustments however may require low-speed adjustments, which ideally should be done by a dealer.
All Other Saws
All other saws are adjustable, but before adjusting a carburetor, ensure your gas is fresh (max 10% ethanol blended min 89 octane), the spark plug is in good condition, the air filter is clean and fitted in place, and the spark arrester is clean.
1 Check Carburetor Tuning
Before we get to twisting knobs, let’s establish if our saw needs adjustments in the first place.
Before tuning a saw, I begin with a few checks.
- I start the saw and let it idle for a few moments to warm.
- Chain brake off, I hit it full throttle and listen for the flutter at wide open throttle (WOT); (some folks may call it four-cycling, dieseling, or sputtering). I want the RPM to come up quickly but level out quickly with a fluttering quality.
- I release the throttle, I listen to the RPM drop quickly to idle, and the chain shouldn’t move.
- While idling, I’ll tilt the saw on its head; on its side, it should run no matter its position.
- Finally, I hit the throttle and check the saw accelerates to WOT without hesitation.
If your saw doesn’t do all of the above, try adjusting it.
2 Locate and identify adjusters
There are three adjuster screws, an idle screw, a low-speed screw, and a high-speed screw; the first thing we need to do is locate them.
Adjusters may be on the pull starter or chain cover side of the saw.
Idle adjustment screw – marked with the letter I, T, or LA. The idle screw adjusts the RPM at idle by pushing on the throttle plate.
Screwing it clockwise opens the throttle plate wider, increasing RPM, and counterclockwise closes the throttle plate reducing RPM.
Low-speed screw – marked L but will always be the screw closest to the engine. The low-speed screw adjusts the amount of fuel the saw receives between idle and initial throttle.
Turning the screw clockwise decreases fuel but increases engine speed, and counterclockwise increases fuel supply but decreases engine speed.
High-speed screw – marked H and located furthest from the engine. The high-speed screw adjusts the amount of fuel the saw receives at full throttle.
Turning the screw clockwise decreases fuel but increases engine speed, and counterclockwise increases fuel supply but decreases engine speed.
3 Adjust Idle
Start the saw, turn the chain brake off, and adjust the idle screw clockwise until the chain starts to move – now back it off slowly until the chain stops moving.
Great, let’s move on to adjusting the low-speed side.
4 Low-speed adjustment
Saws with limiter caps installed offer a window for the adjuster screw; typically, it’s a 1/2 or 1 turn. If the limiter caps have been removed, as a baseline guide, the adjuster screw typically likes to be around 1 1/4 turns out from seated. But all saws are a little different; you can check your saw’s baseline adjuster specs online.
The low-speed screw works within a window, meaning turning it too far clockwise or counterclockwise will likely stall the engine.
When tuning the low speed, we are searching for two points;
- The first one is where the RPM drops off sharply as we turn the screw too far clockwise.
- The second is where the RPM starts to drop off noticeably as we turn the screw counterclockwise.
So with this in mind and with the saw idling, no throttle for this adjustment:
- Turn the low-speed screw clockwise (in); the RPM rises and then falls off sharply.
- Now turn the low-speed screw counterclockwise (out); you’ll notice the RPM come back up, and just when the RPM begins to drop off again, stop adjusting. This is where we live.
If the chain spins after tuning the low side, adjust the idle screw again counterclockwise until the chain stops. A typical smooth idle is about 2800-3000 RPM.
This L-screw setting is usually pretty close to perfect for most saws. However, the best test is how the saw performs; it should accelerate without hesitation, and RPM should drop immediately when the throttle is released.
- Hesitation on throttle means it’s running lean and needs some more fat; turn the screw counterclockwise.
- A hanging idle when the throttle is released means it’s running lean and needs some fat; turn the screw counterclockwise.
- Heavy fuel mist from the exhaust on throttle means it’s running too fat and needs to be leaned out – turn screw clockwise.
- If, when tilting the saw on its nose, it stalls, it’s running too fat and needs to be leaned out – turn screw clockwise.
If your saw passed all these tests, then the low side is good; now for the high side.
5 Adjust the high-speed screw (using tacho)
It is possible to adjust our high-speed screw by ear, and we’ll get to that shortly, but for most DIYers, this can be tricky, especially as many modern saws have, as said, rev limiters, and adjusting the high side by ear isn’t possible anyway.
Getting this correct is important; many a saw engine is destroyed by lean running.
So by far, the easiest and, for some, the only way to adjust the high-speed screw is with a tachometer.
The tacho reads the RPM digitally, and we adjust our high-speed screw accordingly. We will also need the full-throttle RPM spec for your saw, typically about 12,000 for medium-size saws, but it could be as high as 13,800 best to check with your manufacturer what your max unloaded RPM is.
The process is simple and looks like this – with the saw running and tacho in hand, open the throttle to full and read the gauge. If you need more RPM, turn the High-speed adjuster clockwise, and for less RPM, turn it counterclockwise.
You should note while a saw maybe be set to the correct RPM, it may in fact still be out of tune. This can happen when there’s an underlying issue. If it underperforms or operates erratically, hanging idle, revving by itself, is hard to start, etc., have it checked out. It may suffer from a vacuum leak, and such an issue will shorten the saw’s life.
5(b) Adjusting the high-speed screw (manually)
High-speed adjustment is super important; getting this wrong will kill a saw.
The high-speed adjuster, as you know, controls fuel at full throttle. Less fuel at full throttle paradoxically causes the engine to run faster, which some folks might think is better; it’s an understandable but expensive mistake.
Running lean offers less power and will overheat the saw and cause the piston and rings to expand and eventually seize the engine. Running lean means less oil and fuel to help lube and cool the engine. It also, of course, means higher RPM and, therefore, heat.
A lean, screaming saw is a saw that will soon fall silent.
Saws with limiter caps installed offer a window for the adjuster screw; typically, it’s a 3/4 turn. If the limiter caps have been removed, as a baseline guide, the adjuster screw typically likes to be around one and 1/2 turns out from seated. But all saws are different; you can check your saw’s baseline spec online.
The H-speed manual adjustment can be tedious, especially if your saw has an underlying issue, such as a vacuum leak. The process requires listening carefully to the engine’s note as we adjust.
This takes a little practice and finesse; when tuning, you’ll begin to notice how the engine sounds different as we adjust from (clockwise) a lean, high RPM, high-pitched scream to (counterclockwise) a lower RPM with the fat flutter.
The fluttering sound is important as that means the saw is running rich, and that’s how an unloaded saw should sound.
Let’s get to the adjusting.
The process is as follows:
- Warm the saw up and chain brake off.
- Hold the saw at full throttle while adjusting (Max 5 to 6 seconds per throttle burst)
- Adjust the H-screw clockwise and listen for the RPM to increase to a scream. (smooth high-pitched screaming sound).
- Now turn the screw counterclockwise and listen for the change in sound (the RPM will drop, and the scream will turn to a burble or fluttering sound – stop adjusting, this is where we live).
- To test, now move to some timber, and with a sharp chain, begin cutting timber at full throttle.
The saw should now have a ton of power through the cut; if it fades, you need to fatten it a little by turning the H-screw counterclockwise.
When in the wood (under load), the flutter sound changes to a smoother engine note; this is normal as the loaded engine is now working harder and consuming the extra fuel we gave it.
Finally, if the chain is rotating, the idle screw may need a small adjustment counterclockwise.
Warning – Adjusting the high-speed screw clockwise so that the engine runs at its fastest risks serious engine damage. Doing so decreases the gas supply and leans out the saw. If the saw is allowed to run in this state, it will overheat and seize. A lean running saw will scream on full throttle and fade in a cut.
You have just turned pro!
If adjusting the low and high-speed screws didn’t help, then we’ll need to dig a little deeper. The usual issue with carburetors is dirt and or worn-out carburetor gaskets.
There are some other issues like damaged fuel lines, and so an inspection is where the diagnosis process begins but typically ends with a carb cleaning and gasket replacement.
First, we’ll turn our attention to the fuel lines and gas filter; the process looks like this:
- Open gas tank and fish out the filter using some wire
- Check fuel filter condition.
- Check lines for leaks.
In the shop, as part of the diagnosis and repair process, we’ll use a hand-held pump to apply about 10 lbs of pressure to the fuel tank fuel line; a drop in the gauge will let us know we have a problem. It could be damaged fuel lines, bad gaskets, or worn needle valve.
There is no need to remove the carb for this test; just fish out the fuel filter, remove the filter, fit the test hose, and apply no more than ten psi.
Pressure should hold for a few minutes; an instant drop in pressure means we must investigate further.
With the carb removed, try submerging the carb in gas and look for bubbles.
Assuming the lines looked OK, with no tears, etc., we’ll pull the carburetor, clean it, and replace the gaskets, fuel filter, and micro screen. That’ll fix her right up.
The tools we’ll need include:
- Selection of small screwdrivers (some may be specialized)
- Carb cleaner
We’ll also need a carburetor kit with a metering valve diaphragm, needle and lever, and a pump gasket. There are a ton of very similar-looking gaskets, but yours must be an exact part number match.
You’ll find all these tools I use here on the small engine tools page.
Carburetor removal process looks like this:
- Empty gas tank.
- Remove the top cover, remove the plug wire, and remove the air filter.
- Pull the choke and use a clean paintbrush to remove heavy debris from around the carburetor.
- Remove carburetor fasteners.
- Remove intake.
- Remove the choke & throttle levers.
- Remove fuel lines
- Remove carburetor
- Seal the manifold with some kitchen towel to help prevent dirt/grit etc, from entering.
Carburetor stripping process looks like this:
- Use a paintbrush and some carb cleaner to remove heavy debris from the carburetor exterior.
- Remove the metering valve cover and remove the diaphragm – the metering valve employs a rubber diaphragm, a needle valve, a lever, and a spring.
- Remove the needle valve. Be mindful of the spring under the lever – they love to fly.
- Remove the fuel pump cover fasteners.
- Remove the fuel pump diaphragm.
- Using a small pick, remove the micro screen.
- The L and H screws are marked, but the L screw will always be closest to the engine.
- Before removing the L and H screws, count the number of turns until each screw seats (turn gently clockwise) and note that number. We’ll use this information later when reassembling.
Now remove both L and H AFR screws (turn counterclockwise). The screws are different, so be sure to note which is which.
Carburetor cleaning process looks like this:
Avoid using compressed air, as the main jet holds a small check valve that will likely be dislodged and lost without you even noticing.
Use carb cleaner WD40 will work too if your carburetor doesn’t have varnish buildup etc.
Thoroughly spray cleaner through all the passages of the carburetor.
Clean cap orifices and the pump chamber.
Most carburetors only need a rinse and are ready for the new gaskets; however, some may require a more thorough cleaning.
A thorough cleaning requires removing Welch plugs and using an ultrasonic tank to boil it.
This will apply to you if, when removing the metering valve cover and diagram, you find debris inside (wet side) of the diagram. Finding dry sawdust debris on the diaphragm’s outer (Dry) side is normal and doesn’t indicate an ultrasonic tank cleaning is needed.
Fitting carburetor gaskets, diaphragm, needle valve, and micro screen looks like this:
- Make sure the L and H adjuster screws are clean before fitting. Set them to the settings you noted earlier.
- Fit the new needle valve, lever, and spring. If you intend checking the lever height, don’t fit the base gasket yet.
- Check lever height. I’ve covered checking the lever height here.
- Fit the new metering gasket (carb side), then the diaphragm (cap side), and fit the cover.
- Fit the new micro screen.
- Fit the new pump diaphragm (carb side) then fit the gasket (cap side), and then the pump cover. Be sure the diaphragm aligns with the small dowels.
Now you are ready to refit your carburetor.
Carburetors are typically fastened to a manifold which is itself fastened to the engine. But in some 2-stroke engines, the carburetor is fastened directly to the engine.
It’s critical that the carburetor makes an airtight seal at this interface and that the manifold makes an airtight seal where it interfaces with the engine. An air leak will cause erratic idling or a no-start.
The manifold plastic cracks with age, and the gaskets become hard and brittle.
- Clean and inspect the manifold and gasket carefully.
2-stroke engines require an impulse line from the engine to drive the fuel pump. Some 2-strokes employ an impulse line from the crankcase to the carburetor, and some are manifold incorporated as per our image.
- It is also critical that the impulse line or interface is airtight.
In most cases, the manifold gaskets are just fine and may be reused, but if it’s hard best to replace it. Not all carburetor gasket sets come with the manifold gasket, so you may need to order separately.
Refit your carburetor in reverse order.
There are three main reasons chainsaws won’t rev up – they include a clogged spark arrester screen (cleaning needed), faulty ignition coil (replacement), and carburetor problem (adjustment or cleaning).
- 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.