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Chainsaw Starts but Won’t Idle. Tips from a Professional

When a chainsaw starts but doesn’t idle, it indicates that it hasn’t been maintained lately and needs fine-tuning and adjustment. Most gas chainsaw users will encounter this problem with their machines at some point. In this post, I’ll dissect this issue by discussing its causes and the various remedies that can help you adjust your chainsaw’s idle speed by yourself.

Chainsaw starts but won’t idle.

If your chainsaw starts but doesn’t idle, try adjusting the idle (I) and low-speed (L) screws on the carburetor. If it doesn’t solve the issue, inspect your fuel system components one by one, i.e., filters, carburetors, and fuel lines. Also, check if the spark plug is in working condition. Finally, ensure that no vapor lock is formed and the exhaust ports are clean and deposit-free.

The following article comprehensively explains and addresses the factors mentioned above in detail. I will also address how to fix a chainsaw that won’t stay running and why a chainsaw bogs down when pulling the trigger.

Why does my Chainsaw Start but won’t stay running?

If your chainsaw starts but doesn’t idle, the chances are that the low-speed (L) and the idle screw (I) may not be correctly adjusted. Furthermore, the fuel lines or the ignition system could be problematic if the issue persists.

1. Adjustment Screws:

There are three adjustment screws marked on the carburetor. These are low-speed screw (L), idle screw (I), and high-speed screw (H). These screws control the fuel flow to the carburetor at various load conditions.

For example, the L screw determines the fuel entering the carburetor at low/idle RPMs, while the H screw controls the fuel flow at higher RPMs or WOT. The (I) screw is a butterfly valve that directly raises or lowers the idle RPMs by controlling the amount of air-fuel mixture reaching the engine.

When a screw (say L screw) is tightened, it restricts the fuel flow to the carburetor, thus making the mixture lean. This tends to raise the RPMs of your engine to a certain limit, after which they decrease. Conversely, loosening the screw enriches the mixture with fuel and lowers the RPMs up to a certain limit. When the L or idle screws are not correctly set, they could cause the idle RPMs to fall too low, thus causing the engine to bog down and die eventually.

2. Fuel Delivery System:

The fuel delivery system of your chainsaw consists of the fuel tank, fuel filter, fuel lines, and carburetor. A blockage in any of these components may upset the fuel flow to the engine, thus resulting in difficulty while idling. The most common cause of such blockages is stale ethanol-blended fuel, which forms sticky white deposits with time.

These deposits are typically formed at the fuel filter or the carburetor jets, clogging them eventually and inhibiting fuel flow. In some cases, vapor locks formed in the fuel tank due to insufficient venting also result in engine problems. Anyhow, all fuel delivery components must be individually inspected to fix these issues, something we will discuss that in the following sections.

3. Ignition & Exhaust System:

The ignition system contains the spark plugs, the ignition coil, and the exhaust muffler. The spark plugs tend to degrade their spark quality with time, thus being unable to fire the air-fuel mix in the combustion chamber. Moreover, the arrestor screen which captures the spark at the exhaust port has a high tendency to develop carbon deposits. These deposits affect the engine’s working and can be why your chainsaw is not idling.

How do you Fix a Chainsaw that won’t Stay Running?

To fix a chainsaw that won’t stay running, proceed with tweaking the adjustment screws on the carburetor. If it still doesn’t improve, consider cleaning your carburetor and the air/fuel filters. Also, inspect the spark plug and change it if needed.

1. Throttle Screw Adjustment:

As explained previously, the carburetor has three screws (L, H, and I) each for controlling the engine RPMs at low speed, high speed, and idle, respectively. If the chainsaw continues to stall, the problem may lie with the low-speed and idle screws that ought to be adjusted.

– Low-Speed Screw Adjustment:

To carry out the low-speed screw adjustment, perform the following steps:

  • Start the engine: Turn on your chainsaw and tighten the chain. Allow for a few seconds of idling. Keep track of whether the engine runs smoothly or if it dies while idling.
  • Tighten the L screw: If the engine dies while idling, use a screwdriver to tighten the Low Speed (L) screw to raise the idle RPM. As you tighten it further, you will see that the RPMs begin to drop. Create a mental note of where that point is. Loosen the screw from there, allowing the RPMs to rise again until they begin to fall. Also, note the second point’s location mentally.
  • Find the ideal spot: The ideal screw setting should now be anywhere between these two positions. Turn the screw until you get the correct balance, then leave it there. The idle RPMs would be sufficient to keep the engine running at this point. Furthermore, pulling the trigger will result in a much faster and lag-free engine response.

– Idle Adjustment:

After the low-speed adjustment, the chain may start to rotate because the mixture is lean enough to engage the clutch. If your chain begins to revolve, loosen the idler screw until it comes to a halt. This is crucial since an idle chain can be extremely harmful. This step can be skipped if your chain does not rotate.

After performing the L adjustment, the mixture composition should be optimum enough to keep your engine running. Sometimes, the L adjustment may not be enough, and you may need to tune your fuel delivery components, namely, the carburetor and filters.

2. Tune the Fuel System Components:

The carburetor needs to be serviced, but ensure thorough cleaning of your engine’s filters before doing it.

– Air filter:

Remove the air filter cover on the rear end of your chainsaw first by using a screwdriver. Now, remove the filter, and use a brush to clean the dust buildup. I recommend to immerse it in a soap water solution so that it’s cleaned thoroughly. If you notice signs of damage and wear on the air filter, it would be best to replace it with a new one.

– Fuel filter:

The fuel filter sits at the bottom of your fuel tank. Before bringing it out, ensure you drain the fuel tank and store the gasoline in an appropriate container. Take out the filter from the tank’s bottom and clean away the stale gasoline deposits from the filter’s jets using a metal wire. After cleaning, install it back in the fuel tank.

– Carburetor:

Before you start clearing the clogged jets, get a carburetor repair kit. A carb cleaner spray is a fantastic tool for removing sticky residues from your jets and restoring your carburetor to working condition.

  • Step 1: Remove the air filter to expose the carburetor: To begin cleaning your carburetor, remove the air filter. The air filter is usually placed at the back of your chainsaw.
  • Step 2: Remove the bowl and clean it: Remove the bowl nut and the bowl from the carburetor. Expired fuel and its leftovers are commonly found in the bowl. Any residues or old gasoline should be cleaned up.
  • Step 3: Spray the carb cleaner: Spray the carb cleaner liquid on the inside of the carburetor parts. Don’t forget to clean the bowl nut, which is a jet that is prone to clogging. When spray emerges from the other end of a jet, it indicates that any debris stuck has been removed.

3. Examine the Spark Plug:

Lastly, the ignition may be at fault when all the above-mentioned procedure fails to yield results. This essentially comprises a spark plug. Typically, they need to be replaced at least once a year, even if you are an amateur chainsaw user.

Due to continuous firing in use, the spark plug’s electrodes either get damaged or develop carbon deposits which diminish spark quality. If that is the case, replacing the spark plug with a new one is the only option.

Why does my Chainsaw stall when I give it Gas?

A chainsaw stalls when it doesn’t get fuel in the optimum proportion. As the throttle is pressed, the air/fuel charge entering the engine either gets too lean or too rich. This may be due to a leakage in the crankcase and carburetor assembly or incorrect carb adjustment.

Your chainsaw’s engine assembly may have developed an air leakage due to constant wear and tear. Due to the leakage, the air tends to leak out, resulting in the mixing ratio getting upset. This makes combustion difficult to sustain, and hence, the engine stalls.

This effect gets aggravated when the throttle is pressed. This is because more fuel is drawn, and due to leakage, the mixture becomes so rich that the engine doesn’t produce sufficient power causing the engine to die down.

Engine leaks can be detected by employing a pressure test. The test confirms whether there’s an air leak or not, and if there’s, it pinpoints the exact location. If a leak isn’t detected and the engine still stalls, try following the carburetor screw adjustment procedure, cleaning its filters, etc.

Why does my Chainsaw Bog down when I Pull the Trigger?

If your chainsaw bogs down after pressing the throttle, check the filters and remove any debris stuck in them. Check the fuel tank vents and ensure that no vapor lock is formed. Also, it would be best to inspect your chainsaw’s exhaust port.

While the most common causes of chainsaw bogging (carb adjustment, filter tuning, etc.) have been discussed above, let’s investigate some lesser common factors which may be causing this problem in your chainsaw.

1. Vapor Lock:

A vapor lock is formed when the fuel vapors cannot escape the tank vents. When a chainsaw heats up, fuel vapor pressure increases too. If the tank holes get plugged with dirt, the vapor buildup can raise the pressure in the fuel lines. The excessive vapor formation blocks the usual fuel flow and upsets the mixing ratio. Hence, the engine might bog down as the trigger is pressed.

2. Exhaust Ports:

The exhaust ports of your chainsaw are in constant contact with the exhaust gases rich in carbon content. Due to carbon deposition, the ports are likely to get clogged, which increases the backpressure at the exhaust and reduces the engine’s compression. To fix this, remove your muffler and use a propane torch to clean your exhaust ports and the arrestor screen from carbon deposits.

Why does my Chainsaw Keeps Shutting off?

If your chainsaw keeps shutting off, it indicates that the air-fuel ratio entering the engine is not ideal. Probable causes may include a clogged carburetor, faulty spark plug, dirty filters, vapor locks, or a clogged exhaust port.

The factors leading to the problem are:

  • Inadequately mixed oil and fuel: For two-stroke engines, fuel and engine oil are mixed at a 50:1 ratio. When chainsaws get old, most users prefer to run them lean, with lower mixing ratios like 30:1 or 25:1. This could cause starting problems in some chainsaws with malfunctioning filters and aged carburetors, causing them to shut down.
  • Faulty fuel filter: As previously stated, a clogged fuel filter deprives the engine of adequate fuel, leading it to run leaner than usual and eventually die.
  • Defective spark plug: When your spark plugs are faulty, no ignition occurs in the combustion chamber, and the engine’s power delivery gets compromised.
  • Carburetor problems: When carburetors aren’t serviced regularly, they can also make the air-fuel mix lean by restricting the fuel jets, causing the engine to struggle to start.
  • Exhaust ports: When exhaust ports are blocked, the engine’s compression gets affected. Despite a perfect air-fuel ratio, blocked exhaust ports can affect your engine’s performance and need to be investigated if your engine keeps shutting off.
  • Vapor lock: Vapor locks starve the engine of fuel by raising the vapor pressure in the fuel lines; hence, the engine could face difficulty in starting up.