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Chainsaw Idles but won’t Rev. Tips from a Professional

One of the most common issues with a chainsaw is when it starts and idles fine but dies out just when the throttle is pressed. The fuel delivery system and its associated components might be at fault; hence, they must be checked individually. In this blog post, I will help you diagnose the underlying cause and enable you to solve the problem on your own.

Chainsaw idles but won’t rev:

If your chainsaw doesn’t rev up after idling, consider examining the air and fuel filters first, ensuring they are clean, not gummed up. Next, you need to adjust your carburetor’s adjustment screws and optimize the high-speed RPMs by turning the screws. Also, clean the carburetor if needed. If the problem remains, you may need to check for air leakage in the engine by performing a pressure/vacuum test.

In this article, I will explain why your chainsaw idles and won’t rev in more detail and why your engine stalls when you give it gas, including fixes.

Why does my 2-cycle Engine Stall when I Give it Gas?

If your two-stroke starts just fine, it means that the air-fuel mixture being delivered to it is adequate to initiate combustion and keep the engine running at no load.

However, as soon as the engine is loaded by applying throttle, it bogs down immediately, implying that the air-fuel charge isn’t enough to keep it running. Now, this may be due to any blockage in the fuel system components, or it may be due to an air leak.

It could also be possible that the carburetor is tuned to act as too rich or too lean in a high-speed setting. Sometimes, a vapor lock due to a preheated engine could also be causing this problem. Let’s investigate all these possibilities one by one.

Why does my Chainsaw Stall when I Give it Gas?

If your chainsaw stalls after giving gas, you should begin by inspecting its air filter, followed by the carburetor adjustment, cleaning the fuel filter and the fuel lines. Sometimes, you may need to conduct a vacuum test to check if there’s any air leakage in the engine assembly.

To troubleshoot your chainsaw, use the following step-by-step approach:

● Step 1: Air Filter:

We can easily inspect the air filter before examining the carburetor, which could be causing the chainsaw to stall. Like a fuel filter, it screens the ambient air before reaching the carburetor. When the filter becomes clogged, airflow is constricted, and the carburetor fails to produce the proper air-fuel combination, causing the engine to stall.

– Repair: Clean the Air Filter:

Your chainsaw’s air filter screen is located on the backside. Loosen the bolts that hold the air filter in place with a screwdriver. Pull out the filter and inspect it for dirt or deposits. I recommend using a soap and water solution if there isn’t a lot of dirt buildup.

If, after cleaning, it still does not appear to be in good condition, consider replacing it entirely. Air filters are inexpensive, and I recommend replacing them yearly to prevent engine problems.

● Step 2. Fuel Filter: 

Typically, debris accumulates on the fuel filter located within the fuel tank due to low fuel quality. As the name implies, a fuel filter filters the fuel so that the carburetor receives a clean and uncontaminated fuel charge. However, when it becomes clogged, the fuel passage to the engine is hindered, causing the engine to halt.

– Repair: Filter Inspection and Replacement:

Remove the chainsaw’s fuel cap and pour the gasoline into a different container to examine the fuel filter. After that, clean the filter using a dental pick or a thin metal rod. Examine the filter’s quality. If the filter still appears plugged with dirt, replace it with a new one. If it appears clean, keep it that way and move on to the carburetor.

● Step 3. Carburetor:

The carburetor needs to be tuned and properly serviced as a critical fuel system component. It essentially controls the air-fuel mixture’s composition. Hence, the carburetor must be inspected whenever a two-stroke engine undergoes performance-related issues.

– Fix: Carb Tuning:

The carburetor controls the fuel flow at idle, low-speed, and high-speed engine modes using the I, L, and H adjustment screws. Each screw controls the amount of fuel at that particular setting. E.g., the low-speed RPMs are controlled by the L screw. Tightening the screw restricts the fuel flow and increases the RPMs while loosening it decreases the engine’s RPMs at low speed. The same is true for I and H screws.

In our case, the engine starts up and runs fine at idle and low-speed settings. However, as the throttle is pressed for high speed, it begins to bog down. Hence, let’s troubleshoot the problem by tweaking the H screw first.

  • Tighten the H screw: Start your chainsaw’s engine and let it rev for a few minutes. Before pressing the throttle, tighten the H screw by using a screwdriver. After tightening by half a turn, press the throttle and notice if the high-speed RPMs have increased or not.
  • Note the 1st extreme point: The RPMs would certainly increase. If not, perform this procedure by loosening the screw instead. As the RPMs increase after tightening the H screw, keep tightening it until they begin to decrease. This is where the engine isn’t receiving enough fuel and begins to bog down. Please note this point (by remembering the number of turns it takes to reach).
  • Note the 2nd extreme point: Now, loosen the screw and keep the throttle pressed. The RPMs would continue to increase until a point is reached where they are at their maximum. Loosen it further until you notice the RPMs beginning to drop. Again, note this point’s location in terms of the number of turns.
  • Find the ideal spot: Note that the points determined above are the extreme locations. The operating point needs to lie somewhere in the middle. Adjust the screw until you reach that location. As you press the throttle at the new H location, the engine would run at an optimal high-speed setting and wouldn’t bog down like before.

– Fix: Carb Cleaning:

For cleaning your carburetor, I recommend using a carburetor cleaning liquid. If not available, you can use WD-40 instead. These liquids attack the gummy fuel deposits stuck within metal parts and ensure they are clean and debris-free.

  • Detach and disassemble the carb.: It is better to remove your carburetor to clean it more thoroughly. To detach it, first, remove the air filter. Followed by detaching the fuel lines and removing the links holding them in place.
  • Clean the bowl: After removing the carburetor, detach the bowl nut and spray the cleaner liquid on its interior. Remove any stale fuel present in the bowl. Also, spray the cleaner liquid on the bowl nut to dislodge debris.
  • Clean the interior. Spray the carburetor cleaner on the butterfly valve and the remaining interior parts. You can check that the jets are unclogged by noticing the cleaner fluid coming from the other end. After you are done, reattach the carburetor and assemble it back on the chainsaw.

● Step 4. Air Leakage:

The engine’s seals and gaskets are liable to get worn out over time, and once damaged, they may allow a two-way flow of the air-fuel charge across them. As the air leaks out of the engine, it reduces its compression, and the engine begins to bog down.

The air leakage is less likely to occur when your engine starts perfectly. However, even in that situation, it is possible as the leakage effect gets aggravated when the throttle is pressed. This is because, at high RPMs, increased thermal expansion causes more air to leak out of the opening in the gaskets and crank seals.

Fix: Pressure Test:

The pressure test locates the point where the air is leaking from. Once identified, it can be fixed. Usually, the leakage sites are the crank seals and the head gaskets. I recommend purchasing a testing kit to perform a pressure/vacuum test yourself. It contains a manual pump fitted with a pressure gauge.

The pump’s hose is connected at the spark plug’s location, and the engine is pressurized. If the pressure begins to fall, this confirms a leak. The leak’s location can be determined by spraying some soap water at the suspected locations (such as seals and gaskets). If the air leaks, bubbles will be formed at these points

● Step 5. Vapor Lock:

Vapor lock is a condition when the engine cannot draw enough fuel from the tank due to excessive buildup of fuel vapors within the fuel lines. This condition occurs when the engine becomes overheated either due to use at wide-open throttle (WOT) or being kept open in the sun. The engine might start, but as the throttle is pressed, it would not receive enough fuel to keep it running.

– Fix: Open the Cap:

The solution to a vapor lock is relatively simple. Keep your engine off and open the fuel tank’s cap for a while. Let the vaporized fuel escape from the tank and cool down the engine. Ensure the tank vents aren’t plugged to allow safe passage to the vapors when in use.

Can a Seized Chainsaw be Fixed?

A seized chainsaw can be fixed if the engine or its components aren’t permanently damaged. If a chainsaw was seized due to a pull chord or flywheel damage, it probably could be repaired. If other engine components are damaged, it probably can not be fixed in most cases.

Engine seizure is a term that commonly refers to when the engine is unable to deliver power due to the pistons getting jammed in the cylinder. This condition typically occurs due to extreme operating conditions, i.e., using the chainsaw at wide-open throttle (WOT) for an extended period, or overheating it excessively with a lean fuel mix. Typically, engine seizures render the chainsaw useless and sadly cannot be fixed in most cases.

The term seizure is also used when a chainsaw cannot start due to damage to the flywheel or pull cord mechanism. This can surely be fixed. To begin with, you need first to inspect the flywheel and its vanes for signs of damage. Also, check if the flywheel can rotate freely along its shaft.

Next, examine the pull cord mechanism. Check if its spring is loaded correctly and the pull cord isn’t tangled. If you see it getting stuck or damaged in any way, I recommend replacing the pull cord mechanism with a new one. It is quite cheap and readily available from a local dealer or online.