Chain Saw: Engine Will Idle But Dies At Full Throttle

Your carburetor is tasked with mixing fuel and air in the correct ratio, so that combustion can occur, and your chainsaw will run. When the carb becomes clogged (which is often the result of old fuel sitting in it over a long period of time, like over the winter), it will not perform this task effectively. This is particularly true when you attempt to run your chainsaw on full throttle. You may be able to purchase a carb kit, and clean and rebuild the carb, rather than replacing it. But if the carb itself is damaged or corroded internally, which you will notice while you tear it down with your carb kit, you will need to replace it.

The air filter in your chainsaw protects your chainsaw from dirt and debris by filtering it out before it can reach the engine. This dirt and debris can build up and create a clog. When a clog occurs, your engine will not get enough air to operate effectively. In this case, the filter is likely only partially clogged, so it will allow enough air to the engine for it to run on idle, but not enough to sustain full throttle. You should clean your air filter regularly to avoid this issue. Fortunately, air filters are easy to locate on top of the carburetor – clean and replace if necessary.

Like the air filter, the fuel filter is designed to protect the engine from dirt and debris. While the air filter protects it from airborne particles, the fuel filter protects it from particles found in the fuel. If the filter becomes clogged, not enough fuel will reach the engine for it to run properly. Because your chainsaw will idle, but dies when it is revved up to full power, it means that the filter is only partially clogged; it will allow enough fuel to the engine to run on idle, but not enough to sustain full throttle. Disconnect the fuel filter from the fuel lines, and at this point you can either clean or replace the filter to solve this problem. Cleaning and potentially replacing your fuel filter should be part of routine maintenance for your chainsaw. Like the air filter, it is an inexpensive part to replace, but greatly affects performance.

Over time, when your chainsaw has been running on full throttle, carbon deposits can build up in the exhaust port, muffler, or spark arrestor. This can result in one or more of these parts becoming clogged. When such a clog occurs, the exhaust gasses will not be able to exit the engine, and it will cause your chainsaw to die. This type of carbon buildup should be visible, and the offending part should be replaced.

A carb kit can be purchased to help clean and rebuild a plugged carburetor. For more information, as to how a carburetor can cause this symptom, please see our carburetor section. When dismantling your carburetor for rebuilding, it’s a good idea to take photos as you go, to make reassembly easier. Start with the air filter housing filter and air filter, and take off the bolts holding down the carb and gasket. Take care when removing the fuel lines with a twisting and pulling motion, and take off the carburetor linkage. Unscrew the screws at the bottom of the carb and remove it, pulling the bottom off and removing he gasket and diaphragm. Next take off the primer bulb and cover, and make a note of how the lever, spring, and hinge pin go together on the inside of the carb. Clean the internal parts of your carburetor now with carb cleaner, using a soft brush to get inside all the holes. It’s at this point you’ll need to look out for corrosion or wear on the internal parts of the carburetor. If you notice damage or corrosion, it’s time to abandon the maintenance and simply purchase a new carburetor, as this one is beyond repair.

When your chainsaw is running, fuel will travel from the fuel tank to the carburetor through the fuel lines. Generally speaking, the more power your chainsaw requires, the more fuel it will burn. A fuel line blockage or a small crack in the fuel line can let air into the fuel line, and can cause fuel to leak, preventing your engine from running at full power. To replace the fuel lines, simply disconnect them from the fuel filter and the carburetor, and replace them with new ones.