The fuel system on a chainsaw begins with the tank. From the tank the gas is suctioned into the carburetor where it mixes with air, then is pumped further into the cylinder where the spark plug ignites the mixture; any excess gas is purged back into the tank. While all of these areas can leak, a few major areas generally are the culprits.
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The fuel tank offers a simple place to begin, and, when you notice any gas problems, not just leaks, start your inspection and troubleshooting here. The cap screws onto the tank, and a thin rubber seal, along with the suction of the piston, keep the gas in the tank. You'll want to empty the tank and inspect the cap and its seal. You should also carefully inspect the tank for tiny puncture marks, which can occur if you popped the chain off the bar.
Continuing up the fuel system, you'll need to check both the main suction hose and the return hose. These plastic lines get brittle with age and will crack or split when dried out. Replacing these, along with using a sealant on the carburetor's nozzle, will ensure the leak isn't occurring before the carburetor. The fuel hoses will likely need to be replaced on a seasonal or biseasonal basis, depending on how well you care for your chainsaw. Also follow the long-term storage directions as old fuel will cause the hoses to wear out more quickly.
The carburetor draws fuel in, mixes it with air and then pumps it out to the chamber. The intake and outtake valves can lose their seal when the hoses or the small filter inside get clogged, causing a backup in the gas flow somewhere. Dirty carburetors can also cause leaking, again due to the restricted flow of gas somewhere. Carburetor servicing and repair should be left to skilled professionals as improper repair and reassembly can ruin your carburetor.
Gaskets and Seals
On the front and back end of the carburetor are two gaskets. These gaskets provide an airtight seal with the intake manifold and the choke cover plate. These gaskets can also get old and brittle, causing a slight gap in the seal, which will cause your gas leak. You will also need to remove the carburetor to access these seals and will need to clean their seats on the engine and replace them. A carb kit, installed in the back of the carburetor, will help draw fuel into the intake manifold and will extend the life of an old carburetor, so don't throw away your carburetor just yet.