The gas cap on a Poulan chainsaw covers the tank and keeps gas from spilling while you cut. It is also designed to vent excess heated gases to keep the right amount of pressure, which allows the fuel to be pumped into the carburetor via the fuel line. These gas caps can get dirty or damaged when you stop to refuel in the field.
Look for Damage
Unscrew the cap from the gas tank, and inspect it for any signs of cracks or holes. If it is cracked, you will need to replace it. Inspect the rubber washer on the inner side of the cap. This washer helps get an airtight seal and can get lost during refueling. If the rubber washer is missing, you will need to replace it.
Clean the Cap
Pour about a capful's worth of clean gasoline onto the gas cap. Scrape the gas cap clean with a wire brush, and wipe it dry with a rag. Again inspect the cap for any signs of cracking or breaking. If you notice damages, the cap will need to be replaced.
Examine the Vent Hole
The vent hole gets rid of heated gasses, so it may be broken. Flip the cap upside-down, and inspect the vent hole, which is usually in the middle of the cap on Poulan's chainsaw. If the vent hole is broken, ripped open or clogged, the gas may leak in other areas. Replace the cap, and see if it is still leaking.
If the gas cap is still leaking after replacing the gas cap, your problem is probably being caused by a warped or broken fuel tank. Fuel tanks can get warped if they are overheated or come into contact with hot materials. You will likely need to replace the fuel tank unit if it continues to leak.