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Are you running the saw in warm temperatures? If so you're likely experiencing a common issue with this series of saws. What's happening is the carburetor is getting too hot and the fuel is turning to vapor. When this occurs the you will not be able to start the saw, as the carburetor can't get any fuel, often the primer even stops working. I'm also curious of the saws manufacturing date? The 550 has gone through a plethora of updates over the years, including the use of four different carburetors.