The saw starts but not easy runs for about 5

Either the throttle is stuck wide open, or there is a large air leak behind the carburetor to cylinder connection. Make sure the carburetor is securely fastened. If it has a flexible connection to the engine, remove the connection and inspect it for tears or holes. Watch the throttle arm on the carburetor which should move from the idle speed screw to the full throttle limit casting/post when the trigger is pulled and released. If it hangs near full throttle, then remove the trigger mechanism (make notes of the parts positions) and clean out any dirt/sawdust. Check the air cleaner and muffler for plugging. Check the H needle adjustment after repairs by running the engine at full speed after warm up--the engine should 4-stroke unloaded, but immediately 2-stroke when cutting. Check the condition of all of the fuel lines including the line to the filter in the tank looking for decay or other damage. Use original rubber lines for replacement, not plastic substitute. Hope some of this helps!