Saw fires first pull. Starts on 2nd or third pull. Engine idles smoothly. When speed increased engine speed becomes irregular with or without bar and chain. Looses power to the point it will not pull the chain in the cut. Replaced plug , washed air filter, cylinder and piston look like new. Cat,muffler and spark arrester clean free from soot. Fuel from same can as three other saws. Carb adjustment restricted by orange sleeve on high speed screw. I'm thinking compression and ignition ok. Problem seems to be fuel system. How do I service the tank vent? I think it is a small fiber plug on the lower left rear handle. I'm wide open to suggestions. Need a source for parts as well. I would rather keep the saw stock than attempting to modify it as I have no experience.