One of my weed whackers is a Stihl Home owners unit. Im having a problem keeping it running. I have to run it on half choke all the time or it will act like its running out of gas and then stall. It wont stall however if I run it at almost full throttle. No matter what I do to the carburetor adjustments, it acts the same. In fact I dont seem to see a noticeable difference in the way it runs, when I fiddle with the crews. Although right now its running very rich at full throttle. If I turn the choke completely off it runs for a wile, then its leans out and stalls, unless im running it at full throttle. I notice that it runs a little better when it has burned through most of a tank of gas. It runs the worst on a full tank. It almost seems like the gas cap isnt breathing. Any ideas?