My Stihl HS81t bogs down at full throttle. The unit is fairly new, low hours, and I cleaned out the exhaust port etc so there is no blockage there. When I run the unit without the gas cap on the unit seems to run fine. Is the carburetor shot? There is also a black nipple on the gas tank, not sure what that is for or if it needs servicing. Pic with the black nipple, what is it?
https://www.dropbox.com/s/eolndsvllk1hj07/20170328_120625.jpg?dl=0
https://www.dropbox.com/s/eolndsvllk1hj07/20170328_120625.jpg?dl=0