For all the 550xp owners. I finally ran my little 550xp saturday and noticed it had a slight stumble when accelerating off of idle. It never got worse or better the longer I ran it. I remember reading something about this issue with them. So sunday I tore into it. I took the intake horn that air filter sits on off. While the carburetor is still on the saw and you are facing the carburetor, there is a brass plug. I drilled the brass plug out and there is a air adjustment screw in there. I backed it out 1/4 turn and tried it. I ran for 20-30 min and the stumble is gone. So I just thought I would share this with everyone.