Should be an adj screw inside
My 285 had this problem last year, so I took the compass off and there is an adj screw. Problem was, the plastic housing the screw tensioned against was cracked so it did not do much good. Went to Home Depot, found a ground clamp in the electrical section, like you find holding the gnd wire from your cable at home clamping on to a gnd rod. put this onto the throttle lever shaft, with a bolt and handle that goes thru a hole I drilled in the housing. Works very well, I almost never adj it,but now it is easy to do from the outside.