If it only has one needle, it is a low speed needle. The high speed jet is fixed . To stop the low speed from spinning, tighten the packing nut . The needle should be set when running on a boat after the motor has warmed up and The 3/4 out is just a starting point. EDIT: are you sure it has only one needle? Without going out to the shop, I think those have the two needles controlled by what looks like one knob, but there are two. A center knob for the low speed, and an outer knob for the high speed .
