I think that motor has the old plastic idle stop screw at the base of the tower -- down under the spot where the control cable attaches. (If you use remote controls, take off the cables at the motor to make adjustments.) Popping or stumbling is related to the carb mix needle rather than the idle stop, assuming it's a carb issue. Unless there is a typo, you are saying the mix adjustment affects rpms -- a good thing. Typically, leaning the idle mix (clockwise) will increase rpms until you hit the point where the motor lean sneezes or stalls. Turning counterclockwise will richen the mix until the motor stumbles. (1 1/2 turns is an initial setting, btw, and should allow you to run the motor and warm it up for fine tuning the mix.)
If small idle mixture adjustments have no effect, it's likely the carb needs to be cleaned -- in particular, the low speed/idle passages up at the top. Take off the domed plug for cleaning access, and replace it with one from a carb kit. The high speed jet is fixed.
