If you haven't already done so I would try replacing the glow plug. I had a 46FX that started running strange- hard to start and wouldn't idle/pick up right unless the low mixture screw was almost all the way turned in. The plug had plenty of glow when I checked it so I thought it was OK. To make a long story short, in the end I finally replaced the plug and the motor went back to running normal.

I wouldn't mess with the throttle stop screw unless you have to. It should be turned out about as far as it can go where it still holds the throttle barrel in solidly. If you go too far out the throttle barrel can slide out of the carb! But turning it in too far can limit how far you can turn in the mixture screws which can complicate things.