Engine needs to be hot to tune the idle speed and mix, but in your case I would just turn the speed screw in an 1/8th turn till it doesn't stall when you suddenly release the throttle from WOT. Although I would possibly tune the idle mix, because your mix might be off, too.
1. Get the engine nice and hot with a ride
2. Turn the idle screw in 1/8th or 1/4er turn so it's idling very fast
3. Turn the mix screw all the way in
4. Back the mix screw out 1/8th of a turn at a time till the engine will start
5. Turn the mix screw out 1/8th turn at a time and wait 10 seconds to see/hear if the idle speed increases or decreases.
6. Keep repeating step 5 till the idle speed decreases
7. Turn the idle mix screw back in 1/8th turn
8. Turn out the idle speed screw 1/8th or 1/4er turn to the correct idle speed
A tachometer might help if your ear isn't calibrated well enough.
http://www.amazon.com/CyberTech-Digital ... tachometer