Inside the throttle arm you will see a straight slot screw. Turn it clock wise 1/8 to 1/4 (at the most) turns at a time. Shut the motor off the make the adjustement and open the throttle all the way when you do.

Restart and test. I've had a couple students lately needing over 1/2 turn just to get it to idle without snuffing the fire out.

Just tweak and test, its a fairly sensitive adjustment.