When the choke is set, it probably closes the upper choke flap too much. That can be adjusted by bending the link between the choke mechanism and the upper flap. Also, the "high idle" adjustment may have the throttle open too far.
Opening up the choke flap and backing off the high-idle adjustment should solve your problem. If you want to know how to make all the adjustments on your Q-Jet, you need the Chevrolet Chassis Service Manual for the year that carb was produced or for what year car that replacement was meant to be used.