Note that there already is a 'secondary' return spring there, a rotary spring on the front of the rear carb. If you want more return tension, it can be tightened by moving the clamp on the shaft and/or moving the clip that anchors it to the carb body.
But more tension is going to cause the throttle shaft bushings to wear quickly. Much better to find what is binding and fix that, IMO. If the throttle pedal seems too light, try replacing the spring at the firewall.