'64 \6 Choke Bimetallic Spring

Keep in mind that there are two different ones. One for the Carter BBS, and the other for the Holley 1920. Also, that spring doesn't "grab" the adjustment rod and won't pull the choke open. The choke pull-off diaphragm on the carb does that to a point. The bi-metal spring just pushes against the adjustment rod to close the choke. The expansion of the bi-metal spring in the choke stretches as it heats allowing the choke to completely open by the weight (AKA fast idle cam) attached to the choke linkage. There are also two adjustments that can be made for cold running (choke on) performance. One is the tension on the bi-metal spring. The other is by physically bending the rod from the bi-metal spring to the choke flap. Both adjustments are critical for smooth operation when the engine is cold to warming up. Once fully warm, the choke flap should be wide open and not affecting how the engine runs. Depending on where you live/drive (as in altitude and temperature conditions) those adjustments may take some trial and error to get it just right. Start by using the factory service manual to make those settings.