if its a completely original untouched 4 speed car and carb, you should have a dashpot that looks like the one on my 68, you set that dashpot to touch the lever at 2K if i remember correctly from the FSM and it will drop to idle slower than cars without the dashpot. It should drop about 1 to 2 seconds slower than a normal carb. if you dont have the dashpot then maybe timing or throttle blades open to far???