I have a "popcorn" sounding popping noise coming from my exhaust system, only when I let off the gas. Not when accelerating. I have no obvious exhaust leak, but sometimes getting a smell possibily from a "too rich" running carb. I have a 650 holley four barrel on a 350 67 Camaro. I brought the car down to a friends garage, put it on the computer and adjusted the mixture, it seemed to work for about 5 miles down the road and then started back, popping. Any ideas??