Unless its a normal sound from the new exhaust, its caused by fuel burning in the exhaust. How can richening the mixture make it better??? Add more fuel to eliminate an issue with fuel burning in the exhaust...
Popping in the exhaust goes away when you lean out the mixture. The info above is backward, unless its a specific carb flaw that I'm not aware of. If that's the case, the engine would flood at idle too. High volume air draws fuel. A closed throttle plate stops that volume. Velocity past the fuel port decreases with volume. CFM (volume) is required to draw fuel, not simply velocity. If you had high velocity air at idle, you'd either flood or have a very, very high idle. A typical carb flows 6-10 cfm at idle, a Weber 32/36 flows around 235 cfm wide open. At decel (6-10 cfm) the fuel in the pipe is most likely fresh oxygen. Add gasoline and you get popping.
The notion that failed ignition is the problem only adds to the evidence that's its potentially too rich already.
He said he leaned out the mixture, I said richen it... back to where it should be for proper idle.
The BCDD (not used here) is a small carb within a carb that adds gas and air to remove the over rich deceleration. It's like decelerating with you foot lightly on the gas.
I don't see any problem here. The engine has always done this you just couldn't hear it with a muffler and resonator on it.