Popping or backfire on deceleration, New Exhaust system on 1972 240z
 

Just swapped completely the stock exhaust system on my 1972 240z with an MSA PREMIUM EXHAUST KIT:

Motorsport! MSA Premium Exhaust System, 70-72, 74 240Z-260Z (For Stock Manifold) - The Z Store! Nissan-Datsun 240Z 260Z 280Z 280ZX 300ZX ( Z31 Z32 ) 350Z 370Z Parts

My car had the original stock pipe and muffler and had no popping or backfire issues and now after swapping the exhaust system with this one which pipe measures 2.5" i have random popping at the exhaust on decelaration and mostly pronounced when releasing the gas pedal when going down hills. Both of my carbs tuned perfectly at 2.5 turns clockwise and well balanced. The ignition timing is set at 14BTDC as My cam is a SCHNEIDER 274F Cam and not the stock cam.

Should i retard or advance the timing or this will not have any effect?