what would make the exhaust pop when downshifting. It only happens on the reverse downshift pressure. It isn't real loud but only used to happen with fouled plugs. I just put some new acdelco r45ts's. The car is running good minus I can't get a accurate timing reading. I'm going to put a timing tape on it and find tdc and mark it. Right now it thinks I have 35 base. I tuned it by ear and vacuum gauged the carb no issues it screams minus the popping.

Could be pulling air from leaky exhaust manifold of header pipe junction.