I had the "phut-phut" on my previous engine in those circumstances, and no adjustments of timing, valves, carb made any significant difference. It did get better with an electronic distributor (the only benefit!). I get the same thing with my current engine on very steep downhills when I let the engine over-run using engine "braking". My theory is that the worst it can be is surplus fuel burning and then escaping past the closed valves under great pressure. I think a more likely explanation is that as we don't have fuel injection which can be turned off automatically by a computer-control, the engine is forced to accept a bit of petrol in circumstances where none is needed. As the throttle valve is closed with your foot off the pedal, there isn't enough air drawn into the engine to burn the fuel properly, so when the exhaust valves open and let the hot mixture out it burns up spontaneously giving a"pop".
Just my theory, based on (I hope) logic and the fact that it seems impossible to avoid this happening sometimes.:bang: