Having covered 250 miles the running in period seems to be going fine, a few nuts and bolts needed tightening plus an annoying oil leak from an exhaust rockercover which won't seal up yet. And of course head tightness checked at least 3 times
I notice yesterday that having filled up with shell power plus and this maybe a coincidence but shutting the throttle quickly is producing a sort of backfire not loud more of a deep popping.
Not been able to strobe time it yet, premier carbs, tri spark, rebored, new valves and springs.
Is usually caused by air leaks, usually at the exhaust port, but can also be on the carb to head joints.
You have been doing the head retorques, but have you checked exhaust tightness at the same time? Worth doing.
If this happens when shutting the throttle while motoring ie. on the overrun, the mixture may be just a little weak in the pilot circuit so just turn the air screw in a bit until it is eradicated
Strangely I have never had popping due to weakness,A rich mixture (not all burnt) can combine with air from a bad head nut or silencer clamp and ignite in the silencer . An intermittent spark can ignite unburnt fuel and cause mighty backfires ,I was able to do this at will on the Red Hunter Very satisfying when tailgated by some daft motorist..