Because engines are set to produce as little emissions as possible by means of a carefully adjusted mixture and the use of an Air Injection device, they're prone to backfire to a certain extent. To alleviate this the CVK carbs are fitted with an ACV (Air cut-off valve). This richens the idle mixture on deceleration and minimises backfires. These valves sometime fail and have to be rebuilt.
The way they work is that the idle circuit is fed from one gas jet and TWO
airways. The valve closes one of the airways during closed throttle deceleration and consequently a high vacuum, making the mixture richer.
Once you fit an aftermarket exhaust not only is the backfiring more easily heard but if the mixture is not enriched accordingly the backfire gets worse.
Another source could be air leaks at the exhaust joints, ensure these are well sealed.
If the joints are sealed, the ACV valve is working correctly and you still get it, you could try to eliminate it in two ways:
Richen the idle mixture a little at a time until the backfiring goes away or try and temporarily block the Air Injection system like this:
The reference to "balance the carbys" is a procedure whereby the mechanic ensures both carb butterfly valves open at the same time when the throttle is used. It's acomplished by the use of a pair of vacuum gauges connected to the manifold vacuum spigots, normally blanked off by rubber caps. An adjusting screw between the carb operating shafts is turned one way or the other until both gauges read the same. Properly balanced or synchronised carbs mean a steadier idle, cleaner pick-up from low revs and less vibration.