Back fire on over run

I have a 1972 750 roadster which has just developed a backfire when shutting the throttle only in the one exhaust
any ideas what can cause this to happen.

It's often caused by an air leak either between the carb and the head, or somewhere in the exhaust. The first thing I'd do is check the exhaust nuts are tight and the carb rubber has not split ( if you have them ) or the carb or manifold is not loose.

I had this problem fairly recently.

Although Air Leak and or Carb settings are usually the problem, my recent backfire issues was cured by upgrading my Stator / Rotor. The problem was Spark issues. At first i had checked for air leaks, then swapped out my needle jet for one size up. That had "cured" the problem but really just made it overly rich.

Ultimately when i upgraded to the Sparx 3 and returned my carb settings to the original all the backfiring went away.

I'm assuming twin Amals. Could be something very simple like the air screw on the carb on the backfiring side needs to be turned in a little. Try an 1/8 -1/4 turn in (clockwise) and see if the backfire goes away. (This is the screw that you adjust horizontally.)

All informative to me to keep in mind but remind these of 2 British Iron motto's.
Clean the Sludge Trap and Clear the pilot jet hole.