Need help identifying cause of backfire problem

[SK Racing]

Over the last month or so, my 2004 GS500F developed a backfire (thru exhaust) when decelerating on compression, also known as engine braking.

Would anyone know what is the most likely cause? Valves, carb connector leaking, or something else?

It started small as an occasional backfire, but has now become so bad, I can't ride the bike. After a bad backfire, the engine struggles to run for a while, which to me means it backfired thru the carbs as well and affected the fuel levels in the carb bowls.

I have not checked valve clearances, but thought I'll ask here before tackling such a big job.

The engine starts and runs ok otherwise.