CB 500 (1997) BACKFIRE?!?!

Evening all. Can anyone help?? My 500 has developed a very annoying backfire!! It happens when i come off the gas then back on, also when idling in traffic but this sounds more like a wet fart. It has been in the bike shop twice the carbs were cleaned & balanced, new plugs, & new petrol tap but to no avail. Before i bought the bike it stood in a garage for 3 years unused. Any help would be greatly appreciated.

All the best
Bill

What mileage has it done and when were the valve clearances last checked/adjusted?

The idle mixture might be a touch too rich. Is there a chance it might have been previously restricted & still have the oversized jet installed?

Also, what state is the air filter in & does it have a stock can fitted?

Check air filter. How many turns out on pilot screw? And that's also my question to all of you. What are yours settings on pilot screw?

Many thanks to you all, the bike has done 22k for your information!! I shall be checking the Valves.
All the best
Bill

hi i have just got a cb500 1997 been stood for a year backfiring just like yours , did you ever get it sorted out ...