divvy 600 misfire help

[DAGZ]

Hi all' after some advice guys. got a 1998 divvy 600s and got issue with backfire' things i have done and tested are fully refurb carbs (like new). oil and plug change. tested both coils leads and plug caps all showing good on multi meter also tested pick up coil that shows good too. cleaned fuel pipes and tank out. replaced with fresh e5 fuel. bike was running sweet and pulling real well until parked it up for a few weeks' its starts on the button no issue but it back fires when trying to rev it. done carb balance twice. full compression test all good and engine is sweet and only done 24,000 miles. only thing left to do is change cdi unit. but thought they work or don't work lol. new battery too. so guys any ideas please. mot due in a few weeks. cheers all and thanks in advance. ride safe all. no air leaks checked that too