Sudden exhaust popping

Hi! My KZ1000J3 is running better and better! I'm becoming friends with the Mikunu RS carbs.
On my way home from the garage yesterday the bike started popping in the exhaust... It never done that. And i rode the bike a lot that day. It's all new exhaust gaskets, new carb rubbers and new internals and gaskets in the carbs.
I know exhaust popping is common with lean mixture but why would it act like it now? The weather is getting colder here in denmark... But that suddenly ? The plugs seem good and has a good color, am i wrong ?
Would false air do this without showing up on the plugs ? How to check for air leaks on the exhaust side of the engine ? Any thoughts is welcome!

Simon

Could also be that the valves need adjusted.

Will take a look at the valves as soon as i get a chance. But i'm pretty sure they are within specs!

check exhaust leak

points (if you have them) and timing