Webers with exhaust poping badly

[Eljaro]

I have had my 48IDA webers (on a 427 CO with Dove F5 head s and Comp Cam 282S solid lifter cam, 236º duration) now running for a while, and after changing the emulsion tube to a F5 the transition hole got the fix.
I still have the problem with the loud exhaust poping, including flames shooting out of the sidepipes. It was fun for a while, but it is definitely too loud and it will brake the innards of the sidepipes eventually.
I have removed the sidepipes twice and renewed the header gasket and sealed the header tubes which slip into the sidepipes with a special exhaust paste, which seems to have sealed the pipes, since there is no residual sooth now around where the pipes and header pipes meet.
But I can not get rid of the poping, which only happens when getting off the gas, when only the idle circuit is functional and the throtle plates closed.
Any hint as to what can cause this ? How can I check if there is still an exhaust leak?
Jetting is 160 main, 200 air, idel 60 and holder 120, pump bleed 50 and emulsion tube F5.