If you block (as you said, with the coke lid) the return pipe & vent the bov to atmosphere the ECU has already registed the vented air as passing the AFM so yes, the mixtures will richen up.
If unblocked it they would be a little leaner due to some entering air bypassing the AFM. Oh, & dirtier due to it also bypassing the filter shouldn't be the other way around!?
I tried the blocked pipe/vented std bov and found that the car went noticably differently, which means that my car is out of tune
Std bov is way loud when vented & the stalling at idle became rather irritating.
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