by tightening the screw up on externaly venting valves you go against the reason of having it in the first place unless its only there for rice value.
alot of afm sensored cars suffer all sorts of problems when venting them to atmosphere,I had so much trouble with my trust typeR running to atmosphere that i plumbed it back in,it fixed most of the problem but it still never ran right.
after talking to some pretty clued up guys about afm equipped cars i learnt some interesting stuff,afm's are great on standard cars but as soon as you start upgrading turbo's and intakes etc you start messing with its operating parameters,some cars will simply have no problems where others have trouble alot of which owners simply live with,one way of fixing it is to relocate the afm from before the turbo and install it after the intercooler and before the throttle body this way the afm is not affected by the compressor wheels fluctuation especialy with upgraded compressor wheels,running the afm after the intercooler gives it a more consistent reading and is fine with the air being forced through it with boost pressure rather than before the turbo
i sold my car before i could put that into practise so not entirely sure how well that works.