Universal Air Intake ?

[boxster91]

Hi guys

last night i removed the air box on my 03 2.7 boxster and fitted a universal air filter, i left the MAF where it was but i am now having some problems. I belive it is running too lean and if you rev the engine and let it drop back down to idle it sometimes dies or goes very low, and when cold it stumbles badly making pulling away rather difficult! the air box was damaged by the previous owner and i have a new one on order but i do like the difference in performance the air filter gives, any ideas?

Peter