I too had the same problem with my metra......after hours and hours of research and head pounding i figured i would start from the beginning.
Making 2 adjustments really improved the fuel reversion.
step number #1: make sure you have the correct base gasket
thickness!!!
step #2 intake length......this is just something you will have to play with. start out with a rediculous intake and then start chopping.
Before i couldnt run an airfilter. But after fine tuning it runs with no probs. But then after installing my CDI i havent noticed it at all. Probably since the CDI puts out an amazingly high secondary and burns out the overly rich mixture. something else you might check is your float height and your exhaust is not returning the fuel mixture to fast and pushing past your rings.....Hope this helps....piece.
bacon