The stock shield creates an area of low pressure behind it, which is what's causing your problem. I had the same issue, and for a free temporary solution I just took a dremel with a cutoff wheel and chopped the windshield down as far as possible. That got my head in mostly clean, turbulence free air. Now I have a Madstad windshield, which is amazing.
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2013 KLR- klx needle, EM doohickey, EM fork brace, EM drill through subframe, UNI air filter, 16 tooth front, thermo-bob, SW-motech crash bars, JNS skidplate, Tusk panniers, Wolfman Explorer tank bag, Acerbis front fender, Russell Seat, Barkbusters, doubletake mirrors, Cogent suspension