ok .. full face helmet .. just to check ... like a road helmet?.. or one of those newer motorcross style jobs with a visor
it may not matter .. and i have not looked close at the chin bar of these style ones... but i know on my old dirtbike helmet that required goggles... the chin bar was extended somewhat and would let extra ambient noise in ..
even though you have a 125 dirtbike .. and they are noisier than some roadbikes... unless you have a significant air/noise gap in the helmet jaw/chin area id be surprised that enough engine noise found its way in to drown out your voice ...
strong airflow over the mic with a visor slightly open would be worse usually..
you want the stem of the mic held behind the cheekpad... the little foam head , or just the bare mic end sticking out like a nipple (ha ha i said nipple!!

) from the joint between cheekpad and edge of helmet .. these little basic microphones pick up pretty well , but will pick up the loudest local noise kinda thing...
so if your voice is muffled away by a thick cheek pad... but the shell of the helmet is making enough of a collector/resonator for the exhaust noise.. then thats what you going to get!....
you got any short test vids with the problem setup that you can give us a demo of the problem?