Airflow problems can drive you nuts sometimes, but then so can ignition gremlins. Keep at it, you'll get it.
I had the opposite problem on one of my planes; a 50cc Extra with a 62cc Stihl engine on it that I had converted. I couldn't get it to run rich enough. After looking for all the likely gremlins, I happened to notice that when I placed my hand in front of the carb, the engine would go much richer. The Stihl engines tend to have a rather high intake port timing, and as such have a bit of "standoff" or "reversion". That's where some of the intake charge gets pulsed back out of the carb. Normally, this is contained by the air filter and the engine sucks it back in, and the carb is calibrated to take this into consideration. On my airplane, I didn't have room for a velocity stack to help contain the reversion, so it was being blown away and the carb wouldn't go rich enough to compensate.
So, I thought "ok, it will be fine when I put the cowl on". But, no deal. I have a pretty efficient cooling system on that plane, and even with the cowl on, there was enough air flowing past the carb throat to cause the same problem. Finally, I fiber-glassed an air dam into the cowl in front of the carb throat, and then all worked fine.
AV8TOR