you not only have to have airflow coming into the cowl...but airflow exiting the cowl as well..make sure you have an exit at the bottom of your cowling to ensure you have good flow through the cowl..also make sure the engine mixture isn't too lean. you should see a "smoke trail " in the air..adjust the high speed needle so the engine is at peak power by sound, if you lean anymore you hear the engine slow down. stop.. richen until its back to peak power and stop.now richen the needle a 1/4 turn.
hope this helps ya out