As All Day Dan suggested, testing with the cowl on and off is an excellent way to know if you have the problem. You need to determine if the problem is caused by airflow directly striking the vent hole which can happen with front or side mounted carbs. Or is the problem caused by excessive pressure in the cowl? Is the problem constant or does it change with changes in airspeed. Many times the problem is actually a cooling airflow problem caused by excess pressure in the cowl. The fix might be as simple as adding additional cooling air exit area or decreasing the cooling airflow inlet area. On round cowl airplanes such as your YAK, I fabricate a baffle that blocks off all of that huge hole in the front of cowl except the small area just in front of the cylinder. I usually add baffles in the cowl to insure the cooling airflow has to pass through the engine cooling fins and not just around them.