There is no vac leak... when I put the carb on the motor all gaskets were new...and also tried numerous times to find a vac leak(carb cleaner,spray water,propane).. .and the carb just came off my dads Lil Red and ran fine with no whistle,not to mention the other 2 carbs that had the same whistle ..... 4 hole spacer str8ened the air flow in the intake ...I have a thermoquad Im gonna try with no spacer and see what happens ...just for giggles...