Hi, I've got a little backfire that comes and goes. It happens when I shift, especially when everything's cold. A mechanic suggested I take the belt to the air pump off and see if the backfire goes away. I'm guessing the ASV is emissions but what specifically does it do? this is all on the stock spaghetti-covered Aisan carb. He also suggested a Weber swap but I hear those get finicky after they break in. Would carbon on the head cause a backfire when shifting?