Thanks for your answers chaps. I was hoping to try out a few of the theories today but the horizontal viz never quite got to my self-imposed minimum 0f 6km. Mind you that didn't seem to worry an Extra which I just saw heading North at about 200 feet. Rule 5 anybody? Anyway I tend to dublinpilot's theory as the Airtourer needs to be flown "on the step" and the slight reduction in airspeed when I apply carb heat is probably enough for it to come slightly out of trim. I'll report back after the next trip.