When the idle speed drops to 1100 rpms or less I get a back fire on accelerating through the gear changes. On deceleration just the normal savage putt. I checked my plug and it is a nice gray/tan and no carbon buildup. Is this a normal sequence. I have done the Lance carb modification and have a K&N filter. The mixture screw is at 2 turns out.This incident happens when I am accelerate to gain speed for the next gear.