Review on Red CNC Air Fuel Mixture Screw: Adjustable Carburetor FCR Air Screw for KEIHIN Carbs on 4-Stroke Motocross Motorcycles by Pavel Craig

fits well but you literally can't screw it all the way down on the WR400F as the bolt head will collide with the float bowl drain plug as shown rendering it unusable without modification. I'll probably just grind down the screw head and make it smaller so it fits and I can still adjust it with my fingers on the fly. Also, the edges where you have to turn the screw are very sharp, so that's not ideal either. I was hoping for a quick and easy fix, but getting my bike set up correctly was another setback. if you have a wr400f, look elsewhere.