This image is from a Mikuni carburettor, but the same assembly process is used for the Keihin. The pilot screw washer, like most washers, has a flat side and a chamfered side. Place the flat side against the spring to avoid chafing the o-ring. Lubricate the threads before fitting to avoid galling. I use WD-40. You should always lubricate threaded parts wherever there are dissimilar metals for the same reason.