Isn't the washer that stayed on the jet, a split spring-washer that is non-detachable, and comes as part of that standard jet? Mikuni's design, by adding the spacer, eliminated contact between the split in that washer and the carburetor body, and hence meant keeping the jet from falling out was achieved only by the friction between the spacer and the carburetor body. That made it important to tighten the jet firmly, which would not be the case if it didn't have the spacer.
