Check the B20 drawing carefully and note the direction of flow in the hoses. The pipe on the oil filler cap should go to the air filter side of the carbs. Direction of flow is from the filter into the filler cap. If you join the filler cap to the intake manufold that will reverse the flow. On the B20 the circuit is effectively closed with a set direction of flow. It won't work if you have anything "open". The B18 set up will survive with some open pipes as long as the intake connections aren't sucking in air that messes up the carburation. This is why it has a restrictor in the manifold stub when used with the pipe to the flame trap. If you use the B20 system, copy it exactly. Don't mix and match the 2 systems, as each one flows in the opposite direction to the other.