Does anyone know how this works? How it effects fueling etc.

I'm also very interested if above 12mph the valve allows both air circuits to feed air to the carb or just the high speed - the manual isn't specific and I can't test because mine was removed by a previous owner hence interest.

 

It's common for people to think or assume that it has to do with emissions. It's not, although there is something similar to it on CA bikes called the PAIR which does have to do with emissions.

Primarily it's about balance and throttle response. As the bike is moving, it starts to create positive air pressure inside the air cleaner box. The D.A.I. was Honda's way of taking some of that positive pressure and using it to balance the air pressure that is inside the float bowl as well as the lower portion of the vacuum slides.

The speedometer controlled where this balanced air came from. At speeds below 12 MPH, it gets air from the little funnels below the headlights. Above, it gets it from the main air duct. These passages are selected individually, and not combined. Only one is open at a time.

Honda is all about getting that little extra bit, and this was one of the ways they did it.

 

Thanks I'm now pondering this!

As I am using the bike for trackdays I assume I only need the high speed circuit in proper operation - I'm not racing it so I don't need to worry about starts. I guess I should blank off the low speed (currently it's open permanently but not to a duct) to ensure it operates as intended above 12mph. Of course the cheap old track fairing it wears may have completely changed the pressure created but as this system is to match the pressures I it shouldn't change the fueling.

So do both high and low speed feeds go to both diaphragm and float bowl?

 

No, both hoses that are attached to the Air Vent Solenoid go the the air space above the bowls. The other vent that hooks from the right side and not controlled by a solenoid goes to the air space on the lower side of the vacuum slide. This diagram from the manual shows were they go.