ZN1300 (Voyager) motors are VERY different from the KZ motors! About all they share with the KZ motors is their basic dimensions of bore/stroke/cylinder spacing. Their clutches, alternators, left-side cases, right-side cases, back of cylinders, fuel delivery, oil sumps are all different. Having said that, they do share this air system, but only on the U.S. models. And, you are right that that is an air feed system to the exhaust for better emission control.
But, this system is rather sensitive, and any changes can affect idle and power. There is no "pump" to force air through this system except for the fact that the feed port in the exhaust port is positioned such that any exhaust movement past it creates a vacuum (the same way that air flow through the venturi of a carburetor draws fuel into the airstream) that draws the feed air into the exhaust stream to allow continued burning of the waste gasses.
There is a small reed valve that must be kept clean or it can allow backfeed of exhaust gasses into the carbs/injector-bodies if it doesn't check the backflow. In theory, blocking off the air flow with your plates should only change the idle a tiny bit since it is only drawing a small amount of air from immediately in front of the carb or injector body inlets.
In the main manual the theory of operation is shown on pages 180-181, and there is a detailed illustration of the system on page 3-25 in the supplement manual for the ZN engines. This system is only on the U.S. model engines.
I'm fairly adamant about leaving a well-running system alone as the engineers designed it, unless your goal is all-out racing where street-ability is not important. This syetem taps into the main air intake system and will affect it if modified. Here's why:
THis system draws it's air from AFTER the air cleaner on the bike. So do the carbs/fuel-injection-bodies. The air cleaner does create a pressure drop in the air just in front of the carb or injector bodies. So, we have the carbs drawing air from this point as well as this exhaust-feed system. Both systems are affecting the overall "pressure" (actually, a slight vaccum!) right in front of the carb inlet. So, now let's remove the draw from the exhaust-feed system. There WILL be a change in the idle due to a change in this vacuum. Also, if, as the book says, that reed valve, or the vacuum switch, are not in good shape and allow back-flow leakage, you will get some very erratic idle results. We have read several times about how an engine will change idle a lot as it warms up, and sometimes cannot be slowed down even by backing off the idle screw. If the reed valves are warping from heat as the engine warms up, this will change the idle a lot, too, as it will allow more air flow into the exhaust ports.
Your block-off plates will eliminate this system, and it's attendant problems, altogether, but you will need to make adjustments, especially to carbureted models. With slightly less vacuum at the carb inlet, the jetting might need to be adjusted. Not sure if/how the fuel injected engine would be able to be compensated since there are no sensors for this change. I suspect there would be a lean-out effect since there will be less vacuum, hence a denser air supply feeding the injected system.
Oh, what a complex web we weave when we try to clean up our air!
Nick, thanks or presenting your plates! It has forced me to re-examine the system they affect and really come to understand the hows/whys/whats and why-nots of this system, and possibly why my '85 ZN slowly increases idle speed as it warms up! AND, I have learned that this system has nothing to do with venting the crankcase, so my apologies to you about my suspicions about the effects of your block-off plates.
(as he takes a couple of bows and steps down off his soap box!)
