There aeems to still be confusion on the 'net, despite this subject being long documented. It's possible I was the first to publicly address it (all the initial mentions on vintage Honda forums are mine), but Honda actually originated the procedure in a racing service bulletin for their (1980s-era) 900F based road racing parts kits. So the procedure, namely, defeating the factory Keihin carburetor air cutoff system, is at least that old, and is factory-sanctioned. This article is by way of clarifying and justifying the procedure. What, how, why.

The aircut valve's job is to momentarily block the air side of the idle circuit (its air bleed, in other words) to make conditions for exhaust afterburn ("pop") less likely. To eliminate popping on decel. The aircut system is actually integral to the idle circuit, thus the most complex circuit in any 70s-80s Keihin carburetor. The aircut system has several openings. These include the fuel passage, the air passage ("bleed") that mixes air into the fuel passage, the discharge outlet inside the carburetor bore, the pilot screw cavity, and the three "bypass" ports immediately under the throttle plate. Yup, seven openings, more than any other carburetor circuit, and all of them connected to just the one idle jet. The aircut valve is planted across one of these seven openings, specifically the air bleed passage, before that passage terminates at atmosphere. You can see then that the aircut valve is actually a blocking device. At the right time, it shuts off the idle circuit's air bleed. This results in momentary extreme richness. This extreme richness makes the air/fuel mixture enter the combustion chamber burnable thus not go into the exhaust unburned where after mixing with other exhaust gases combusts. Pop! That's all the valve does, momentarily richen the idle circuit on decel to eliminate afterburn.

The first misunderstanding that needs busting is why to defeat the valve. What's the benefit? The benefit is simply to avoid this delicate, oft-failing valve from being a future maintenance concern. Not to mention cost. A GL1100 or 70s-80s DOHC inline four carburetor rebuild can cost $150 in just aircut diaphragms. A failed aircut diaphragm will pass fuel across the vacuum barrier and directly from the idle jet into the carburetor bore, resulting in gross richness at idle and low speeds. A diaphragm is failed by merely having a crease in it, which in short order will become a hole, as the very active valve opens and closes each time the throttles are opened. What's worse, all replacement aircut diaphragms are inferior to the (unfortunately unavailable) factory original because they are made cheaper and thus are more prone to failing.

So defeating the aircut is preemptive maintenance. That's it. There is no performance advantage, or disadvantage. Defeating the aircut does not solve any problems, or create any. The procedure does not change mixture strength in the idle circuit, or any other circuit. It does not require a compensation elsewhere in the carburetor (at least none you shouldn't have already done irrespective of the procedure). Don't look to an aircut defeat to correct a poor idle, that problem is due to many other possible issues. Don't look to it to improve fuel economy. Don't expect it solve anything at all. It won't. It can't. It isn't meant to.

Bypassing the system is as simple as blocking off the vacuum source, the valve's trigger. Do this by substituting a solid piece of rubber in place of the tiny d-ring at the vacuum port. Leave the diaphragm in place even if failed, as in all cases it serves as a gasket for the aircut valve cover, and in some cases is also needed to keep the aircut valve open. Did you notice the difference here? Keihin carbs have two kinds of aircut valves. The two types are piston-valve and trap door. U.S. model 70s-80s DOHC fours, the CX500, CX500, GL1100, and a few others, are all piston valve type. If it weren't for the need for a cover gasket, this type would allow removal of the diaphragm altogether during defeating. The other type, the trapdoor, found on the CBX, GL1000, GL1200, CB1000C and CB1100F, as well as many Euro model 70s-80s DOHC fours, requires the diaphragm be left in place to keep the trapdoor in its passive, open state.

Lastly, and amazingly, one Internet aource describes the aircut valve as "extremely important to starting and idling" and "when dysfunctional responsible for severe engine damage or failure, preignition, head gasket failure, blued exhaust pipes." Really? You didn't mention male-pattern baldness! Such incredible nonsense! The aircut valve just isn't that important a part.

Nor is it even a necessary one. So, to recap, defeat the aircut valve only to save the hassle and expense of future repairs. Don't do it to solve a performance problem. It doesn't screw anything up. It doesn't add anything or take anything away. And it does not by itself necessitate any other changes to the carburetor. And it's not a time bomb.

Original aircut valve article