A vacuum petcock, or "auto valve" as Honda calls it, is/was found on many bikes during the 70s and 80s. Honda products were somewhat unique however in that unlike Yamaha, Kawasaki and Suzuki, no U.S. model Honda carried an auto valve until the 1980 model CBX. The reason it appeared was in fact the previous year 79 CBX's tendancy toward number one cylinder hydrolock (and subsequent connecting rod bend), which was caused by a 6-1 float bowl overflow hose system that encouraged capillary action resulting in wholesale carburetor bowl dump, with the number one cylinder taking the brunt of it due to its being the lowest point on a bike on its sidestand. The 80 and later CBX bowl overflow hoses were changed to a 6-2 system and also the auto valves fitted as backup insurance. An accessory (Hondaline) valve was also made available (optionally) for U.S. 79 model owners, and may have been fitted before sale by some Euro importers, as many UK and Australian models have them. And like the other Japanese brands, Honda has fitted auto valves to nearly every road model since.
Like some other bikes the CBX is known to be hard starting at times. Specifically, after sitting for a week to 10 days. This is because the float bowls are wide but shallow, and well vented to atmosphere, and coupled with a hot runnning engine (300F+ oil temp), this adds up to rapid fuel evaporation over that short period of time. Thus the bowls have to refill each time the bike is started if sitting for this long or longer. Some owners have resorted to removing the 80 and later auto valve and this does in fact shorten the refill time. But there still is that refill need, and nothing is going to overcome it completely. Some have resorted to using outboard engine squeeze bulbs, some even electric fuel pumps, and these are mildly successful. There are unfortunately some additional reasons for CBX hard starting, such as a poorly designed choke linkage that tends to hang open, weak ignition, and not the least, a carburetor design that does not use a dedicated start circuit the way many other carbs did, but instead relies on the idle jet for starting fuel discharge, a circuit that is the smallest in the carburetor, making its discharge feeble and easily overcome by varying conditions.
All this is to say that there is no complete solution to hard starting on the CBX. My guarantee to my CBX customers is that short of that 7-10 day non-use limit, your engine will start easily every time, and keep doing so within that time. After 10 days, the conditions already described will lengthen the start scenario progressively with longer periods of vehicle disuse. And should the bike actually be stored, stretching into weeks and months, the starting will of course get harder yet, as now the fuel is not only evaporating, but also beginning to gell in the tiny idle jets (making them effectively smaller), and remember it is the idle jets that the bike starts on. Use a fuel stabilizer or race gas to slow fuel gelling and all but eliminate this last scenario.
Connecting rod from a Honda CBX.
As for defeating the auto valve in order to enhance cold starts, there are many ways to do this. Simply removing the valve and adding a tee fitting is what many do. Others modify the valve itself, and there are a couple ways to do this. In any case, care should be taken. The manual tank-mounted petcock should be turned to the "off" position each time the bike is parked, for insurance against hydrolock. For more protection, remove the bowl overflow hoses, or fit individual hoses, or if a 79 model CBX and the hoses are important from an aesthetics standpoint, at least fit the later 6-2 hose system.
Coming back to the other reasons CBXs can be hard to start, starting is optimized on the CBX by making sure the idle jets are clean and to proper size (this really requires removal), by ensuring that the choke system works properly (which I do during a rebuild -- I also offer a mod that ensures positive choke function), by remembering that a week to ten days lowers the float bowl fuel levels and thus more time is needed for starting (so patience is needed, or an auto valve bypass), by fitting high performance ignition coils and maximzing plug gaps as a resulting benefit, by ensuring the CBX's ignition pulsers are gapped properly (as they age they require closer and closer gaps), and by not allowing the fuel to get stale.