Auto valve?
The vacuum-operated fuel shutoff valve is ubiquitous on modern carbureted powersports vehicles. However, Honda products were somewhat unique in that unlike Yamaha, Kawasaki and Suzuki, no U.S. model Honda had an automatic fuel shutoff ("auto") valve until the 1980 model year. And it appears that the famous, endearing, quirky and greatly misunderstood CBX1000 was the bike that brought the vacuum fuel valve to the market. This came about because the CBX's 1978/1979 spectacular introduction was marred by persistent instances of smashed engine crankcases--I did a few warranty case replacements myself while at Honda dealerships. I took the picture shown here, at Rice Motors in West Covina, California in 1978.

Cylinder hydrolock
When the machine was parked on the sidestand, the engine's unusual width resulted in the number one cylinder hanging very low. Should the manual fuel valve be left on, fuel could migrate to that cylinder and more or less fill it, resulting in hydrolock when the engine was subsequently started. In virtually all cases, the rider reported noticing nothing unusual, perhaps reluctant starting, maybe a slight noise. And he would ride off. However, the number one connecting rod was now bent, and like almost everything else in the CBX engine, if was of very lightweight construction. That rod was at risk. Few actually failed, but all bent. The ones that failed snapped and sawed or punched the engine cases.

CBX overflow hoses
Though never publicized, it was discovered early on that the carburetor's overflow hoses encouraged the movement of the fuel. Their 6-1 arrangement created a capillary action which resulted in the number one cylinder getting doused. In 1980, Honda did three things: 1) They inaugurated the fitting of a vacuum fuel valve on the CBX and on that year's DOHC fours. 2) They revised the CBX's carburetor overflow hose system by changing it from a 6-1 system to a 6-2 system. 3) And they issued a version of the vacuum fuel shotoff valve that would fit the 78/79 models. This valve was available for sale through their accessory division, Hondaline. No bulletin. No recall. No customer letters. The little communication there was was limited to the manufacturer/dealer network. And we were told in unmistakable terms to remove the overflow hoses on the bike in the picture.

Three kinds of valves
On the CBX, there are actually three different kinds of vacuum fuel valves. 1978/1979 models had no valve unless someone purchased the retrofit one. The retrofit valve is round and is unique in that it has only one fuel outlet nipple. The second, 1980 valve, on both the CBX and on the first-gen DOHC fours is also round but has two output nipples, because those carbs have two fuel entry points. Then in 1982 a different, third valve appeared on the CBX and DOHCs. It has a square body and two output nipples.

Valve anatomy and rebuildability
The vacuum valve is very simply made. All three valve types have a rubber-tipped plunger inside that turns the fuel flow on and off. This plunger is contolled by two diaphrams, one on the vacuum side and the other on the plunger/fuel side. All valves also have an air vent nipple on the zinc alloy valve body with a long hose attached, as well as a vacuum nipple that attaches via hose to one of the carburetor bodies. Honda never offered a rebuild kit for these valves. However, Honda did issue a rebuild kit--basically just diaphragms--for motorcycles made in the 1990s and later. This kit happens to work on the CBX and DOHC valves, but only the square-bodied version. Don't attempt on a bad round valve to "make it work". It won't. You also don't want to disassemble a working round valve. It will stop working on reassembly.

Availability
Honda has long since run out of replacement auto valves. Occasionally, if you wait long enough, you will find an NOS one on David Silver or CMSNL--put in a request. But the last one I waw was going for $400 and that was in 2022. Buying one of these valves used is not wise, as they are all over 40 years old and have failed, and the round ones cannot be rebuilt. A square one, whatever condition, would be a good find, but be aware the square valves take a different mounting bracket than the round ones, so you must hunt for that bracket at the same time.

Three tests of the auto valve
Attach a hose to the valve's fuel inlet nipple. 1) Gently blow into the hose to check that the valve is closed internally. Attach a Mityvac to the vacuum nipple, and after creating vacuum 2) Look for vacuum loss on your tool. There must be none. Lastly, 3) before removing the Mityvac, test the throughput of the fuel inlet by again gently blowing on the hose as you did at the beginning.

Symptoms of a failed auto valve
The most common symptom of an in-service bad valve is gradual leaning of the carburetors because the valve is not holding vacuum and therefore not opening all the way. I have seen scores of these valves that wouldn't hold vacuum for even a fraction of a second, and many many more in various vacuum-holding condition. Even a valve that holds some vacuum but bleeds it off--at virtually any rate--is bad because the valve is already choking the fuel supply.

Starting
Forums like to bemoan the fact that the auto valve affects ease of starting the engine. It does. Here is how that happens. First, realize the carburetor float bowls on the CBX and the DOHC get hot because the engines get hot. On a summer day after a good ride, you park your bike in your garage and come out an hour later and the place smells like gas. The fuel is evaporating and in just a couple days all the gas will be gone. On a vacuum valve equipped bike, the carbs can't refill until the engine is rotating by the electric starter. For that reason, starting scenarios can be frustrating. Should you remove the valve to make starting easier? No. I think most riders can benefit from the added engine protection. However, what you can do is do the hemostat trick. The next time you're starting the engine after a week or more sitting, have a pair of hemostats in hand. Crank the engine for 2 seconds as if to start it, and before letting go the button, clamp the vacuum hose using the hemostats. Turn the key off, go inside to grab your coffee, helmet, whatever, and within about three minutes the float bowls will be filled and the bike should start right away. Don't forget to remove the hemostats.