vacuum petcock on 81 cbx

[kbart1]

anybody have any advice on either using or not using the vacuum petcock on an 81. bought the bike and discovered it's missing. keeping my eyes open for one. what are the dangers in not having one? i am looking into having my carbs restored soon and think i MAY need this part before i get them done?

thanks!
kent

[daves79x]

The vacuum valves were installed in the '80 and later CBXs to prevent hydrolocking of a cylinder due to a leaky float valve. Lots of guys eliminate the valve and just remember to turn the petcock off EVERY TIME. Just buy a brass 'TEE' fitting and it will work just fine, but I would continue to look for one to use as it's your safety valve to prevent hydrolock.

Dave

[Will]

anybody have any advice on either using or not using the vacuum petcock on an 81. bought the bike and discovered it's missing. keeping my eyes open for one. what are the dangers in not having one? i am looking into having my carbs restored soon and think i MAY need this part before i get them done?

thanks!
kent

My experience is that that vacuum petcock is the cause of a lot of hard starting. It got replaced with a plastic tee originally designated for automotive windshield squirt system. Now if doesn't start, that variable is out of the equation.

= Bill =

[Z Grand]

The one on my '79 certainly causes hard starting issues, but to minimize the possibility of hydrolock it is worth it. Blowing on the vent hose to allow gas to flow past the diaphram into the float bowls helps it start a little quicker if the bike has been sitting for a week or so.

[Louis]

anybody have any advice on either using or not using the vacuum petcock on an 81. bought the bike and discovered it's missing. keeping my eyes open for one. what are the dangers in not having one? i am looking into having my carbs restored soon and think i MAY need this part before i get them done?

thanks!
kent

Have plenty used and NOS ones if you are still ion need of one. Just email me at usedcbxparts@hotmail.com Thanks, Louis

[Mike Nixon]

I took mine off years ago on my 81. it does help starting I think, but a few extra things are needed to optimize starting as well. Modifying the choke system, for one, adjusting the ignition pulser gaps, for another.

[spencer]

a few extra things are needed to optimize starting as well. Modifying the choke system, for one,...

OK Mike, I'll bite. Will you explain what modifications you make to the choke system?

[kbart1]

i thought i had this answer somewhere but can't seem to find it? can anyone confirm that the auto petcocks are different for each year? will one advertised as a 79-80 unit work on my 81?

thanks a lot!
kent

[daves79x]

Kent:
I think the retro-fitted '79 ones had only one outlet to suit the '79 carbs. All other years had 2 outlets and are the same.

Dave

[cbx6ss]

I had troublestarting my 81. I replaced the valve with a brass "T", and that helped some. Then one day a wise CBX advocate suggested opening the petcock for 3-5 minutes before starting. The explanation was that this allows the float bowls to top off with fuel before starting. Next, twist the throttle WOT 6 times (6 cylinders, what else?). This causes the accelerator pump to pump fuel into the carb throats. I set the choke full and touch the button. The bike starts immediately every time.

[EMS]

Then one day a wise CBX advocate suggested opening the petcock for 3-5 minutes before starting. The explanation was that this allows the float bowls to top off with fuel before starting. .

This, of course, only works if you do NOT have a vacuum valve installed.

[Mike Nixon]

a few extra things are needed to optimize starting as well. Modifying the choke system, for one,...

OK Mike, I'll bite. Will you explain what modifications you make to the choke system?

Sorry I am so late in responding. I seldom visit any more. I modify the choke to make it fully manual and not semi-automatic the way it is stock. The semi-auto thing is so the engine can be revved while still on the choke, a feature Keihin started putting in their carbs in the 1960s. The CBX version of the sysetm is more troublesome than most because it relies on very weak springs. Also, the design of these carbs is such that the choke shafts drag easily, overcoming the springs, making the choke hang up. This is especially true as the carbs collect grime around the choke shafts, corrosion build up in the carb castings, and especially if someone banged on the carbs or treated them roughly when installing, which I find a lot of folks do. Rough handling tends to unsettele the delicate alignment of the choke shafts and makes them that much more prone to sticking. Don't bang on, clamp with clamps, or incorrectly lever your carbs.

[EMS]

Sorry I am so late in responding. I seldom visit any more. I modify the choke to make it fully manual and not semi-automatic the way it is stock. .

Mike, this is very interesting. And I wish you would find the time to elaborate a little on this subject for us.
I do indeed find the choke mechanism puzzling every time I work on a set of carbs off a bike. Especially when I try to get both sets of butterflies left and right to react the same to the action of the lever. The spring connection is a nightmare.
Do you take these out and make a solid connection to the lever/cable for both sides?
In fact on both my 79 CBX and my 1982 CB1100R, the choke is essential to make the bike start, but once it is running, there is absolutely no difference whether I pull the choke out full or leave it off completely. The engine revs the same as opposed to my 81 which responds to the choke with higher rpm.
I know all this must be terribly boring for you as you must have done this a thousand times in a different life and since have moved on. But as you see, a lot of good stuff just gets lost with some of you people "getting out" of CBXes.

[Mike Nixon]

Thank you for the kind words. I used to do this mod for no extra charge as part of my rebuilding service. I recently quit including extras such as this so that I can compete with other rebuilders price-wise (because it appears so many customers choose a rebuilder solely on the basis of price). But I still believe every CBX should have this done, for the reasons I have named. I did it to mine so long ago I have to take a moment to remember when it was. 1983, when I bought the bike, within probably the first few months. You mention DOHCs, I have done it to them too, though it is a different modification as the shafts join differently, and the DOHC carbs don't really need the mod like the CBX does as their shafts don't jam nearly as easily. I have done it to a few though, for customers' benefit. The DOHC carbs do indeed have a fast idle, as does the CBX of course, but it is quite a bit simpler. If yours is not working it needs adjusting. Like the CBX it is often out of adjustment (maybe from the factory?) and needs attention. This is just one of the dozens of things that separates the average from the more meticulous rebuilders, that is, checking and adjusting the fast idle.

I really am sorry, I don't want to give away what I do since it is a service and thus a commodity of my business. It's sad in a way that I have to be like this, but with everyone and their brother getting into the carb rebuilding business, I have to work hard to preserve the value of my work, and whatever I can do to make it stand out. My apologies. I am planning to include the mod in a booklet in the future, but that is going to be quite a ways off. Incidentally, contrary to what some have said, I may indeed be doing carburetor rebuilding only part-time (which totally discounts all that I did as a dealer tech) as I build my business, but it is in fact a legitimate business I pay taxes on, so let's settle that now -- my "second job" takes up 30+ hours a week -- it is not fly-by-night). The mod is not that hard to figure out though. And the way I do it, it is reversible. No one has asked me to however. Keihin actually built in some unused pieces in the linkage that naturally lend themselves to being used for the mod. Works great. The only thing you have to remember is, with the choke fully manual, you can't really ride the bike on the choke any more. The choke plates won't swing out of the way like they were previously designed to, so if the choke is still partly deployed, the engine will of course starve for air enough to not rev up. But as I say, no one has yet complained (though one customer did express some surprise...).

Honda has had problems with this flapping choke affair from the beginning. The flaps used to back out of CB450 carbs, and others, the worst being the XL350 dual purpose bike. The system the CBX uses does away with the flaps, by making the whole choke plate flap instead of only a little trap door. But the result is the same and in the CBX' case the system is more trouble than it is worth, in my opinion, which I realize not everyone shares.

Mike Nixon

[EMS]

Mike, fair enough! As far as I'm concerned, I will not hold that position against you. Especially after the headwind you have received from other "know-it-alls", even on this site here. While most of the people in the CBX community accept you as one of the few great experts without an agenda, some others feel the need to challenge your position and experience in order to prove they are in the know.
Everybody has the right to protect his own assets. And specific knowledge is an asset!
Thanks for at least pointing out the issue!