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Old 08-20-2012, 05:40 PM   #1
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Default 85 cb700sc Nighthawk vacum / fuel line problem

I have bought a disassembled 1985 cb700sc Nighthawk
I have cleaned and rebuilt the carbs, As I am installing them I find on the bottom of one carb a brass tube like an overflow or vent, then on top all four carbs have a vent tube, I think these four go together and ty in with the bottom to a vacum / pvc emision control sys that is missing. On the intake pipe on the no#2 cylinder is another brass tube that I believe goes to the diaphram on the fuel petcock. But where does the vent tube on the gas tank underneath go??

Anyone able to confirm or direct me as to where these all go would deeply be appriciated.

Russell
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Old 08-20-2012, 08:55 PM   #2
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If this is a California model, there was a vapor recovery system; the fuel tank vent went to one input of a bottle-like device, which had something in it to adsorb the fuel vapors, and the vacuum line to the other input, to draw the fuel back out of the device and burn it. If not a CA model, the tank vent would probably go to an air filter box. I would guess that the four tubes on top of the carbs are bowl vents, and these normally go to the air filter box, so only clean air can enter the carbs that way, and any vapors released go into the intakes. I'm surprised that only one has a tube on the bottom, which I would guess is a drain; if there is a screw near it, it is very likely a drain.

This is all relatively-informed guessing; unless someone with first-hand knowledge of this bike chimes in, you really need a manual which shows routing for all those tubes.
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Old 08-21-2012, 07:33 AM   #3
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WintrSol,
Thanks for the info // confirmation, The brass tube on the bottom of the #3 carb is like a vacum or vent tube, not the bowl drain. I descovered on the right side cover on the inside has a diagram of emissions control, but I do not have a canister, or a entry port on the intake filter box for other hoses to hook up to except at the bottom the crankcase hose and very bottom has a weep drain hose for excessive liquid.
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Old 08-21-2012, 10:16 AM   #4
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Just for my curiosity, can you take a clear photo of that diagram and post it? It would certainly explain a lot, to me, anyway.
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Old 08-21-2012, 10:56 AM   #5
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Russell,

I just so happen to be rebuilding the carbs on my CB700sc as well. I actually rebuilt the head in January, and now I can't remember where everything goes. I actually have the fuel coming into the brass ports on the top of the carbs, but I think that's completely wrong now! On my bike I have all 4 of the lines on the top leading into a "junction box" (for lack of a better term) and then, likely erroneously, I have the fuel feeding in here. If the fuel doesn't go here (which it likely doesn't, since it should go in the fuel rail) then I don't know where the extra vacuum line goes. But at least that might be part of the puzzle for you as well.

I have the factory service manual and I'll take a look tonight.
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Old 08-21-2012, 11:14 AM   #6
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PS just found this post from a while back, it may be the answer we need: donthave15postsye: showthread.php?t=104119
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Old 08-21-2012, 11:48 AM   #7
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Here is a picture of the California emmisions on the inside cover, I dont see any tank or pvc sys on my bike, could have been tossed by previous owner.

Gringz, The top tubes go together and are vent relief for the float bowl area, they do run into a common header with about a 3/8" connector.
The fuel goes to one port on back of carbs between the #2, and#3 carb just above the float bowl. It is a stubbt Tee between the carbs. It is connected to the other cabs via a pass through with short tubes between the carbs.

I have been rescuing older Honda's from varnish and dust death. I like to tinker and this gives me a chance to tinker, ride and enjoy a multitude of bike styles. I do sell them after putting a couple hundred miles on them so someone can enjoy them. So for me to buy a service manual is not feaseable money wise or tiime wise. I will have it fixed and running before I was to recieve it and that's $30 to $40 I can use elsewheres.


I can take a picture of the carbs and or what I have so far and ppost or email if needed.

Russell
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File Type: jpg Honda diagram 002.jpg (56.6 KB, 11 views)
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Old 08-21-2012, 11:51 AM   #8
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Ahh that may be your answer. My bike wasn't fitted with an evap canister, likely because it wasn't required at the time by law. So I imagine they are just vented into the open air.

Does the bike you have have an evap canister (and presumably the PCV valve)?

Let me know what you need from the manual and I'll post a picture.
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Old 08-21-2012, 11:59 AM   #9
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No on that extra stuff. I just added a automotive PVC valve into the intake box next to where the fuel petcok sits. I piped it to the header to the top of the carbs. It may have been to the atomosphere but it needed a filtering device of some sort. Since I had the prior items on the bench I used it instead.

I still need to find out where the small line from the tank itself (like a vent tube) goes to. Then the vacum tube on the diaphram on the petcock where it goes to.

Thanks
Russell
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Old 08-21-2012, 12:23 PM   #10
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That's an easy one. There is a brass nipple that connects from the 2nd carb boot to the vacuum nipple on the petcock.
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Old 08-21-2012, 02:50 PM   #11
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Now I need to know where the tiny line fron the tank(looks like a vent tube) goes to. it is about an 1/8" off right side of tank, seperate from fuel valve stuff.

Russell
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Old 08-21-2012, 02:55 PM   #12
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Not sure on that one, I'll have a look tonight.
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Old 08-21-2012, 03:28 PM   #13
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OK, the line from #2 carb opens the valve labeled P.C.V., which stands for 'Purge Canister Valve'; that valve opens, and lets the other four vacuum lines draw air through the canister and, with it, any fuel that was collected from the tank evaporation. The drain on the canister is in case any liquid collects in it. Without a canister, the tubes should just be capped; the 4 upper tubes can be used to test the carb sync, BTW. You can confirm that they are vacuum lines easily enough with a gauge on one.

Your crankcase vent should go directly to an air filter box; none I've seen on bikes have valves in them.

If you have a vacuum-operated petcock, I think it should T into the line that controls the P.C.V.; in your case, just route it to the #2 carb.
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Old 08-21-2012, 03:31 PM   #14
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Thanks Wintersol!
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Old 08-21-2012, 05:31 PM   #15
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Thanks all for the help on this, seems there are a few out there needing this info too..

Russell
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Old 08-21-2012, 05:35 PM   #16
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What are the odds?
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