1982 cb650sc carb questions

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82cb650
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1982 cb650sc carb questions

Postby 82cb650 » Wed Feb 03, 2016 3:21 am

Soo i traded an older honda civic straight over for this 82 honda nighthawk CB650SC custom hardtail. And it appears that at some point It was owned by a tinker tweaker and I'm not sure if it's missing things or not here's a pic of the carb tree that hose that's capped goes to nothing. There's only one hose that's straight from the tank to the fuel bar and that's it
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Folsoml
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Re: 1982 cb650sc carb questions

Postby Folsoml » Wed Feb 03, 2016 6:00 am

Welcome to the forum!

When these bikes were first designed, there was a device on the top of the carb bank that acted as a vacuum fuel shut off--or to be more accurate, it was a vacuum fuel "allow flow." When the bike was not running, the valve in this device would be closed, and fuel would not flow through it. When you started the bike, vacuum pressure from the engine would open the valve and allow fuel from the tank to flow to the carbs. Over the years, these devices would break down, so bike owners would simply remove them and run the fuel line directly from the tank to the carbs. The only down side to this is that you must always ensure that you turn the petcock off whenever the bike is parked--otherwise your float needles may fail and allow the cylinders to fill with fuel. Now to answer your question: A previous owner of your bike removed this device. The hose you see that is capped off is the vacuum hose that would have opened the fuel valve.
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Volker_P
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Re: 1982 cb650sc carb questions

Postby Volker_P » Thu Feb 04, 2016 3:29 am

Welcome here! :D

Here are some pictures that indicate the complete set of hoses and devices:

viewtopic.php?f=4&t=4036
viewtopic.php?f=4&t=4162

As writtten, without the vacuum shutoff the nipple at the #3 intake must be plugged. This has been done by the PO in a strange way (I do not trust screws fit in hoses like that being really tight from experience).
However I am not sure if it would work without the hoses to the "vacuum tees" that are located between carbs 1+2 and 3+4. Don't be too surprised if your carbs leak without the hoses there (seems like some do and some don't) or even with cracked T's.
To find out what I am talking about now, please refer to:

viewtopic.php?f=4&t=8865&start=7
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82cb650
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Re: 1982 cb650sc carb questions

Postby 82cb650 » Tue Feb 09, 2016 11:27 am

Thank you! That's good to know. My second question would be, when I first got the bike it would not go above 3000 rpm giving it a top speed of about 45mph. When I checked, each carb had a 1/4 turn difference from the next. I bottomed them out and started at 2 turns out. Started it, and I started by 1/4 turn back in and it seems to be running fine. It has instant throttle response and I have not yet tested top speed as it cruises fine now. So to my question: is 1 3/4 turns out where it should be? Or am I still missing on how it could be.

Here's a pic of the bike in case you wanted to know 8) 8)
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Folsoml
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Re: 1982 cb650sc carb questions

Postby Folsoml » Tue Feb 09, 2016 12:13 pm

Your problem is likely your air filters, or lack thereof. The CV carbs were designed to work with the stock airbox. Anything else will give you less than desirable results.
Have a problem with your CB650? Have a technical question? Click here!


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82cb650
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Re: 1982 cb650sc carb questions

Postby 82cb650 » Tue Feb 09, 2016 8:43 pm

Folsoml wrote:Your problem is likely your air filters, or lack thereof. The CV carbs were designed to work with the stock airbox. Anything else will give you less than desirable results.


I think I worded it wrong, as of now it runs like a dream. Solid throttle response and satisfactory acceleration. But since it's my first bike I have nothing to compare it too. My question was . Does 1 and a quarter turns out on all of them sound right? What are yours currently set at?

82cb650
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Re: 1982 cb650sc carb questions

Postby 82cb650 » Tue Feb 09, 2016 8:51 pm

But yes, the lack of intake resistance is probably affecting performance, especially since the valve cover breather is open

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Volker_P
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Re: 1982 cb650sc carb questions

Postby Volker_P » Wed Feb 10, 2016 2:49 am

These idle screws really do nothing but fine tuning to the idle mixture. If it idles all right at a reasonable level it is just fine. Depending on airbox, altitude etc. this setting may differ so everyone has to find his own here. So 1 1/4 turns out sounds all right.

The issue that you really should worry about now is that your mixture at load is too lean, too. This is not only affecting top performance but the engine may overheat, too. Have a look at your plugs. A bit of white is all right, but if they are really bright mixture is too lean which asks for larger main jets or a more restrictive air intake. Probably nothing to worry as long there is snow around and you do short trips at low rpm but engine overheating damage may occur at an extended turn at higher rpm.

An open valve cover breather may collect moisture. Of course you must not plug it but a hose of some length may be helpful.
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