Topic: 1981 CB650C High rev troubleshoot help

[Samgo]

Hey yinz,
   I have a 81 cb650c as the title says and I am running into a problem where when I start the bike it idles super friggin high.  Between 5000 and 6000 rpm. I just bought the bike for pretty cheap and assumed it would be a quick carb cleaning and be back on the road. Turns out it needs a new battery, rectifier, neutral switch, turn signal wasn't working (short), and this dang high rev issue.  Woof!
Things to know about the bike and my trouble shooting work thus far.  I am in Tennesee. It came to me with a 4 into one mac exhaust. No muffler, just a straight 3" Steel pipe. I messed around with the carbs for a while and after no luck I had The carbs cleaned and serviced at a shop just to double check my work and make sure it wasn't a fault on my end. Still super high idle.
The idle screw is turned all the way out and the choke cable or throttle cables aren't connected. Still cranks right up and revs high.
Any thoughts or help is much appreciated.

[DaveBarbier]

I would go through the full 3k mile service check list but it’d bet the jetting and needle height are probably stock or close to it and not setup for the intake and exhaust setup. Generally with open exhaust you need to enrich the fuel mixture.

[scunny]

Have you got a little free play on the throttle, if not your throttle cable/s need adjusting.

[Hutch]

Hey yinz,
   I have a 81 cb650c as the title says and I am running into a problem where when I start the bike it idles super friggin high.  Between 5000 and 6000 rpm. I just bought the bike for pretty cheap and assumed it would be a quick carb cleaning and be back on the road. Turns out it needs a new battery, rectifier, neutral switch, turn signal wasn't working (short), and this dang high rev issue.  Woof!
Things to know about the bike and my trouble shooting work thus far.  I am in Tennesee. It came to me with a 4 into one mac exhaust. No muffler, just a straight 3" Steel pipe. I messed around with the carbs for a while and after no luck I had The carbs cleaned and serviced at a shop just to double check my work and make sure it wasn't a fault on my end. Still super high idle.
The idle screw is turned all the way out and the choke cable or throttle cables aren't connected. Still cranks right up and revs high.
Any thoughts or help is much appreciated.

I had the same issue, solved by new carb boots. 

[Samgo]

Hey yinz,
Its been a while and life has been super busy. I checked the jetting and it is stock.
I looked over the electrical connections to the coils and it is connected correctly. I got a fresh battery and just to make sure took the carbs over to a shop and had them double check that they were in good working condition and clean and functioning properly. I reinstalled the carbs and did not connect the throttle cables or the choke cables. I also turned the idle adjustment all the way out.
Fires right up and revs super high.
I sprayed starter fluid all around the boots on both sides to see if it was an air leak. No change or increase in rpms so as i understand it isn't leaking anywhere. 
Anyone got a good lead on where to access some tables or advice on how to jet it properly/ set the needle jets?
I have also been told by a friend here in town that my pulse generator could be and issue. Anyone know what or how this could be the case?
Thanks

[scunny]

It has to be carb related, the amount of fuel being delivered determines the revs. Sorry no help on the CV carbs, they are a dark science to me.

[DaveBarbier]

Post up a picture or two of the bike and carbs. Always helps to see what you’re working with. There could be something obvious that you’re overlooking like maybe the carbs aren’t stock to that machine or the carb boots are installed backwards, etc.

What is your intake setup? Complete and stock air box with filter or 4 individual pod filters?

If you haven’t already you should verify timing is set right at both the F and advance marks. Advance should come in around 3k rpm. The manual will say exactly.

Have you checked spark plug gap? What plugs are you running? Should be D8EA with gaps between .024”-.028”.

Do you have the factory service manual? If not, get it. Any manual actually. Doesn’t have to be the Honda one but that is the best. eBay sells great reprints.

You want to find out what’s inside the carbs in terms of jets, slide needle height, float height and pilot mixture screw settings and then compare it to what it says should be stock for that bike. That means you need to take the carbs off again unfortunately.

Once you take them off, shoot some pictures of them and inside the bowl and under the top CV slide caps. You want the rubber under there to be soft and have no cracks.