Vintage and Classic Hondas

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Durf

Need help???

I've been lurking the forums for a while now for advice and help and up until now I been able to find answers. I have a 1981 CB 750C that is nearly stock. The carbs have been rebuilt, compression checks out great, good spark, and no vacuum leaks. The carbs still have the stock jetting and I am currently running the stock airBox. It also has the stock 4 - 4 exhaust. Here's the problem. The bike starts up ok and idles fine once it has warmed up a little, but after riding it for about 5 minutes the idle runs high. (3000rpm) At first I thought it was the throttle cable sticking but I checked that and it's fine. I can hear the butterflies close when I use the push cable if it's turned off so I don't think they are staying open. Any wisdom would be greatly appreciated.

TOOLS1

Re: Need help???

Administrator

Sounds like a vacuum leak. I would start with the rubber boots between the head, and carbs, they are usually the problem.
TOOLS

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1976 CB 750-A X 2
1977 CB 750-A X 4
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1976 CB 750 F
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1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
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Durf

Re: Need help???

That's what I thought so I sprayed to boots and they're fine. I also went as far as to remove them and reinstall them thinking that maybe I just need to crank them down a little more. No luck don't vacuum leaks normally lead to an inconsistent idle, meaning going up and down or is it different for the motorcycle?

Durf

Re: Need help???

I forgot to mention that after sitting for about three to four minutes the idle will return to 1000rpms.

TOOLS1

Re: Need help???

Administrator

It might be the Sync off, but there are other places that a vacuum leak could be. Have you checked the little plugs for the vacuum ports? How about the vacuum operated petcock?
TOOLS

Durf

Re: Need help???

I removed the vacuum petcock and haven't really given much thought to the vacuum ports since trying to sync up the carbs. I'll have to check.

shinyribs

Re: Need help???

Administrator

Hi Durf. Welcome in finally. :)

Do your vacuum plugs have the sealing washers in place? They're aluminum crush washers like you'd use on banjo bolts.

Truck

Re: Need help???

In reply to this post by Durf

You can buy the little rubber caps at Advance. they come in a pack of 10 for about $2.

It's only illegal if you get caught.

If at first you don't succeed, use more lighter fluid

95% of Harley Davidsons ever made are still on the road... The other 5% made it home.

New Baltimore, Va '82 CB900c, 1980 CB985F/K 'Mutt"

Durf

Re: Need help???

In reply to this post by shinyribs

Yes, it did have the metal washers. I'm really perplexed by this one. It had to be a vacuum leak right?

Durf

Re: Need help???

In reply to this post by TOOLS1

So it is a vacuum leak but it is at the throttle linkage. So my next question is... is there an o-ring in there? When my dad sprayed the intake boots we got nothing, but when he sprayed the linkage it revved like mad. What do I need to do to fix it? P.S. Thanks everyone for taking the time to help, I really appreciate it.