Slow to return to normal idle?

[grazi]

Hey all,

Looking for some tech advice. Quick background: 79 CBX. My first CBX and have owned for a few months. Have owned lots of other Honda CBs of the DOHC style. Stock airbox and exhaust. Carbs gone through by previous owner earlier this year. Starts easily with choke (though idles at about 3K with choke one...not sure if that's normal).

After warm, the bike settles into a nice even idle at about 1100 rpm. Problem is when driving or revving (parked), the throttle is slow to return to normal idle. It will fall quickly back to 3K, but then returns slowly back to 1100...probably 3-4 seconds. You can not open the throttle quickly and have it return as quickly. Adjusting the idle speed makes no difference. Choke is off, cable is free and free play is correct. It runs great off idle with no hesitation or stumbling if you ease into the throttle and pulls VERY hard above 4K. If you try and grab a big handful from idle it's slow for about a second as it climbs to 3500-4000 then takes off. It didn't do this when I first test drove it before purchase, though it wasn't an "instant" return throttle...more relaxed, but consistent.

Any thoughts as to what causes the the hang up? Not sure what is normal but this sure seems like it's not.

[steve murdoch icoa #5322]

An air leak can cause the revs to be too high and be slow to return.
I would check the the carb boots for cracks or loose clamps, maybe the connectors to the air box.
It could also be something as simple as a dirty throttle sleeve.
Welcome to the site, Grazi.

[grazi]

Thanks Steve. I'll check those both out.

[daves79x]

Welcome! You have a pretty common problem, which can be caused by several things. It's most likely the carb 'cleaning' the PO did. You likely have some plugged idle circuits and the idle has to be turned up too high to make it run. You said that backing off the idle screw does nothing - which likely means that the butterflies are way out of sync. Someone probably tried to correct the condition by adjusting the sync screws.

Another cause is obviously hung-up throttle cables, either by incorrect installation or incorrect routing. Then there's the choke cable - if it does not allow the choke plates to completely return to horizontal, the bike won't idle down. Make sure there's a bit of slack at the carbs.

Also, do not let the bike fast idle on choke - you'll turn your headers blue real quick.

Rarely, the ignition advancer can hang up. causing a high idle

Lots of things to check - most all of them carb-related.

Dave

[grazi]

Thanks Dave. Lots to look into. The previous owner id replace add the cables when he had the carbs done. But routing could for sure be a possible cause. I assume the idle circuit is overidden by the jet system above 3K and that's why it runs so well above that range. I have all winter to dig into it. Thanks again.

[daves79x]

Any CBX that runs at all runs well above 4000 rpm - you are on the mains. It's under that and at idle that causes us all the grief. But having all the low-speed circuits and accelerator pump working and carbs properly sync'd is the real trick to making these bikes run as they were designed. Sad to say, many newer owners don't know how nice these engines should run, mostly because of carb problems. Keep sorting through your problem, but you can't do much with the carbs on the bike. Be ready to remove and install them a few times until all's well. Don't worry, you'll get good at it!

If/when you take the carbs off, let me know and I'll walk you through a few checks.

Dave

[grazi]

Thanks again Dave. It's a long winter in WI without riding so I'm sure I'll be contacting you.

[grazi]

Hey again,

Start with the simple stuff first, right? I dug into the right switch control and found some interesting stuff. Old, stiff grease everywhere, some stray grip adhesive on the tube where it rides in the switch housing, but most interesting, the rear screw female thread on the housing top was split (from over tightening at some point). That lead the the front two screws being WAY over tightened and slightly pinching the the tube in the housing. Surprised they didn't split.

I disassembled all of it and cleaned all the grease and glue out of the tube, housing and handlebar. (Note: I've found the best performing throttle tube/housing setup is dry, as any amount of lube eventually attracts dirt and gums up. But open to other's opinions). I super glued the split threads and retapped the plastic. I have yet to reassemble to check performance.

I'm not convinced this is the main problem, as Dave points out some other probable issues with the idle circuits. But it's always amazing how many little things can contribute to a problem. And proof again more problems come from over tighten stuff. I'll keep you posted when I can reassemble later this week (work gets in the way). :)