Wandering idle...

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Wandering idle...

Postby seawall » 19 Sep 2019, 16:23

Here's the problem. I have a 550B1 which has developed a carburation issue. It has always been a pig to start, requiring full choke and about five or six blips of the button before it successfully stays chimed up. Once you get going idling is a complete lottery. This can vary from stalling, to banging away at 2000rpm plus. Tweaking the idle stop below the carbs makes little difference other than accentuating the issue each way. On occasions it does gradually drop from around the 2000rpm mark down, whilst one is sitting there fuming, almost like something is sticking.

It went in with a local mechanic for a few days to try to solve it but he drew a blank, and has a with a similar issue. His 'solution' was to fit new carbs.

Just in case he had missed the obvious I fitted new throttle cables a couple of weeks ago. It didn't say anything in the manual about needing tentacles to do this, mind you. Result - not a blind bit of difference.

Any thoughts or solutions appreciated. It's not quite making it a mare to ride, but it's not so much fun at the moment.
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Re: Wandering idle...

Postby hugojose » 19 Sep 2019, 23:43

If it is about carburation, there is not much to it. The floats valves and cleaning. This means float sealing and controlling fuel level. Any "rebuil kit" is about changing everything rubber + the mix screws. You may also have a vacuum leak.

Also, the Kehins CVKs don't have 'choke', they have fuel plungers (enricheners), that means instead of choking the air, they add fuel, and it is very rough to control, even when everything is fine, specially after a week of not running. If you have some other issue, it gets worse.....I'd consider checking the carbs, floats, fuel level, cleaning (spraying all openings with carb cleaner), and checking for vacuum leaks.
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Re: Wandering idle...

Postby Freddy » 20 Sep 2019, 00:36

seawall wrote:Here's the problem. I have a 550B1 which has developed a carburation issue. It has always been a pig to start, requiring full choke and about five or six blips of the button before it successfully stays chimed up. Once you get going idling is a complete lottery. This can vary from stalling, to banging away at 2000rpm plus. Tweaking the idle stop below the carbs makes little difference other than accentuating the issue each way. On occasions it does gradually drop from around the 2000rpm mark down, whilst one is sitting there fuming, almost like something is sticking.

It went in with a local mechanic for a few days to try to solve it but he drew a blank, and has a with a similar issue. His 'solution' was to fit new carbs.

Just in case he had missed the obvious I fitted new throttle cables a couple of weeks ago. It didn't say anything in the manual about needing tentacles to do this, mind you. Result - not a blind bit of difference.

Any thoughts or solutions appreciated. It's not quite making it a mare to ride, but it's not so much fun at the moment.


Strip, clean, and readjust the carbies. It should be taken as a near certainty that an infrequently used carburetor engine around 20 years old will have significant buildup in the carburettors.

Not a difficult job, other than getting them in/out, which is not that difficult once you've figured out the tricks. I recommend against using aftermarket overhauls kits, other than just the gaskets and o-rings. The quality of the jets etc in these kits is all over the place, and the parts unlikely to be an accurate replacement for the originals. The only 'metal' part you may need to buy is a float valve (unlikely) but if you did, use the expensive OEM part.

My principal is because carburetors are finely tuned precision components, never use anything but original or identical genuine replacement parts. Not unless no other option is viable, or you deliberately want to make changes.

P.S. Find a new mechanic, and make sure you have a small amount of free-play between the take-up of the two throttle cables. About 2-3mm 'roll' at the throttle grip.
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Re: Wandering idle...

Postby seawall » 20 Sep 2019, 07:32

Thanks both. He did use some sort of overhaul kit but I imagine that it wasn't up to much. A job for me then, when the weather gets rougher.
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Re: Wandering idle...

Postby Ivor » 20 Sep 2019, 21:50

I had similar problems which might help. I had one of the vacuum hoses come off and also a couple of the carb to air box rubbers had come adrift. (and you're right about having tentacles for them throttle cables.... jesus what a job that is!! :shock: )
i hate being bipolar its awesome :/
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Re: Wandering idle...

Postby Freddy » 21 Sep 2019, 05:42

The easiest way to do a throttle cable on a 750 (either one) is remove the carbies. Might be the same for a 550 or 1100.

To make removing the carbies easy on a 750 easy all the crap that is in the road of the throttle cables should be removed anyhow (clutch cable, vacuum lines, rectifier, coils, etc). With the carbies out the throttle cables are easy to disconnect, and with all the other 'clutter' removed the rest is just straight forward.
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Re: Wandering idle...

Postby Hungry Wilf » 25 Sep 2019, 23:27

Hi mate, sorry to read about your problems.. starting issues and intermittent idling and running.. dirty carbs and fuel is the obvious place to start, but there are other things worth looking at.. how are the spark plugs? I think someone mentioned the carb rubbers, they can be an issue.. air filter? Maybe the carbs just need balancing? Hopefully your mechanic has covered these? If the carbs are repeatedly getting blockages, maybe the tank needs de-rusting.. it wouldn't be the first! Good luck with it.
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Re: Wandering idle...

Postby retrorider » 16 Oct 2019, 10:29

Hi all, seawall ive had the same sort of issues with my 750 c3 keihin cvk32 carbs i found that they have air leaks through the throttle linkage butterfly shaft which has a felt washers fitted i bought another bank of carbs and they have the same leak! as yet i havent found any info on how to sort this problem.ATB.
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Re: Wandering idle...

Postby Freddy » 16 Oct 2019, 12:10

Just to put some perspective on wear/age concerns. My bike has now down 185,000 hard kilometers. Yes, there is some wear in the butterfly shafts which becomes obvious when you try and accurately sync the carbies.

Set a perfect sync at idle, open the throttle slightly and the sync goes way out. I work around this by setting the sync with a slightly open throttle and wherever it is at idle, it is.

Regardless, the bike idles perfectly, starts first spin, no choke regardless of the temperature (only since fitting larger main jets), and runs well. 185,000 kilometers of wear hasn't affected performance.
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