IDLE PROBLEMS CB400/4

[Nurse Julie]

Hi Chaps
Having problems with Hettie, the 400/4.
When first started from cold she idles fine at about 1100 rpm but when hot the idle increases intermittently to about 2000 rpm and then gradually settles back to 1100 rpm.  Have had the carbs off today as I thought it may be  dirty idle jets. Dismantled the carbs, blew everything out with airline, no blockages. Floats are where I set them last year, 21mm, re bench synced. Mixture screws at 2 turns. Couldn't find evidence of any air leaks on any rubber bits, refitted carbs.... still the same problem. Checked the timing / points all seem ok.
Where should I look next?
Ta in advance
Julie x

[hairygit]

I'm sure Trig will give the rubber parts a thorough inspection for you! Has the bike been laid up long? I more than a couple of months, always worth checking the advance weight pivot points on the timing unit, often overlooked and neglected of any sort of lubrication. The springs are not very strong when it comes to returning to static timing (idle speed), and sticky or rusty/dry pivot points can cause those symptoms.

[Nurse Julie]

I have been riding her recently Hairy but to be honest she did do it sometimes last year as well. I will look at the bits you suggest and report back.

[Norniron]

My 400 does the same Julie!
Im always fumbling at the idle wheel on the carbs at the lights

[Nurse Julie]

Haha, what, and burning your hand like I do when I have a fumble  You have electronic ignition though Dave and pods fitted, so yours should run perfectly.

[Norniron]

Its a 40yr old bike,il forgive it for having a few ailments.
Yea its got electroinc ignition but the pods make it a nightmare to set up Juls

[mike the bike]

Mine too. After 40 odd years there's probably wear and tear that makes them not 100% reliable.

[Nurse Julie]

Yes Chaps, I hear what your saying, maybe I'm expecting too much from the old girl. It is a minor problem I know and she still rides very well, it's just a little niggle I would like to get rid of if possible.

[Clem2112]

Just a thought... I had this once on another bike.
Is there enough free play in the throttle/cables ?

[Nurse Julie]

I will check that also.

[Nurse Julie]

Update. Cables definitely have enough free play and work just fine. Took the advance / retard unit apart, cleaned it up, put new washers and springs in and...........exactly the same problem. Never mind, I took her for a lovely fast ride around the Lincolnshire Wolds and other than this small problem, she is running so well, so I'm going to leave it at that for now.
Thanks for your suggestions.

[Piki]

I have the same problem with my 400F.........

[K2-K6]

Some further thoughts to consider;- usually you need about 2 degrees or more advance to get the engine revs to rise from just ignition so unless the a and r unit is obviously not dropping back as revs come down then it's unlikely.

Normally you're looking at an unmetered air supply of some source, this could be through the rubber intake seals as first consideration but it's not the only route.
The top of the carbs and the throttle spindles also have a route into the inlet that would not get fuel metered as required by going down the side of the slides.
You may be able to detect one carb leaking over the others by trying to trim the idle circuit one at a time, going richer or leaner should get a similar response in rpm change from each carb, if you get one with low or minimal effect as you change it then it could be that one requires more in depth inspection.
Also if you assume that high tickover shows its running lean in some way, do a test with all the idles a quarter turn richer to see if it gets a more consistent tickover.
Check also the engine crankcase breather, obviously it should be breathing!  If it's badly restricted, then crankcase pressure builds and it can, if too high, pass air up past the pistons during the intake stroke ( with the carbs almost closed at tickover, it'll have quite a vacuum above the piston and positive pressure in the cases will try to go up past the rings) this is also unmetered and can make it run leaner than ideal.

[Nurse Julie]

Thanks for your input K2-6 but the carbs have been stripped and rebuilt twice, the breather pipe is not restricted, as I have fumes in the airbox and she runs a little rich, rather than lean.As it is a high mileage, 46K, well used engine, the crankcase pressure is a little high as I get a little smoke from the breather tube. Have been out on her this evening and she's absolutely flying and a majority of the time, revs dropped back to 1100rpm. Its probably the intermittency of it that p••••s me off more than anything else.

[K2-K6]

I know what you mean about tickover, it's just something that bugs us.

My brother in law ran a 400/4 years ago, commuted on it, went on holiday, etc etc it got to 107,000 mls before he replaced it! Even then it was mostly cycle parts that generally wore out, corroded etc. Cam chain was rattly though.