Topic: Cb400f idling rough

[Doom64]

Hello everyone, I have had my 1977 400f for a while now and it has always idled rough, I had new Pistons, piston rings, cam chain, skimmed barrels, electronic ignition and a few other bits about 1500 miles ago, when it was first fired up the timing was set up and the carbs balanced.
When that was all sorted it started first push of the button but it slowly started getting worse, it got hard to start and idling got rougher it would sit at 1200 and go up to 2000.
Last week the timing was adjusted so now that's correct and today i cleaned the carbs out and they were a bit dirty but one carb was dirtier than the rest with a couple blocked jets (cleaned the emulsion tubes too).
Took it out for a spin it runs better but it still idles rough going up and down, the rubbers are all on correctly.
The previous owner put all new jets and emulsion tubes in but i am not sure if they were genuine Honda ones, could that make a difference?
Could the carbs need balancing again?
When it's not idling it runs lovely and accelerates smooth and fast.
Any help with be great as this it getting on my nerves 

[gwallis]

I also have a 1977 CB400F and have experienced the same problem.  When first starting I use the choke and the idle goes up and down until I make some kind of small adjustment to the choke or until it warms up.  Other than that the bike runs very strong!

Any info on this situation would be very helpful.

[innovativems]

I would definitely sync the carbs


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[gwallis]

Yeah... I did that several months ago, with my Carbtune, and they were 'right on'.  Hard to imagine that they would be out of sync with only a few miles on them.

Oh well... it's not a major problem... nothing that I can not live with at this time.

'Thanks' for the reply!

[BomberMann650]

Yeah... I did that several months ago, with my Carbtune, and they were 'right on'.  Hard to imagine that they would be out of sync with only a few miles on them.

Oh well... it's not a major problem... nothing that I can not live with at this time.

'Thanks' for the reply!

I was informed the vacuum carb sync should be checked/adj. when you do your 3k service.

[Doom64]

Hello thanks for the replies, I synced the carbs today, I done it when the bike was warm and two were out, it idled nicely but when I took it for a spin the idling was better but it was still going up and down, should I redo them tomorrow but take the bike for a ride first then adjust them?

Also I was just reading the throttle stop screw can make the bike idle funny if not adjusted right, is that true?
When I push the throttle round the revs go down a bit.

[flybox1]

Recheck everything for vacuum leaks. 
Pull the boots and inspect each one.  Over-tightening them over the years can introduce tears not seen until the boot is bent.

[Tim2005]

Does it have the stock airbox and filter?

[Doom64]

I will check the boots thanks.
Yes it has stock airbox and filter but the exhaust is a shorter reverse magaphone but it still idles rough with the standard end can on.