Topic: CB400F Very high idle

[CB400Finhawaii]

I am working on a 1975 CB400F.  I have had it around 9 years and working on it the last two.  Except for the few times I have started it, I don't think it had been started in about 20 years.

I had rebuilt the carbs with new parts, but the bike was really running rich.  I was told that the new brass parts weren't as good as the original brass so I took out the new brass parts and put the old brass parts back in.  When I put the carbs back on, the bike idled really high (4,000 - 5,000 RPM).

I carefully bench synched the carbs and am confident that they are done correctly. All the carbs seem like they are running high, not just one or two. I can synch the carbs once I get the bike running smoothly, if needed,     I thought I might have an air leak between the carbs and cylinders.  So I replaced the rubber boots between carbs and cylinders, but still have the same high idle issue.

The throttle cables are not binding and not causing the high idle.  I took off the open throttle cable, but the bike still idled high.   I don't think it is a throttle cable issue, as I have checked it several different ways.  Off the bike, the carbs look like they open and close correctly and in synch.  The idle screw is all the way out.  The slides on all carbs look like they are at the 1/8 inch mark, just as I had set them.  Off the bike, the carbs look like they open and close correctly and in synch The high idle happens with the air box connected as well as off the bike.

Does anyone have any idea what is causing the high idle?   Are there other things I should check?

Mahalo
Mike

[bryanj]

It is very easy to get the bench sync set too high, you need to be adsolutely certain that the slides are hitting the bottom, also i have seen carbs with the slides in backwards

[Nurse Julie]

The 400 carbs have a 'fast idle' component fitted, have you set it correctly?.

[Rayzerman]

I am not familiar with the 400 carbs, but if I can offer my experience with the 350F carbs.  There is no fast idle feature except when the choke is used, the choke cam pushes on a set screw.  Set the slides at 1/16" to start.  Ensure the "fast idle" screw on the choke is not engaging (350 gap for it is 0,012") when choke is off (best done off the bike as it is difficult to access on bike).  To start have the main idle screw backed out to the point of no contact.  When you sync the slides on bike, it should be only a tweak of the slides.

I think equally important is to go over the points setting and get the ignition timing right.  Best be as anal about it as you can.  Hopefully you have an OEM point set.

Other 400 owners will perhaps have better advice if there's something unique to the 400 carbs.

[BenelliSEI]

“The slides on all carbs look like they are at the 1/8 inch mark, just as I had set them“

It’s been ages since I worked on a 400F, so I may be way off base….. if you set the slides at 1/8” and the idle is too high, maybe knock that down to 1/32” and work from there? Or bottom them out to start.

[Mark1976]

It is very easy to get the bench sync set too high, you need to be adsolutely certain that the slides are hitting the bottom, also i have seen carbs with the slides in backwards

   I think that Bryan is on the right track as the last several posts regarding this issue (specifically cb400f 's) have been just that, an improper sync. Post a pic of the rack's sync adjusters on or off the bike.

[Ozzybud]

1/8 is way too high. Slides need to be bottoming out so the idle set screw comes in to play. Likely your idle set screw has zero effect.  Lower all the sync set screws and re sync.

[bryanj]

1mm is about bigest i would use thats 1/3 of a 1/8, using 1/8 just about guarantees a too high idle.
If you go to the uk forum and look in tricks and tips there is a how to right up i did years ago, sorry dont have a copy to post here

[HondaMan]

I am working on a 1975 CB400F.  I have had it around 9 years and working on it the last two.  Except for the few times I have started it, I don't think it had been started in about 20 years.

I had rebuilt the carbs with new parts, but the bike was really running rich.  I was told that the new brass parts weren't as good as the original brass so I took out the new brass parts and put the old brass parts back in.  When I put the carbs back on, the bike idled really high (4,000 - 5,000 RPM).

I carefully bench synched the carbs and am confident that they are done correctly. All the carbs seem like they are running high, not just one or two. I can synch the carbs once I get the bike running smoothly, if needed,     I thought I might have an air leak between the carbs and cylinders.  So I replaced the rubber boots between carbs and cylinders, but still have the same high idle issue.

The throttle cables are not binding and not causing the high idle.  I took off the open throttle cable, but the bike still idled high.   I don't think it is a throttle cable issue, as I have checked it several different ways.  Off the bike, the carbs look like they open and close correctly and in synch.  The idle screw is all the way out.  The slides on all carbs look like they are at the 1/8 inch mark, just as I had set them.  Off the bike, the carbs look like they open and close correctly and in synch The high idle happens with the air box connected as well as off the bike.

Does anyone have any idea what is causing the high idle?   Are there other things I should check?

Mahalo
Mike

Hey, Mike: do you know Terry?? He's pretty good with the 400F, lives by you. I rebuilt his bike's head and cylinders some years ago, he rides it all over there.

[rsharp-CB400F]

I just got done with my CB400F carbs and I did the 1/8 pin during bench setup.
When I started it idle was about 3000 and it was directly that fast idle cam screw.
I fiddled with that and got it to just contact when the choke lever starts to lift and I was fine.
Sync was pretty good right from the start with all around 20 except one at 24.

[Bodi]

The slides have to close completely with the idle screw backed out and the choke cam not touching using the link adjustments.
Then use the idle screw to open the lowest one to 1/16 and adjust the rest via the link adjustments to match.
This should get you a startable setting.
Adjust the idle screw to get it idling ok. Then adjust the choke cam for around 4K at full idle but make sure it isn't touching with choke open.
And I think it was said before but the slide cutout must face out, it won't idle at all with them reversed.
For vacuum sync, pick the one with highest vacuum and don't touch it, adjust the other three to have even readings.