Topic: CB400F Hanging Idle & rough running - out of ideas

[HondaMan]

One of the things Ihave run into with the O-ring-sealed mainjets is: the O-rings don't seal because they are too thin. They should be almost difficult to get into the mainjet's housing, requiring being wetted or lubed (I use ordinary grease). I have found that in every rebuild 'kit' someone has sent me with their 500/550/400F/350F (and 350 twin) carbs for rebuild have had too-thin O-rings, letting fuel sneak past the mainjets during idle or low-speed (less than 3500 RPM) operations.

The way I first discovered it: I turned off the tank's petcock to let the carbs use up the fuel before I pulled the carbs (again) to find out why they ran rich.

Sound familiar?

The thickness of those Orings should be 1.1mm if OEM or 1.2mm if the carbs are worn. Hard to find, sometimes: I've used 3.9x1.2mm and 4.0x1.1mm O-rings, and once 4.0x1.2mm (in a CB500/4 carb set) just depending on how much corrosion the mainjet holders had suffered.

[telerick]

Yes smells very petrol when running (not so much pure petrol smell more extremely rich / acrid exhaust smell). It's so rich it leaves soot on anything behind the exhaust pipe

Winding out air screws helps slightly but barley touches the issue

I tried increasing floats to 24mm but the problem still persists. It seems the fuel level has to be very low in the bowls for it to even out, way out of spec

Could be on to something there HondaMan - I have had the issue present with 3 different O-rings (original jets & o-rings, new (non oem) jets with the o-rings they came with, and original jets with another set of new o-rings from an assortment box) - not to say any of them were sealing properly

Could, as other people have said, a worn needle jet have similar symptoms too letting fuel by at idle? I'll try changing the O-rings first as that will certainly be less expensive that a new set of OEM needles and jets

As a side note I also removed the silencer in case it was causing a restriction, and it made no difference

[Nurse Julie]

I have the O ring kits in stock here in UK in Viton. Correct sizes, 100's of kits sold worldwide and I use them on all my own bikes and customers.
If you've already bought a kit from me and you are still having issues , that would aid your diagnostics, suggesting worn parts.

[Ozzybud]

This sounds frustrating. The fact that it is hanging up at a high idle makes me wonder if it has the same problem I had on my 400F after returning the carbs to stock settings and jet sizes. The previous owner had done everything to make it super rich and was fouling plugs
 When returning the needles to their stock positions I could not believe how intricate the linkage was. All went well and I bench synced them as close as I could get.At initial startup the idle was about 2000. Turning the idle screw down only could get it to about 1500.With the idle adjustment screw all the way out until it wasn't touching at all. When riding the idle would hang up at about 3000 and slowly go down. It would only drop to 1500 when you pushed the linkage down with your fingers. Off came the carbs again. Looking down the bores they were all even but would not close all the way no matter what.It turned  out that when messing with the linkage it changed everything not allowing the slides to close all the way. I backed off each adjustment screw one full turn. Re bench synced. Now the slides could evenly close all the way and the idle screw  could come into play. Giving the idle a hard stop.It worked as designed after that.

[telerick]

Can you point me towards where I can buy your O-Ring kits Julie? I had already purchased some based on the sizes recommended before I saw your post but would be good to get some nice Viton ones in the future

I think it is now fixed! Going to go for a ride tomorrow to confirm - but it actually needs choke to start and seems happy on air screws 1.5 turns out, where before it would not come close to running with anything less than 3 turns on the air screws. Idles super smooth and appears to run as it should now, no more rich smell or sooty exhaust

I think there were a few things at play

Primary issue: Aftermarket needles and needle jets (possibly worn too) - The problem went away after fitting new Honda needles and jets - I suspect PO had installed an aftermarket carb rebuild kit at some point as I noticed none of the jets had the Keihin logo and the old needles had a different number on them as well as a shoulder missing at the top of the needle compared to the Honda ones.

Secondary Issue 1: Oversized aftermarket slow jets installed by PO - The slow jets that came in the bike had a significantly larger bore than the replacement ones I got - You can actually see the difference by eye, and confirmed with carb cleaning wires - a wire that wont go through the replacement slow jet is loose in the old ones, despite them both being labelled '40'. Installing new slow jets made the issue slightly better, but didn't solve it.

Secondary Issue 2: Too small main jet O-Rings - This one was caused by me replacing the main jets. I suspect the poor running / backfiring whilst riding and some of the idle issue was caused by fuel being drawn up past the main jet due to the replacement main jets coming with incorrect size o-ring. After replacing the o-rings which are nice and tight now, the backfiring went away but the idle issue was still there

Thanks everyone so much for your help and suggestions. Without suggestions of worn needle jets and undersized main jet o-rings I would not have even thought of that. I left it to last to replace the needles as the needle sets are pretty expensive and didn't want to replace them until it was pretty much the last thing left - I can't work out how the bike used to run fine like this! All I can think is the PO managed to tune the air screws perfectly to mask the issue, and / or the corrosion on the needles and jets whilst it was sitting was enough to push it to the point it could no longer be adjusted out.

[Nurse Julie]

You can contact me either via the UK Forum (Nurse Julie) or email to:- cbclassicmotorcycles@outlook.com