IDLE PROBLEMS CB400/4

[Nurse Julie]

Bloody hell, 107k on a CB400/4, I bet he had a sore arse  I think they are one of the most uncomfortable seats Honda ever produced. I suppose being a 70's sports bike Honda guessed all riders would have their bums out of the seat with knees and elbows scraping the tarmac 

[Erwin83]

If you only bench-synced the carbs, it's probably time to do an actual vacuum sync.

Weird idle definitely points to a poor synchronisation.

[Clem2112]

I can get my carbs to within 1cm Hg but no better..... minimal knock after air screw adjustments.
Still think the rings and  bores are bedding down after 1000 miles.

Max arse tolerance on the wooden 400f seat was an hour when I was young and supple !

[Nurse Julie]

Max arse tolerance on the wooden 400f seat was an hour when I was young and supple !

Haha, you did well at an hour. I had my first 400/4 in the late 70's and found the seat awful even back then 

[Norniron]

If you only bench-synced the carbs, it's probably time to do an actual vacuum sync.

Weird idle definitely points to a poor synchronisation.

I vacuum sync my carbs every year and they still do this

[Nurse Julie]

Well Dave, if you send me over the gauges you 'acquired' I will have a go. But in the meantime, it's such a fantastic evening, I'm off out for a ride and a rough bit of idle isn't going to stop me having fun.

[kevski]

Have you checked the vacuum take off screws, as they only have a copper washer they can sometimes let by, i always put a twist of ptfe tape around these.

[Nurse Julie]

I have got new aluminum sealing washers in place and I'm sure they are not leaking as only idles high when hot.

[K2-K6]

It's an interesting problem, certainly with others saying the same in trying to get a constant idle.

Also the bit about running rich confuses me as anything I've worked with will usually hunt and drop rpm, eventually stalling if running rich at idle. Usually you'd have to give it more air to keep it going.
I always associate increasing idle with extra air assuming everything else is ok.

They use an air bleed on many fuel injected cars to control the idle with just a small amount of air bleed to raise even something like a V8 of 4lts, it's a surprisingly small amount. It's why many latter engines have o-rings on the dipstick, as when you pull it out it changes the idle as crankcase is vented into the intake systems.

S'pose it could be drawing some small amount of air down the inlet valve guides when hot if they've got a bit of wear of them.

[Clem2112]

Air getting down the inlet valve guides sounds possible especially when hot.

Extra hot and slightly oily? air is also pulled in from the crankcase via the air box.
This must vary according to what volume is getting past the rings.

Might be worth checking the drain tube and the manifold vent tube underneath, the handbook says that one of these needs a squeeze periodically !

[kevski]

I have got new aluminum sealing washers in place and I'm sure they are not leaking as only idles high when hot.

That's indicative of air leaks.

[Nurse Julie]

I have got new aluminum sealing washers in place and I'm sure they are not leaking as only idles high when hot.

That's indicative of air leaks.

The bike does not have any air leaks.

[Clem2112]

Think Kevski is referring to internal leaks ie blow-by gases ?

[K2-K6]

Rising idle rpm says it does, it's whispering to you Julie 

[Nurse Julie]

It must be the faintest of whispers then  . You are going to have to take my word on this, there really is no evidence of an air leak anywhere on that bike. I may be looking at this problem with only a basic knowledge of all things mechanical but, him indoors has also gone through everything possible and at this stage cannot find an obvious problem. And if Trigger can't find the problem (without stripping the engine) I stand no chance