Won't return to idle/slight oil seep...

[Mikep328]

My new to me CB400 does the "won't return to idle" thing.  I noticed this the first time I rode it.  It will idle OK after starting/warmed up but when out riding, it will hang at 2500+ RPM.  Blipping the throttle doesn't help.  I have found extensive posts here re that issue so I'm going work my way through the suggestions/opinions.

 I had a Norton Commando that did the same thing and it turned out the mechanical advance weights were sticking in the advanced position when the throttle was reduced to the idle position.  So I'll check it with a strobe light and verify that the weights are functioning properly.

I don't know if the carbs were worked on previously.  There are receipts and mechanic's notes for an engine rebuild but no mention of carbs and no parts receipts for carb parts.  So I need to at least check them for proper linkage adjustment, etc.  Also will check sync though when it is idling at 1200 RPM after start up, it runs smoothly so I'm thinking the sync is reasonably close.  Popping the throttle open when running down the road at say, 3000 RPM will result in a major bog; opening the throttle less aggressively it pulls well.  I should go through the carbs I guess but if I can sort out the not-returning-to idle with external adjustments, I'll delay that..

Also noted a seep from the oil pan gasket.  I checked and all bolts were all adequately/evenly tight so seems like a new gasket is in order. 

A problem I have for mechanic work is that our bikes and car are in an apartment building multi-car garage.  So there is no workbench or electricity and everything I do has to be contained within the parking slot which also contains our MiniCooper and two other motorcycles.  Obviously, things like the carbs can be removed and worked on indoors but much care has to be taken with any operation that might end up with oil or fuel being spilled on the community garage floor.  Replacing that oil pan gasket might be tricky under the circumstances...

[Nurse Julie]

Double check to make sure it is the sump gasket that's leaking. I chased the same leak years back only to find it was an oil seal, gear change I think, that was leaking and weeping down and around the join of the sump pan.
Also, if you do take your carbs apart, I sell complete O ring kits in Viton which you will need to prevent issues with Ethanol in fuel.
Where do you live?

[Mikep328]

Thanks!  I'll check to verify where the oil is seeping from.  We live in Bath.

[Nurse Julie]

Thanks!  I'll check to verify where the oil is seeping from.  We live in Bath.

Not local to us then 😂😂😂. I was going to say if you live fairly local you could come and work on your bike in our workshop but Bath to Lincolnshire is a bit far 🤣🤣🤣

[Laverdaroo]

Im in Compton Martin near Chew Valley Lake (BS40) and Multiman Tim isnt far from you either. Let me know if you need a help or  bit of space, we're all here to help and youre smack in the middle of both Tim and me if youre struggling.

[Multiman]

As Roo says I'm close, Corsham.
I had similar idling issues with my 400/4 and between Roo and myself we eventually got it sorted.
Very happy to come over and help remove the carbs and get them stripped for you.
Inspection of the brassware and rings should reveal where the problem is.

[Trigger]

My new to me CB400 does the "won't return to idle" thing.  I noticed this the first time I rode it.  It will idle OK after starting/warmed up but when out riding, it will hang at 2500+ RPM.  Blipping the throttle doesn't help.  I have found extensive posts here re that issue so I'm going work my way through the suggestions/opinions.

 I had a Norton Commando that did the same thing and it turned out the mechanical advance weights were sticking in the advanced position when the throttle was reduced to the idle position.  So I'll check it with a strobe light and verify that the weights are functioning properly.

I don't know if the carbs were worked on previously.  There are receipts and mechanic's notes for an engine rebuild but no mention of carbs and no parts receipts for carb parts.  So I need to at least check them for proper linkage adjustment, etc.  Also will check sync though when it is idling at 1200 RPM after start up, it runs smoothly so I'm thinking the sync is reasonably close.  Popping the throttle open when running down the road at say, 3000 RPM will result in a major bog; opening the throttle less aggressively it pulls well.  I should go through the carbs I guess but if I can sort out the not-returning-to idle with external adjustments, I'll delay that..

Also noted a seep from the oil pan gasket.  I checked and all bolts were all adequately/evenly tight so seems like a new gasket is in order. 

A problem I have for mechanic work is that our bikes and car are in an apartment building multi-car garage.  So there is no workbench or electricity and everything I do has to be contained within the parking slot which also contains our MiniCooper and two other motorcycles.  Obviously, things like the carbs can be removed and worked on indoors but much care has to be taken with any operation that might end up with oil or fuel being spilled on the community garage floor.  Replacing that oil pan gasket might be tricky under the circumstances...

Do you have pods fitted ? They can give problems with bogging 

[Mikep328]

Thanks guys for offers to assist: I really appreciate it!

 I did a bit of checking this morning.  The timing per strobe is dead on and the advance mechanism is working correctly.  I futzed around a bit with the airscrews/idle speed adjustment and managed to make a bit of improvement.  It didn't hang up at high RPM when revved up/back to idle but since it was raining I didn't care to go out to see how it acted on the road.  If it's dry tomorrow I'll do that and, if no change, pull the tank/examine the various linkage/cable settings/adjustments.  I'll check the valve clearances as well.

Again, thanks!

[McCabe-Thiele (Ted)]

Depending on your ignition system might be worth checking the timing on 1&4 and 2&3 to check they are synchronised before taking the carbs off.

For how easy it is to get at compared to the carbs removing the mechanical advance - clean & lube if you have not already done so. Something that is fine when cold can start binding when hot.

[K2-K6]

Thanks guys for offers to assist: I really appreciate it!

 I did a bit of checking this morning.  The timing per strobe is dead on and the advance mechanism is working correctly.  I futzed around a bit with the airscrews/idle speed adjustment and managed to make a bit of improvement.  It didn't hang up at high RPM when revved up/back to idle but since it was raining I didn't care to go out to see how it acted on the road.  If it's dry tomorrow I'll do that and, if no change, pull the tank/examine the various linkage/cable settings/adjustments.  I'll check the valve clearances as well.

Again, thanks!

Hopefully in addition to the investigative pot, the "idle air screw" on these carb  can play significant part in this performance, tickover and throttle opening response.

They effect the mixture much further up into rev range than appreciated by some. If not rich enough they can give that diving into a hole flat spot described. Also too lean and the idle takes it's own good time to get  back down to base.

Honda manual advice is to wind them out (one cylinder at a time) to hear maximum rpm, slowly a little further until it only just starts to falter from being too lean, then turn the screw back in again (making it richer) until the revs drop by 100 rpm from the peak you obtained. Follow that with successively each of the other three cylinders.

If this doesn't work in a linear fashion (no response possibly) it would indicate that in that idle circuit there's some restriction that you'd have to get sorted to be fully operational.

The good thing about this (as opposed to just setting them 1 1/2 turns out) is it allows you to see and hear if one or more is not working properly.

[K2-K6]

To add detail, the idle circui,  both air and fuel, have to be ABSOLUTELY on the money for clean unimpeded flow on these carbs, else you'll chase your tail forever 

They meter really fine amounts, are very accurate and work impeccably when all clean and set well.

[TrickyMicky]

I've owned my 400 for over 30 years, and I've found that because there is virtually no flywheel effect, it does not like 'ticking over' until really warm. I made the mistake of increasing the idle speed using the large thumbwheel under the right side of the tank. This was fine until the engine was hot, and then, like yourself it was "idling" at 2500 revs! If your choke lever is set up right, there is a fast idle position as you start to lift it.  You say that it's possible the carbs may have been removed, might be worth checking the operation of the two throttle cables, when I replaced mine it took a couple of days to get them settled in correctly. Hopefully this might help before you have to go tearing everything apart. Good luck. Mike.
  PS. I heartily second the comment re. the gearbox oil seal!!

[Mikep328]

Good info, thanks!  Today I'm going to check the fact idle/choke linkage and throttle cable adjustment.  Regarding adjusting the carb airscrews...is there any reason to re-install the four black plastic airscrew limiters that were on the carbs?  I don't really see any need for them now.

[Nurse Julie]

"Regarding adjusting the carb airscrews...is there any reason to re-install the four black plastic airscrew limiters that were on the carbs?  I don't really see any need for them now"
No, they are not needed. Don't throw them away though, keep them safe.

[K2-K6]

Black airscrew limiters, I'd view them as obviously limiting movement, but think they look to do tbat specifically in relation to situations that we probably don't encounter in these lowlands 

For screws set at sea level, then riding up to high altitude, they seem to give a set adjustment to compensate without having to do the "turns counted out" thing. In other words you could easily set them at one end or t'other in that scenario. And maybe an early nod to restriction on adjustment for emissions legislation in some market.

Outside that, they dont seem to do much, but some may want them for originality purposes.