IDLE PROBLEMS CB400/4

[K2-K6]

I was trying to think of a way of testing any theory of valve guides or rings as a source without dismantling.

If while it's hot and idling too fast, then connecting a vacuum to the engine breather should possibly prevent internal transfer via the above bits. If the revs dropped during this it would indicate internal wear.

I don't disagree with you vis a vis external leaks not being present, but on any petrol engine I've worked on, uncontrolled idle is always too much air from some source.

[Nurse Julie]

I know where your coming from and more diagnostics will take place. But, it only happens intermittently when hot. I was sitting in a fuel station tonight after a very fast and hard 40 mile ride, revs were idling at 1100rpm for at least 3 minutes, then all of a sudden, revs dropped and engine cut out. So, restarted the bike, rode another 6 miles, pulled up and the revs went up.Once the engine is very hot, I get exhaust gasses' into the airbox, indicating a small amount of pressure.
Edit...Its strange how other CB400/4 owners on the forum also have this problem and we can't all have the same air leak.
Edit, edit...for anyone that has read this post I originally put ' idling at 2200rpm' typo on my part. I have now changed the text to show the correct rpm at the time, which was 1100rpm.

[kevski]

could even be spark plug threads blowing by or should i say drawing in, once hot and then resealing, then it could be one of many things, i am sure you will eventually find it.

[K2-K6]

We love a good air leak on here 

After your grilling you'll be able to withstand a full round of mastermind, specialised subject; unspecified air leaks within 1970s Honda 400 engine architecture. 

Thinking about it, I was trying to flip the problem to see where I got. Sooooooooo, if you had the throttle tickover adjustment set so it gave a high idle when hot ( as in it was properly being held there) then consider it's too rich on idle screw adjustment to give stable idle over a full heat range.
To test that, you'd have to lean the idle jets off by half a turn which should speed up the tickover, then drop the tickover master screw back out to bring rpm back in range. Maybe this would give a more linear setup over a wider heat range.

Then get it hot again to see what you've got.

[Nurse Julie]

We love a good air leak on here 

After your grilling you'll be able to withstand a full round of mastermind, specialised subject; unspecified air leaks within 1970s Honda 400 engine architecture. 

Thinking about it, I was trying to flip the problem to see where I got. Sooooooooo, if you had the throttle tickover adjustment set so it gave a high idle when hot ( as in it was properly being held there) then consider it's too rich on idle screw adjustment to give stable idle over a full heat range.
To test that, you'd have to lean the idle jets off by half a turn which should speed up the tickover, then drop the tickover master screw back out to bring rpm back in range. Maybe this would give a more linear setup over a wider heat range.

Then get it hot again to see what you've got.

Well, they say great minds think alike, we did try that the other day and it made no difference at all 
I think I'm going to have to start right at the beginning again with the diagnostics and just keep plugging away until I find the problem. I took her out this morning and she was running like a bag of shite at low revs and running very hot. Gave up on her this evening and we took a pair of Honda CD250U's for a blat around the Wolds, great fun.
As for Mastermind, I won't sign up for it until this problem is sorted 

[K2-K6]

It'll be interesting if you find what's giving the problems. As you say we've thought along the same lines.

Have you checked the resistance of the plug caps to see if they're ok, also you could try it on a higher octane fuel if it's on std.

My mastermind specialisation would probably be something like VAG group passenger car diesel engine oil specifications from the period 2000 to 2008 
Yeh,  I know I need to get out more but my household to-do list is so long that I need a bike to get from one end to the other of it 

[royhall]

One of my bikes had an idle problem. Part of the problem (but not the only one) was using resister plug caps and resister plugs. This doubles the resistance and caused part of my problem. Silvers website specify the resister plugs so the problem may be widespread. Sorry to say, the other part of the problem was an air leak at the isolator rubbers, and it caused an identical problem to yours......... On a side note I have always used NGK plugs until recently. A friend of mine said to try Denso plugs, and they make a significant difference to the running and idling on all my old bikes. I am told Honda specify Denso plugs as standard, but not sure that is true.

[Nurse Julie]

Thanks both Roy and K2-K6, your thoughts give me more things to try.

[Orcade-Ian]

I tracked down air leaks on my old Brit stuff using 'easy start' - any leaks of unmetered air on the inlet side show up with a corresponding increase in revs.  I also used it to good effect very recently on the supercharger elbow seals on my Daimler when all else had failed to find the rough running - there is a heck of a lot of potentially troublesome pipes in that there engine bay!
I also found a flattened o ring on one intake rubber (carb to head) on the 350/4 a few years ago.
I have never tried it on inlet valves on these fours and apart from getting splashed with hot oil, might work with a tappet cap off and a squirt of easy start aimed at the spring area.
Be careful though - it's very volatile and an old farmer friend used to say 'don't use it too often, them engines can get to depend on it!'
Not sure of any long term damage to rubber components - perhaps Ash might know better?
Ian

[Nurse Julie]

Thanks Ian
I will add that to the list of things to try.

[K2-K6]

I saw someone starting an old engine the other day and instead of using eazistart, which they didn't have, they used Lynx! Yep the boys perfume.

I suppose for air leak testing on these old bikes you could use Brut for that proper 70's vibe. It's only a pound a tin in Savers chemists, so cost effective too. 

[Clem2112]

Honda might respond better to Hai Karate 

[K2-K6]

Think you may have something there Clem.

Perhaps Brut should be used for old Suzuki two strokes and BMW boxer twins given their promotion by both Barry Sheene and Henry Cooper.

Maybe a little Chanel No 5 for an old Motobecane?

 

[Nurse Julie]

Plenty of ideas there chaps but no way am using my Chanel No.5 for diagnostic purposes!!!. No diagnostics going out at the moment as away in Prague with 4 girlfriends, all Nurses, so will fettle more next week.

[Johnwebley]

Plenty of ideas there chaps but no way am using my Chanel No.5 for diagnostic purposes!!!. No diagnostics going out at the moment as away in Prague with 4 girlfriends, all Nurses, so will fettle more next week.

  you have a good "fettle" in Prague with the nurses!!

 then you can come back and work on the CB !!

  Have fun !!