SOHC.co.uk Forums > CB350/400
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 :D
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. ;D
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:
--- Quote from: K2-K6 on May 25, 2017, 06:07:22 PM ---We love a good air leak on here :D
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. ;D
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.
--- End quote ---
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 ;D ;D ;D
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