Topic: Rings or valve stem seals? Blue smoke!

[Mooosman]

Hi all,

I tried searching, but didn't really find what I was looking for.

I've got a 78 CB400T, and it is blowing blue smoke out of the exhaust at idle. The bike has about 13,000 miles on it, and runs beautifully! Plugs are not oily, light brown in color. Starts, runs, and makes good power. The smoking starts only after the bike is very warm, like after sitting in traffic. It will puff blue smoke at idle, but when you take off and get moving, it cleans up and there is no visible smoke. I observed from another bike when my dad rode it.

I live in Las Vegas, and the bike gets ridden in 100+ degree weather. I use Honda 20w-50 in it.

Could this be piston rings or valve stem seals? The bike doesn't smoke at all on startup, and there is no smoke trail when the bike is going down the road. It only blows blue smoke when the bike is hot, and it's sitting at idle.


Thanks!
Nick

[RRRToolSolutions]

A leak-down test is perfect for this diagnosis, but my gut says it is rings. The valve seals will allow oil to drain down into the cylinder of an open valve or puddle on the top side of the valve if it's closed - then at start-up that oil gets burned and exits as a cloud of blue smoke. This is you've guessed is at start-up. Consistent smoking is almost a sure sign of ring wear.

Get you a manual and this winter take it down. A machine shop can help you with taking measurements for comparison to wear specs given in the manual. Check the cylinder wear and cam wear before buying parts. You may need an over-bore of .5mm - usually the first over size. I just did a 4 cylinder KZ900 and th ecost of gaskets and rings was $200. You should be able to beat that. Worse case, you'll do a re-bore and hone and you'd be looking at $400. Not bad for 30 more years of fun.

Gordon

[TwoTired]

Just thinking out loud...

I agree that a compression test is in order for a better diagnosis.  If not a leakdown test, then both a dry and wet compression test.

But, vacuum is highest in the intake tunnel during idle.  If a valve guide seal was leaking, it would pass more oil under high vacuum idle conditions when throttle valves are more closed.

Seems like if it was rings, it would smoke worse when the engine is cold, as the pistons/rings don't fit the barrels as well as when it is hot.

Also, hot, thinner oil will slip past marginal valve guide seals easier than cold thicker oil.

If the bike has lived in high temps it's whole life, those valve guide seals could be pretty well baked and hard after 30 years.

That's my armchair analysis $0.02, anyway.

Cheers,

[RRRToolSolutions]

Two - you reminded me about the compression test -

Check your compression on all cylinders - recording the results. Now add oil tot he cylinder through the sparkplug hole and check each cylinder again - if there is significant increases - then your oil has sealed the wear gap - you need rings.

Guides bad enough to pour that much oil into the cylinders as to burn blue would be surprising.

Regards,
Gordon

[Mooosman]

As far as I know, the bike has lived its whole life in the desert southwest. So, it has seen high temps all its life.

I'll do the compression check. I'm guessing that, for now, it wouldn't be advisable to put in thicker oil, right? 20w-50 is pretty thick as it is...


Nick

[RRRToolSolutions]

Nick - adding thicker oil to the engine to keep the oil pump from getting it to the rings is not the way you want to go.

Take it down this winter and fix it. You'll enjoy the project - you'll love the result.

Regards,
gordon

[Mooosman]

That's what I'm thinking. Guess I'll start piecing together all the parts I'll need. Might as well go through the head as well.

Until then, I'll just run it and deal with the smoke...

Nick

[bryanj]

400T would nip up and damage pistons/rings if they got hot, also there were quite a few bad generators where the timing would wander and pistons would eventualy hole, you could never find anything wrong with electrics but Honda always told you to fit a new generator assembly complete

[tbpmusic]

...
..
.
Nick -

I have a 79 CB400T2 in the garage - can't get a title for it, so it gets parted out.
Let me know if you need anything, we'll come to some sort of agreement.
The bike only had 2,700 miles on it, it's a shame about the title...

Oh yeah, the wheels and forks are gone, transplanted into my '70 CB450 project..

bill2