Topic: 1975 CB400f White Smoke from Breather?

[csantelle]

Hey Guys,

I'll start off by stating the problem: a significant amount of white smoke emanating from under the carbs. I think it's coming from the breather although I'm not sure as I haven't had time to really investigate. Here's some background info:

I had a bad leak from the head gasket and low compression so I took the bike to a mechanic who a family friend used for a restoration on his CB750. Work done included, among other things, a head gasket replacement, new piston rings, cylinder honing, replacement valve guide seals and intake manifold o-rings, and carb work.

 I received the bike yesterday after a short test ride (didn't notice any issues but again, the test ride was short) and went for a ride today. About 15 miles into a 30 mile ride I noticed a good amount of white smoke coming from what I think is the breather tube from the top end. It's hard to tell exactly where the smoke is coming from as the engine was hot and I couldn't really dig in and locate the exact source and didn't have time tonight to investigate. I will say that the smoke only occurs when the engine is running so I don't think it's anything "burning off."

I'm a little concerned and am hoping someone could help diagnose the issue.
Thanks all.

[scottly]

Take it back to the mechanic. If there is a problem due to his workmanship, he will correct it, if he is an honest man.

[csantelle]

One thing I didn't mention - the mechanic is an 8 hour drive away. It's kind of a long story as to why I brought the bike to a mechanic that's so far away but it is what it is.

That said, I'd like to diagnose the problem and see if I could fix it myself. Any thoughts?

[BenelliSEI]

White smoke after a rebuild is not good! Really white smoke can be condensation or water. Any chance your tank was improperly stored and there is water in your gas? It’s unlikely, but I would drain it out and put in fresh.

[csantelle]

The tank didn't have much gas in it when I got it back so it's got a full tank now.

Any way to remove condensation? Maybe a fresh oil change?

[bryanj]

More likely to be rings or honed too much, these engines need an incredibly tight piston to bore clearance

[csantelle]

@bryanj

Sounds like I need to request a complete do over from my mechanic then. I'm kind of crushed - prime riding weather and have been waiting forever to get back on the road and now this.

[Tim2005]

Any chance of a video of this? bryanj is mostprobably right, but on the 400 the breather should be routed back into the airfilter- though this is often disconnected- so you wouldn't see any smoke from it 

[bryanj]

Before taking it back first check that there is not too much oil in it, even a small overfill will cause smoke.
Second check oil level before every start and see if the engine is using oil.
I only say these as you have stated it is 8 hours to take back BUT i would contact the man immediately to say it is smoking badly and where from giving him a chance to make his own suggestions. People say it takes time for rings to bed in but i have never known that on a Honda, i have seen rings fitted wrongly and rings damaged on assembly  but that should be covered.

[trigger]

It will be down to a few things : Was it honed too coarse, piston to bore clearance is beyond spec, damage to pistons, rings not staggered or pistons not cleaned in the ring grooves 

[scunny]

What does the oil look like, any sign of condensation, sludgy on dip stick ?

[csantelle]

I did call my mechanic who basically said that the breather tube may have come loose from the airbox and to route it back. As to the excessive amount of smoke, he said it's tough to tell without looking at it but to check if he accidentally overfilled it with oil and if not, maybe the rings aren't bedded in yet.

I store the bike a few miles away so I'll check the breather tube routing and oil level tonight.

Anyone know of a good Honda mechanic in NC? May start a thread on that if that's acceptable...might even be good to have a tagged thread with names and locations of reputable mechanics by region/state, although I realize this is more of a forum geared towards folks troubleshooting issues on their own.

[scottly]

Is there any smoke out of the tailpipe? To check the oil level, the bike should be upright, like on the center-stand. Unscrew the dipstick, wipe it off, and plug it back in without screwing it in, and then pull it out and read the level.

[csantelle]

Had a chance to check the oil (engine cold, on center stand, dip stick unscrewed and resting on threads after wiping clean)- it's just shy of the high mark but within correct level. @scunny oil looks clean, not sludgy, thin or discolored. Then again, it was changed after the rebuild and only has about 60 miles on it.

I do find it strange that I'm only getting smoke from the breather though. I'd think that if oil was getting past the rings I'd also be getting smoke in the exhaust as scottly mentioned but I'm not.

I'll try and get some video in the next couple of days. I'm sure that'd help folks diagnose.

Again thanks for your help and suggestions!

[scunny]

That's what I was wondering, oil leaking past the valves, would smoke out the exhaust on start up after sitting a while, for the rings it would result in smoke out the exhaust when accelerating.
I could be wrong, but don't think so in this case.

[ekpent]

Won't hurt to take a quick peek at your spark plug color also to get an idea as to whats going on in the combustion chamber.