QuestionHello!
I have a 1972 Honda CL350 twin that I've recently completed an engine rebuild on (from the crankcase up). This included: new(er) pistons and cylinders, new piston rings and pins, new gaskets, new oil seal at end of camshaft, new timing chain and guides, new(er) cam and followers, new(er) valves and springs that were checked to spec and seated in the original head, new valve seals, new spark plugs, and a new points plate unit. Additional work completed during the course of the rebuild included removal of rust from inside of tank, new fuel lines, new petcock, new air filters, rebuilding the carburetors, and a new condenser (the old one finally gave up the other day). The engine has around 10700 original miles on it with around 200 added since completion. I've done my best to break in the engine in a methodical and varied manner, as I'm dealing with many new parts and/or parts from different motors.
However (and here's the actual question), I'm still noticing a light white smoke coming from the crankcase breather tube (none from the exhaust) after the bike has warmed up and run for 10 or 15 minutes. It seems to be more pronounced at idle, becoming more so the longer it sits and runs. It never really billows, just a steady puff at times. I noticed this when I first purchased the bike and hoped the rebuild would get rid of this. Yet it persists, perhaps not as much, but that’s a tough call. I do not profess to be mechanically gifted (this is the first time I've worked on an engine); however, I feel that I have been very patient, sought knowledgeable advice when needed, and followed my Clymer's manual to the best of my ability.
Other than this white smoke, I feel the bike has performed beautifully; smooth operation, idles well, and no oil leaks. I’ve spent a lot of time working on this motor, and I feel fairly familiar with it, yet I just don't understand what could be causing this. Do you have any idea what this could be, or more importantly, do you feel that this indicates anything serious that needs attention? I truly appreciate any wisdom you might impart (my apologizes on the length, I simply wished to be thorough).
Thank you,
Dillon
AnswerDillion, sounds like you have done a careful and thoughtful rebuild of this vintage classic machine, especially for a first time effort.
The breather tube exists because the pistons are taking turns moving up and down, each pushing and pulling 175cc of air on each rotation. When an engine is cold, the air inside the crankcase is whatever the ambient air temperature is, until the engine begins operation. With just the oil temperature rising into the 200 degree bracket, when fully operational, the air inside becomes heated to that temperature and more. The temperature differential between the air inside and the outside air temperature will give a large differential value, resulting in a vapor effect...
In general, white smoke is water vapor related. Blue smoke is oil related and black is excessive fuel byproducts.
Being that the piston rings are probably not fully seated at this time, you will probably see a mix of air and a bit of crankcase blowby, seen as visible vapor from the breather tube. The rings may not be seated for a good 300+ miles or more, especially if you are being conscious about the break-in requirements.
It is vitally important that you keep an eye on the ignition timing. If you installed new points, the rubbing blocks will wear down in the first few hundred miles, causing the gaps to shrink and the ignition timing to retard. In these cases, the engine has to work harder due to being out of spec, and the extra work shows up as excess heating. This overheating condition also applies to a sticking or stuck mechanical spark advancer, as well.
Having said all of that, it isn't clear if you have fitted pistons that are within recommended clearances or not. Usually new pistons and rings can be installed with about 1.5 to 2.0 thousandths of an inch. If you put in loose fitting pistons, the piston will tend to rock in the bore and that will cause the rings to not stay squarely in contact with the cylinder walls. Anything that contributes to piston/ring sealing instability will result in excessive blowby through the breather tube.
Stay on top of the ignition timing and watch the color of the spark plugs. If they are getting a coating of oil on them, then the rings are not fully seated (with correct clearances) or there is excess oil working past the rings. Any rings that are installed upside down will contribute to oil consumption and blowby.
If you can find a shop with a leakdown tester, they can check the engine to see where and how much pressure leakage is occurring, at least at this time.
If your engine has any serious problems, it will generally show up as smoke out the exhaust system, along with more and more fumes coming from the breather tube. If the plugs are staying clean and oil consumption is minimal, then the vapors are fairly normal, given the circumstances.
Hope this clarifies things for you, somewhat. Enjoy the ride on your classic Honda!
Bill Silver
