Topic: 750k6 breather hose smoke question

[jay_m]

15,000 miles on the odometer
bike runs like top, with steady idle and tremendous acceleration
recently changed oil/filter, filled to the correct level and checked on level ground on center stand

after long drives (an hour or more seems like the typical threshold), i notice that the breather hose by the back wheel, just above the center stand is smoking, and there are small wet splotches around the edge of the tire. the smoke is white, and appears to actually be condensation, as it has no detectable smell, is clear, and has the viscosity of plain water when collected. but there must be a little oil in there, as it stains the tires--perhaps its just vapor picking old oil up from the inside of the lines.

i dont think my petcock is leaking, and the throttle response is so good that i have a hard time thinking that theres a carb/fuel supply issue thats draining into the oil. but then again, what do i even know?

in any event, the hoses are not pinched, and are connected properly. given the low mileage on this bike, and the fact that its running so well, what are the most likely causes of this, and how can i stop it? even having a hint of oil by my tire makes me uneasy.

also, heres a video of the bike idling after a long enough ride to create the smoke/condensation. note the marks on the sidewall of the tire, as well.

IMG_0648.MOV.MOV

[TwoTired]

White smoke is water vapor.

Water is a by product of combustion.  Amount determined by humidity of the air ingested.

Each time the engine cools down water collects on the internal wall of the engine case.  (Just like water forms on the outside of you beer glass.)

The condensation turns to water vapor when as the temperatures increase.  It is more visible on exit if the humidity is high and can't be readily absorbed by the outside air.

So, to get all the water out of the engine case, you must heat the water to a temperature that induces vaporization and hold that temperature for as long as the water still exists in the engine case.  (usually 15-20 minutes at full operating temp.)

Then, if you have piston ring blow by, the water from combustion gets pushed into the engine case.  Where it either exists condenses on the engine internal walls or exits the breather tube.

So, my question for you now, is what was the relative humidity during the video?
And how long had the entire engine case been at full operating temperature for the video capture?

Cheers,

[jay_m]

hey twotired-

it wasnt particularly humid when this video was taken. but i have also experienced this phenomenon when the air was distinctly dry (earlier on in this season, when it was still almost wintry outside). like i mentioned above, this video was taken after around an hour of riding the bike, so the cases had been at operating temperature for some time. it also has happened after a full day of riding, with the final stretch of riding being about 2 hours straight, both highway and city. i'd hate to think my rings arent seated properly, seeing as the bike has such low mileage and is still on the stock rings as they came from the factory, but i guess anythings possible with a  bike this old.

[Markcb750]

Pretty normal, I installed a PCV** canister and airbox from an F0 on my K6.  I no longer have the smoke spewing on my pipes and giving people at trafic light something to point at and tell me about...

Some of the smoke is water, some of it is other combustion gasses which escape the valve seals which are not perfect even when new.

Keep riding.

**dyslexic I eb. 

[jay_m]

so is this particularly endemic to the k6?

tell me more about this canister you installed? my main concern (apart from the annoyance of people always pointing it out), its the dripping on the tire that i really want to prevent.

[Markcb750]

Common to all the 750 SOHC motors.  some fixes out there, but if it is running OK, ride it.

The canister and tubing connect to the valve cover via the tube you see billowing smoke and a connection point on the 75F0 airbox.  It sits above the swingarm & below the battery-box.  It has a filter element and a drain to allow water and other liquids to drain out.  at the top is a nipple to connect to an airbox.  (I made a connection to my original airbox and found that without the F0 metal deflector it melted the filtration element I placed to use as a tubing connection.


Best to look at the parts drawings and maintenance manual for a F0 or a K7 K8. because the K7 & K8 had improved carburetors I do not think the airbox is interchangeable to the K6 as the F0 & F1 are.

[MCRider]

I would agree to reroute the tube into a catch can, and just ride it. The downtube infront of the oil tank, above the brakepedal may have room for a small 4-6oz OJ can or the like.

I don't think that its a proper condition of the motor. Fresh and new there would not be any visible smoke from any orifice. Water vapor from the pipes or elsewhere should dry up in short order, and not continue on.

Even though it only has 15,000 miles, as I've mentioned many times, its no longer an issue of low mileage with these bikes. Sheer age, the passage of time, is taking its toll. In particular the rubber parts get hard from age, outgassing the stuff that keeps them soft.

I'm speaking of the valve guide seals (tip of hat to Markcb750). They can be hard and leaking, even though the motor feels and in fact is healthy otherwise.

So, catch can it is, till you can afford to take it apart and put an 836cc kit in. And new guide seals. 

[jay_m]

Cool, thanks for the advice. So this isn't necessarily a symptom of immanent (or otherwise approaching) disaster?

[andy750]

Jay Ill compare my K2 overflow pipe gasses to yours (sitting outside work just now) so you can have a Boston control bike. Plus I have a new head with new seals etc etc...(and havent seen any residue on my new rear tire) so absence of gases may be indicative of your engine. However even if you do have worn seals Id ride it without worry until you see oil dripping out of your exhaust. This happened to me....valve oil seal on #3 went, pipe went cold, plug fouled with oil and no smoke....just oil dripping from end of exhaust....unfortunately this was 1000 miles from home...fortunately the mighty CB750 will get you home on 3-cylinders...just not very fast (about 55 mph).

Ill update later after I ride home....

Andy

[MCRider]

Cool, thanks for the advice. So this isn't necessarily a symptom of immanent (or otherwise approaching) disaster?

IMO...nope.