Topic: Blue smoke - crankcase breather?

[jawntybull]

An interesting puzzle...

On my last couple of rides, fellow riders have pointed out blue smoke is coming from my exhaust particularly under load. I've had the bike a couple of months and have been working through the problems. It runs strongly and compression is good (150psi).

I had a good look this morning. No smoke on cold start, then after a few mins the smoke started, particularly as I held the throttle open at around 3000rpm and above. The garage was getting a bit hard to see in...

Came back an hour later once the air had cleared, started her up still warm, ran her up to 3000rpm, but now no smoke?

When I go back through the last couple of weeks and look at what I've done, the list includes forks etc and the tacho oil seal. I wonder if the crankcase is pressurising and now that I have fixed the tacho seal there's nowhere for the pressure to go but past the rings or valve seals? When I shut the engine down I heard a hissing sound; there on the front of the engine I can see oil and air bubbling out from behind a tappet cover for a minute or so. Que!

So I check out the crankcase breather hose; it appears to run unobstructed into the seperator, but it runs into the top not the side fitting. My manual shows the crankcase hose going into the side fitting on the seperator and the airbox hose coming out the top. Does anyone know if there is a valve in between these two fittings? If so, having the hoses reverse piped would block the pressure coming out of the crankcase. I plan to pull the seperator out on the weekend but am interested to know if I'm on the right track...

[Kickstart]

I just checked two of my 75 CB750F's (as best I could see)... and the hose from the top of the valve cover is piped into the side of the separator, and the hose to the air-box comes off the top.

I took one of these apart over 10 years ago, as I recall it's not really a valve but a spring pushing down on a foam filter element (maybe that makes it a valve?)

Also, I had a similar oil/smoke problem on a 1990 ZX6, and it was because I put too much oil in.  Under high RPMs or heavy load, the oil pressure (or pressure in the case?) increased enough to push oil past the rings - causing smoke.  Our bikes have a separate oil tank, so maybe this wouldn't work the same way... but if changing the hoses to the separator doesn't work, I would make check to make sure you don't have too much oil in the bike.

[jawntybull]

Thanks Kickstart, just pulled the seperator out and there's no valve; lots of gunky stuff in there though. I thought the hose itself might be blocked so I blew air into it, pressurising the engine, and managed to spray myself with aforementioned gunk as I took my mouth away - ugh!!!

I then took a tappet cover off and blew air both ways through the breather hose, so it seems I'm barking up the wrong tree. I did more revving etc and it seems the smoke eventually appears if you hold the throttle open at 3-4000rpm for long enough, then it blows as a steady stream. Rings? Strange given the compression test came up clear though.

[Kickstart]

Yeah, I would think with your compression it wouldn't be the rings either.

I would check to see if you have too much oil.... maybe drain all the oil (both drain plugs), see if more than 4 qts comes out.

Maybe the valve seals (valve stem seals?)... I'm really out of my league here... but I thought I read something somewhere about bad seals on the intake valves allowing oil to be sucked into the engine.

[jawntybull]

I know the oil's ok as I replaced it recently and measured the amount I put in. The bike's been using oil but I put that down to the leaking tacho cable; on one ride it managed to spray all the way back onto the airbox. Perhaps its valve seals as you say; I believe they are a known problem on these bikes and I don't know if they have ever been replaced. 35 years is a long time for a rubber seal to hold  I'm out of my depth a bit too but it makes sense that as the revs go up the vibration and motion around the seals may lead to oil getting through.

[jawntybull]

A bit more testing this morning and whilst there is a bit of smoke at higher revs, its much less than before. Oil leaks have stopped from the valve cover too; no more bubbling air/oil mix coming out. Perhaps the crankcase breather was blocked (recall the gunky stuff that popped out pretty explosively when I pressurised the hose end with my mouth  ).

I guess the valve seals are worn but not too badly, and the blocked breather hose was pressurising the crankcase and valve cover, forcing oil through the seals and making the problem worse. Does this sound right? Anyone?