78 kz400 crankcase breather question

Just a little info about the bik. It's a 1978 KZ400 B. The bike came with pods and straight pipes. It's run pretty good for the most part except some mid rev bogging and idle issues. When I bought the bike, I decided to change the oil because I didn't know when the previous owner had changed it before. I used Kawasaki specific 5w-30 oild and filter. For a while I noticed the there were little spurts of oil on my exhaust pipe. Seeing that there was a hose coming from the crankcase breather, I assumed it was coming from that. So I shortened the hose a little and added a breather filter to it. That was about 200 miles into owning the bike. Today, I looked under the bike and had seen a little oil spot. Then I noticed that oil was caked on almost everything. The swing arm, the center stand, the inner fender... So I turned the bike on and let it warm up a bit and watched the sight glass fill up. Then it stated to go low, real low. While the bike was running, I looked at my breather filter and oil is spurting out of it, like it's full.

I've only put a little over 400 miles on the bike. I live in Ohio and the temperatures have been anywhere from as low as mid 50's to high 90's. Most of the miles have been ridden at night when it cooler out.

Any idea as to why my crankcase breather is spewing oil?

how are you checking oil level?
side stand or on center stand?
sounds like its too full

Observing the oil level when the engine is running will only show if oil is circulating; you cannot at all judge the oil level with the engine running. To check the oil level let the bike sit on the centerstand for an extended period so all the oil has plenty of time to flow back into the oil pan. Then make sure the oil level is between the two lines on the oil site glass - near the top line is good, but over the top line is too much oil. Don't even try to check oil with bike on side stand or engine running as you will get the wrong level every time. Ed

650ed wrote: Observing the oil level when the engine is running will only show if oil is circulating; you cannot at all judge the oil level with the engine running. To check the oil level let the bike sit on the centerstand for an extended period so all the oil has plenty of time to flow back into the oil pan. Then make sure the oil level is between the two lines on the oil site glass - near the top line is good, but over the top line is too much oil. Don't even try to check oil with bike on side stand or engine running as you will get the wrong level every time. Ed

I did have it on the center stand, but I was checking the oil with the bike running. I did just check it and it's about half way on the glass.

P21, I was thinking I over filled it too, but when I did the change, I put 3qts in which it's require 2.9L =3qt

If the oil is about halfway between the marks it shouldn't be blowing oil out of the breather. You may want to perform a compression test and a leakdown test to determine if the piston rings are letting cylinder pressure leak past them into the crankcase. Ed

[strike]Did you replace the oil filter when you did the oil change?[/strike] never mind my mistake

Is there a reason for using 5W-30?

martin_csr wrote: [strike]Did you replace the oil filter when you did the oil change?[/strike] never mind my mistake

Is there a reason for using 5W-30?

You know what? Now that you asked that, I'm wondering "Did I use 5w-30?" because I'm pretty sure I used 10w-40 like my oil cap says to use. But now I'm questioning myself because I don't know why I would have said 5w-30.

650ed wrote: If the oil is about halfway between the marks it shouldn't be blowing oil out of the breather. You may want to perform a compression test and a leakdown test to determine if the piston rings are letting cylinder pressure leak past them into the crankcase. Ed

That's what someone else had suggested I do. Thanks for the help!

napalm wrote: ... Seeing that there was a hose coming from the crankcase breather, I assumed it was coming from that. So I shortened the hose a little and added a breather filter to it. That was about 200 miles into owning the bike. Today, I looked under the bike and had seen a little oil spot. Then I noticed that oil was caked on almost everything. The swing arm, the center stand, the inner fender... While the bike was running, I looked at my breather filter and oil is spurting out of it....

Would toss the breather filter that was added to the crankcase vent hose. And not replace it. It serves no useful purpose, and has since clogged with oil which is disallowing required free breathing of the crankcase.

Route the crankcase vent hose -- without any filter -- so that it exits to open air underneath the bike, preferably in a location that won't allow any escaping oil mist onto the rear tire.

The crank case is vented to "exhale" any pressure build-up due to blow-by into the crankcase.
The crankcase only exhales, and doesn't "inhale" through the vent.

I suppose a fresh new medicated filter might help prevent any bad germs inside the crankcase from being expelled.

Good Fortune!

Patton, your GB500 inhales through the vent every time the piston goes up, but you're right, as per usual, regarding the twin. They have 180 cranks so that every time one piston goes up, the other is going down keeping the crankcase "neutral" relative to atmospheric pressure given perfectly sealed rings. Paul

slayer61 wrote: Patton, your GB500 inhales through the vent every time the piston goes up, but you're right, as per usual, regarding the twin. They have 180 cranks so that every time one piston goes up, the other is going down keeping the crankcase "neutral" relative to atmospheric pressure given perfectly sealed rings. Paul

The KZ400 twin has a 360 crank.
But even if it was a 180, or even an inline four, upward piston's motions don't equal downward piston's motions. That's why 180 crank twins and inline-fours don't cancel vibrations. The pistons move much slower at the bottom of the stroke. It's the result of being connected by a connecting rod. If pistones were somehow connected directly to the crank, then their up-down motions would cancel.

Am uncertain of the GB500's factory routing of the crankcase vent hose, or the extent to which it also inhales into the crankcase. But guessing -- perhaps incorrectly -- that the hose might exit into the air cleaner.

The FSM addresses removing the "plug" from the crankcase drain tube to empty any deposits from the tube.





Am thinking the crankcase drain tube serves only "collection" and "drain" functions, and not "venting" function.
In other words, there is no breathing through the crankcase drain tube.

Am more familiar with Kawasaki fours where factory configuration includes a short hose between the crankcase breather and the air box.
And where an incidental advantage is slight suction from the air box to help draw from the crankcase.

Many British twins came from the factory with a hose running from the crankcase vent and along the fender to exit near the rear of the bike. I don't know if the design was later changed with models having an air box.

Good Fortune!