78 kz400 crankcase breather question

loudhvx wrote:

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.

Well, I've been corrected again :blush:
Sounds like I should keep quiet and let the more knowledgeable folks respond. Loudhvx, do you mean to tell me that both the pistons on these little twins move up and down at the same time? :huh: Seems like it would shake itself to death! or does it have a wee counter balancer?

Regarding the GB500, I just figured that every time the piston goes up, it would naturally create a vacuum in the crankcase and that if there is a vent on the crankcase, it would naturally "inhale" because there isn't another corresponding piston going down to make up the difference. That's the way my Yamaha single operates any ways!
Paul

Patton wrote:

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!

It was that way before I put the filter on it. But it still doesn't explain why it seems like there's so much oil coming from it.

As suggested by 650ed, 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.

A leakdown test injects air pressure through the spark plug hole to pressurize the combustion chamber.

Can perform a poor man's leak down test by introducing compressed air into a spark plug hole and listening to where it escapes from the combustion clamber. This may be done at TDC or any other cam positions where both valves are fully closed. Top dead center (TDC) is when the piston is at its highest position. An easy way to tell TDC is by alignment of the T mark with the case mark when viewed through the timing window.

May introduce a spurt of compressed air by using a rubber cone-tip blow gun (rubber air nozzle) held into the spark plug hole. And of course keep holding it in position to keep air from coming back out the spark plug hole while listening for escaping air at other places. The air compressor should not be running while listening because the noise will likely drown out any sound of escaping air.

May alternatively use a portable compressed air tank.

Air heard escaping from exhaust port indicates exhaust valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from carb intake indicates intake valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from crankcase breather indicates loss of compression past rings into crankcase (likely due to worn piston rings and/or cylinders).

Air heard escaping from head gasket area indicates indicates loss of compression past head gasket (perhaps due to a blown head gasket).

A leaking valve may sometimes be resolved by adjusting the clearance to within specs.

A leaking head gasket may sometimes be resolved by torquing the head fasteners when the engine is stone cold (such as after sitting overnight).

Excessive blow-by, usually accompanied by poor compression, usually indicates need for a top end overhaul.

Good Fortune!

Thanks. I guess I now know what I'm doing with my weekend. Haha...

slayer61 wrote:

loudhvx wrote:

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.

Well, I've been corrected again :blush:
Sounds like I should keep quiet and let the more knowledgeable folks respond. Loudhvx, do you mean to tell me that both the pistons on these little twins move up and down at the same time? :huh: Seems like it would shake itself to death! or does it have a wee counter balancer?

Regarding the GB500, I just figured that every time the piston goes up, it would naturally create a vacuum in the crankcase and that if there is a vent on the crankcase, it would naturally "inhale" because there isn't another corresponding piston going down to make up the difference. That's the way my Yamaha single operates any ways!
Paul

Yes, both go up and down together, as they do on the 440 twin and 750 twin as well. And yes, dual counterbalance shafts. The kz 400 twins are noted for their smoothness over Honda CB/CL 350/360 twins. (Plus the simplicity of only dealing with one ignition system.)

But 180 crank twins and inline-4's don't cancel their air movement either because the piston moves much faster near the top of its movement compared to the bottom of its movement. Inline triples and sixes are the smooth motors. The inline sixes are often smoother than V-8's. I don't know about their air displacement in the crankcase, though.

But really, because of blowby, air travel in the crankcase is almost always outward (on my inline 4's and twins I've seen). I use a long hose so if there is any inward suction, it will only take in what had just come out. But it seems to only breath outward for the most part.

In napalm's case, I think Ed's suggestion of compression test should be his first priority, (as he says).