Topic: 1978 CB550K breather question

[Johnie]

Who has an original unmolested 78 CB550K? I need to know where the breather tube connects from the valve cover. I see some vented to open air, some to the nipple on the airbox next to the battery, some to the air plenum behind carb #1? Can you guys who have this bike help me out? My Honda service manual seems to show it to the airbox nipple next to the battery. Then this bulletin below shows it to open air...I am confused and want this to be correct the first time. As a side note, I have a feeling the PO vented it to open air as the hose is about 10 inches to long when I vent it to the nipple in the air box by the battery side.

http://manuals.sohc4.net/cb500/service_manual/HSM500550_15.pdf

Engine #2100717
Frame #2100707
Build date 6-77.

[TwoTired]

One tube mounts top of engine to side of air breather.
Bottom air box fitting has reduced diameter tube which routes to bottom of bike. It is a drain and has a pinch slit in the end, to block air ingress.
The air plenum fitting also is a drain and routes to bottom of bike.  It was also a reduced diameter (to 1/8") has some open cell foam stuffed into the end.

The 76 supplement show a regression in design, reverting to pre-550 configuration.  I never saw a 76 configured this way, and was the only year, AFAIK. My F model had all the routing details of the rest of my unmodified CB550s, prior and after 1976.

Cheers,

[Bankerdanny]

On my '77, which is the same as your '78 the tube ran from the output in the valve cover, between the exhaust manifolds, under the carbs to the bottom of the air box. There is a metal tube that comes out the bottom that it attaches to.

[Johnie]

Thanks TT. Banker Danny - my air box does not have the metal tube out the bottom of the air box. I only have this side nipple in the pic below. Looks like that is where it goes.

[TwoTired]

On my '77, which is the same as your '78 the tube ran from the output in the valve cover, between the exhaust manifolds, under the carbs to the bottom of the air box. There is a metal tube that comes out the bottom that it attaches to.

Sorry Danny, that's just not correct.  One: the tube routes between intake manifolds to the bottom *side* of the box.  Two: the very bottom of the Airbox is a condensate drain.

Check out Chapter 8 of the Honda Shop manual.  Shows the connections for proper air flow.  It does not show the pinch tube detail for the drain.  Or the air plenum safety drain with the foam stuffed in the end.  The pinch tube is shown in chapter 16 fig. 2-14, but it doesn't show connection there.

Cheers,

[Bankerdanny]

As usual you are correct TT. What I am picturing is the earlier airbox I have in my parts bin.

[Gene]

At least we have an answer. Thanks for bringing this up.

[Fezzler]

On my '77, which is the same as your '78 the tube ran from the output in the valve cover, between the exhaust manifolds, under the carbs to the bottom of the air box. There is a metal tube that comes out the bottom that it attaches to.

Sorry Danny, that's just not correct.  One: the tube routes between intake manifolds to the bottom *side* of the box.  Two: the very bottom of the Airbox is a condensate drain.

Check out Chapter 8 of the Honda Shop manual.  Shows the connections for proper air flow.  It does not show the pinch tube detail for the drain.  Or the air plenum safety drain with the foam stuffed in the end.  The pinch tube is shown in chapter 16 fig. 2-14, but it doesn't show connection there.

Cheers,

This is how I connected my 78 and plan to keep it that way.  Did I see in Addendum however they added the option to just let the hose drain to the ground and not through the bottom of the air box?  Wonder why.

[TwoTired]

Did I see in Addendum however they added the option to just let the hose drain to the ground and not through the bottom of the air box?  Wonder why.

Yes, the Chapter 17 supplement shows the straight dump to atmosphere for the 76 K2 models.

I think Honda was trying to cheat on the tail pipe emissions test to reduce hydrocarbon output out the exhaust, as the test only measured tail pipe emissions.  The K2 routing dumped crankcase hydrocarbons out to atmosphere before the they could be sniffed in the tail pipe.  Must have been on the hairy edge of acceptance.

The K model got PD carbs and new pipes in 77 (K3), as well some small changes to the air box intake horn to further refine emissions.  The F model configuration, even with it's high pressure exhaust muffler, was discontinued after 77 model year.  I don't think the older carbs still used on 77 F model could be tuned in under the new emissions standards.

I speculate there was an import restriction to the US to restore the air breather function, once the EPA noticed the change.  Both my 78 CB550K and the 77 CB550F had the air breather routing same as my 74 CB550K, matching the book specs.  I know for certain these 3 bikes were in stock configuration when acquired as I knew their original owners.

I never found a factory configured 76 CB550K that had the crankcase vapors dumped directly to the atmosphere.  Though, they could exist, I suppose.  I certainly didn't see them all.

Cheers,

[Johnie]

Hey TT...any chance you have an extra plenum drain hose and extra air box drain hose? This bike came with no plenum drain hose with the foam. The air box drain hose broke off at the reduced diameter.

[TwoTired]

Hey TT...any chance you have an extra plenum drain hose and extra air box drain hose? This bike came with no plenum drain hose with the foam. The air box drain hose broke off at the reduced diameter.

I wish.  None of my drain hoses have lasted more than 15 yrs.  I have to make replacements using the old ones as pattern.  The bottom pinch tube cap can be reused on the made up hose for the air box drain if you select the right tube diameter I.D. and O.D.  If you don't have the cap, you can get a vinyl cap from the hardware store and cut a slit in it with a razor saw.

The plenum drain gets nested tubes at the end to attain equivalent diameter to original (~1/8").  And then some open cell foam to stop bug and dirt ingress, but still keep the plenum from filling with gas during a carb malfunction.

Cheers,

[Johnie]

Again thanks for the help TT. I think I can make some up here.

[strynboen]

for mee it looks like the konds drain is in the bottom..and the 90 studs is the korrekt place to konnekt the breather 12 mm line to pos 23
breather line is 12mm#420 mm

my data is for a 78 k3

[Bankerdanny]

I need to go through my pictures of my '77 that are on my home computer to see if I took a shot of the airbox in place. I recall a metal thing in the bottom filled with crumbling foam.

[TwoTired]

I need to go through my pictures of my '77 that are on my home computer to see if I took a shot of the airbox in place. I recall a metal thing in the bottom filled with crumbling foam.

You mean inside the filter box at the bottom, right? That's the separator.  The shop manual shows that doesn't it?
I'm sure the parts diagram does.

Cheers,