Topic: Smoke from the breather hose

[IAmCitizenMe]

I noticed the other day as I was tinkering with my bike as I often do, that smoke was coming out of the air box when it was running. The tube that runs from the head to the airbox was blowing smoke. The smoke looks blue, so I'm guessing it's oil. But My oil level has stayed consistent ever since I changed it a while ago. I am considering getting pods and losing my dry-rotted airbox, so I was messing with the air system when I noticed the smoke. Any ideas? Is this normal?

Chris

[cookindaddy]

I'll take a shot at this. I think that it would be normal. The purpose of the tube from the head to the air box is to scavenge oil fumes and recycle them through the engine, much as a PCV (positive crankcase ventilation) system does on an auto engine. If your oil level is consistent, then all is good and your emissions are probably being lowered as a result. You don't say what bike or year you have but with the CB750, I think that this system was introduced in 1977. You might want to look at the layout of the rest of the system (in the first few pages of the Honda Service manual) and check the condensate drain hose from the little vacuum cyclone thingy under the battery box.

[IAmCitizenMe]

Thanks! That makes a lot of sense. I have a 78 550. So, if that's the case, when I put the pods on the carbs, what do I do with that hose?

[cookindaddy]

Well, that's the rub isn't it? You loose the scavenging action unless there is another way. If you plug the hose from the head, then case pressure can build up and potentially cause oil leaks. Perhaps others will have ideas for you. I cleaned out and restored the air box and the cyclone thingy and mine is as stock as I can make it.

[IAmCitizenMe]

I guess I'll just have to spill those fumes into the environment, then. haha.

[MCRider]

Thanks! That makes a lot of sense. I have a 78 550. So, if that's the case, when I put the pods on the carbs, what do I do with that hose?

Seeing as how prior models simply vented that hose to the atmosphere, you can do the same.

Now back to the smoke. While its true oil vapor vents out of there, it should not be visible in a well running engine. Maybe at night you could see a little in the headlights, parked at a stop light.

If you can see it in the daylight, then its probably too much. It would come from poor sealing valves both at the valve seat and / or the valve stem seal, pressurizing the cam box. Or if the piston rings are not sealing, pressure goes down into the crankcase and back up thru the cam chain tunnel and out that hose. Or a combination of all of the above.

If your oil level is maintaining OK, then its not a clear and present danger, and you can likely run it for many more miles, thousands probably. But you may want to start saving up for that 836cc kit.  

[cookindaddy]

Or you could check the compression?

[Sporkfly]

I have a '77 CB550, the airbox setups are identical. When I first had my bike running I didn't have this hose connected (oversight) and noticed a fair amount of smoke escaping. Since that point, I rebuilt the top end: honed cylinders, new rings, reground valves, valve seals, etc., checking specs on everything of course, and I've gone to pods (I was missing many parts for the airbox, couple of holes to boot, so I may be reworking it this winter to go back to the stock system). I have a filter on the end of the breather hose as it stands. The smoke escaping was reduced after addressing the top end, and here is what I found with my engine: it was a combination of old dry valve seals, poorly sealing valve seats and ring gaps out of spec. Compression before rebuild was around 115-130 (mind you with a fairly long hose) and after was 145, even with the long hose/adapter setup.

I'd say do a compression test and from there it might be your own personal judgment on whether to go in and check specs.