Smoke from crankcase breather

Alright, after lots of reading and mixed answers, time to ask the question myself.

Just recently picked up an 85 Maxim X 750. The bike has pods and instad of a breather tube from the crankcase going to the airbox, there's a fancy looking breather filter on it.

I noticed the other day back from a ride that there was some smoke coming out of it - probably equivalent to a cigarette being held down there. Not high pressure and not scary volumes... but smoke nonetheless.

I've checked the petcock functionality to make sure I wasn't dumping gas into the oil and I've done 2 oil changes without a significant increase (or decrease) in oil quantity.

I can't seem to find a definitive answer on this... My engineering background tells me smoke is bad but on the other hand, there's no sign of oil consumption and no oil coming up through the breather. Plus, normally this would be piped into the airbox and I would never know.

Should I be concerned!?

 

A compression test would be a good start.

 

Huh... Yeah, only possible source I can imagine would be smoke exiting out a cylinder into the crankcase?

 

There will be some blow by of the piston rings - that's why the tube is normally routed to the air box so the fumes can be burned. Some "smoke" is normal.

As stated, compression test is the best way to know for sure.

 

Right on... I'll do a compression test one of these days and check.

I'm around 36 000 km and bike pulls good... I would have expected oil to be splashing out of the breather and blue smoke if there was serious wear, but maybe my assumptions are incorrect.

Thanks for the help, and I'll throw up an update when I know more.

 

Howdy Mikeyman, I think you have mistaken "oil vapor" for smoke. As these engines run and heat up they can cause the oil in the crankcase to give off this vapor, it's normal and harmless if I go by your description. It's also normally sucked into the air box.

 

When I just turn the bike off I can see smoke out the oil filler hole if I take the cap off. Just a little bit, like you describe. Since the oil filler hole and the breather tube connect to the same space, I'd expect smoke out of your little filter, too.

 

mikeyman, I literally have the exact same setup as you. Pods on the carb, with a fancy filter and tube going off of the crankcase ventilation opening. I must admit that my bike does the exact same thing as yours, and I even have pictures and a video I might upload later.

My bike only has 18,000 miles on it, so I also figured it wasn't a matter of engine wear. I'll pull the plugs sometime and do a compression test to make sure. But I do have one comment:

I wonder if this has anything to do with the type of oil I am using. I am using a synthetic blend, and I can't remember the viscosity, but I'll post that later.

I wonder if switching to a thicker oil would do anything. Also, I plan on going full synthetic eventually.

Cheers.

 

Different oil won't cause smoke from the crankcase.

Viscosity won't be the issue, but the composition of the oil would be.
Wet-clutch motorcycles need to use an oil that is JASO MA certified (or at least compliant).
Check the oil thread in my signaure for more information.

On to the smole out of the breather. You were asked about blow-by. This is a possible cause of your symptoms. Do a compression test with a good compression gauge. Do one dry test, if the compression is on the low side do another with a tablespoon of oil in each cylinder and see if the compression rises.

The other thing to note is that a small amount of oil does vaporize in the crankcase. That mist will get pushed out of the breather and tends to look like smoke. This is a normal condition and would be remedied by fiting a longer breather hose so that the oil mist has time to condense and run back down into the crankcase.

 

Thanks k-moe for the quick response. It's nice to be able to hear from a guru. I'll keep your suggestions in mind and I'll see if I can do a compression test soon. Just got the bike about three weeks ago and I've already driven it about 1000 miles. I'm loving it so far.

 

Your water level staying up? Small water leak into crankcase will cause smoke or steam. Oil not chocolate milkshake colour I hope.

 

I have the exact same set up an exact same symptoms on my 82 XJ650. Just completed a compression test and all cylinders are within spec. I could imagine your bike would be the same. Like everyone else says though, I still recommend a compression test. It only takes about five minutes.

 

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