Topic: smoke and oil comming out of oil tank breather

[simmo400]

Hi all

well I'm a newbi and have a 1971 cb750 in my garage. Probelm is that excessive oil comes out of the oil tank breather tube enough to have a bottle rigged up to catch it!!! Also there is excessive smoke and some oil comming from the top rocker breather tube that is routed down near the back wheel. Sitting at   traffic lights it looks as if you are attempting a burnout!! I am assuming that there is a hell of a lot of pressure in the crankcase comming from somewhere. Does anyone have some suggestions? fixes? before I open up the motor? If I do what am I looking for? The motor was only rebuilt 18,000 klm ago  so I cannot see why the engine would be worn out.

Any suggestions, opinions would be appreciated

Simmo400   

[gerhed]

On occassion my top rocker breather smokes enough that I originally thought I was having an ignition fire !
I try to ignore it now.

[crazypj]

Engine re-built 18,000Kms.
Did you have it bored?
How much rpm do you use?
On  bored 550 (591 conversion) I found new rings needed every 10,000 miles max but that was operating around 8,000 rpm or more most of the time (about 16, 250Km)
 If you've been too gentle rings/cylinder may be glazed and need some heavier loadings to fix
PJ

[simmo400]

Thanks for your comments and suggestions. Bike is used very little and to be honest I am not sure if it was rebored as the reciept I have from previous owner only shows new rings! I think I will have to bite the bullet and strip it down. So am I right in thinking that this problem is blowby from worn rings?

Simmo400

[medic09]

It seems too simple, but how much oil do you have in the bike total?  Dry sump bikes (like the our Hondas, or my Yamaha SR500) will dump oil out the breather when they're overfilled.  Typically this will happen when they've wamed up and are running at speed.  Scared the hell out of me first time I saw it.  Check your oil just to be sure it's not that simple...

[simmo400]

Thanks for that but I am sure the oil level is fine, however I will double check because yes some things are that simple. Despite this a great deal of smoke comes out of the rocker breather as well which to me indicates something in the motor.

[scondon]

    If you were blowing a lot of smoke from your exhaust I would think that a teardown might be a good idea. I would think that valve or ring wear would have to be so bad to cause what you describe that the bike would run very poorly and foul plugs in very short order.

     Smoke from the rocker breather could just be contaminated oil or semi dried oil burning off if the bike is not used for long periods. It gets pretty hot in the rocker area and anything that is not clean free-flowing oil, which is constantly refreshed, will start to smoke. If any water or moisture gets in there it can turn the oil to sludge and you will smoke quite a bit.

   Oil level should be checked after several minutes of running the motor. Shut off the bike and put it on the centerstand before checking measure stick. When the bike sits for a while oil can drain from the tank into the engine giving the appearance that oil level is low. If you were to fill the oil on a "cold" bike, chances are you will overfill it. Too much oil and it will spit out the breather as medic points out.


    If you have not personally changed the oil and filter on the bike then I would definitely do that first before considering tearing the engine down. Take it for a good 10-20Km ride to get things good and hot, then change oil and filter and ride a bit more. Or, if you are absolutely sure that the oil level is fine and the oil is clean and uncontaminated then take it for a good long ride as it might just be blowing smoke 'cause it's peeved for being made to sit unused for a long period.