SU-MO-KIN!

[jamman18]

Noticed that in town riding my 1977 cb750 runs fine but if I take it on the highway in 5th, it smokes a lot.  Is this something that will cause damage or just result in fouled plugs and crappy gas mileage?

[shultz_1978]

Has it always been like that are ju8st started? may be oil ring or rings. Could even have some thing to do with the oil value seals.

Is coming from the tail pipes? 1,2,3,or 4 or all four ?

[jamman18]

4 in 1 exhaust and yes it smokes out of it .  Damaging or just a nuisance?

[shinyribs]

I'd check your crankcase vents/breathers before I got too worried about it. It might be something small like that.

[jamman18]

I'm not familiar with those;  details on location and what to look for ?

[shultz_1978]

One on the back of the valve cover just above 2-3 carbs. Other is on the back of case right in front of the back wheel. 

[jamman18]

Thanks!  What is their function and how would they relte to the smoke?

[LukeM]

The breathers take the vapors inside the engine/crankcase, and route them through the intake (airbox and carbs).  Being that there's an oil component in those vapors, it's possible that may be the source of the smoke.  It's like the guys who run LARGE amounts of SeaFoam through their engines and turn them into mosquito foggers.  In your case, if the smoke is fairly light, I'd say don't worry about it.  If it only happens with heavy acceleration or going up a hill, then you may be getting some oil seepage past your valve guides.  If it smokes all the time and fouls your spark plugs (like my old '72 Cutlass V-8 used to do), it's probably rings or scored cylinders.

Maybe a video would help the diagnosis.
Luke M

[jamman18]

It does mainly happen on higher speeds like 65, not around town.  I would take a video but I might crash when I am turned around at 65.

[shinyribs]

jamman18 wrote

Thanks!  What is their function and how would they relte to the smoke?

 What Luke said. Also,if they are plugged off and not working at all the crankcase pressure can build up and will create blow-by past the rings= smoke out the tail pipe.

[jamman18]

The one in front of the rear tire has a tube that is routed back and downward and has a plug on the end....do I need to get rid of the plug?

[shinyribs]

That vent on the rear should be connected to the vent tube of the oil tank. But it definitely shouldn't be plugged.

[jamman18]

Did a bit more observing this am and I would say at idle you can slightly make this smoke out but then if you rev the engine, it increases quite a bit to where you can see it for sure.  The color is white as far as my eyes can tell.....   Seems to be something that is happening not just under load at high speed on a hill but just revving it up as well.

[jamman18]

Also, there seems to be a build up of oily residue on the inside end of the exhaust pipe if that means anything

[shinyribs]

Textbook blowby. Either the crankcase is not breathing properly or the rings are getting worn.

[jamman18]

45000 on the engine at this point.....

[jamman18]

If the rings are worn will it cause damage or just tick people off that are behind me?

[jamman18]

Today it's not smoking at all........no rhyme or reason to this.  Anyone ever use Restore oil treatment for this problem?

[tcazes]

maybe try a little marvel mystery oil in the gas as well as take the plugs and put some in there... it will smoke like hell at first but it should go away. that will help your rings out if they are possibly just dirty and maybe stuck

[jamman18]

Read that running hotter plugs would help as well