Topic: White smoke after warm-up

[78CB750F1]

I have been riding my recently completed CB750 F1 cafe racer around a lot recently for longer and longer distances.  The past few rides I have noticed that after about 20 minutes stop-and-go riding, I get a lot of white smoke from the exhaust.  It only starts at this point and it is most noticeable at idle when a cloud starts developing around me!  My CB750 has a stock engine with Keihin CR carbs and Kerker exhaust - recent changes that may have an impact is an oil change and spin-on filter conversion (1 week ago).  Thank for any help or suggestions.

[sparty]

What does the smoke smell like?  My bike smoked a little after my 836cc rebuild, but it was determined to be the anti-seize compound burning off of the exhaust bolts.  It has a distinct odor.

Sparty

[78CB750F1]

Sparty - I don't know what it smells like!  I will have to check.  I don't think it is anti-seize - there is a fair amount of smoke. 

[sparty]

Have you checked your exhaust gaskets?  Did you replace them with the build?  You may have a leak if you didn't.

Sparty

[dusterdude]

if its a true white smoke,its water,condensation

[78CB750F1]

Sparty - I didn't replace the exhaust gaskets because I couldn't get the old ones out!  I tried everything but they were wedged tight in there after 30 years.  I didn't see any leaks after I installed the Kerker so I assumed they were okay.  If it was the gaskets leaking, wouldn't it be smoke coming out of the header connections?  I have white smoke from the exhaust that hangs in a cloud around the bike when I am stopped at a light.  I almost asphyxiated an old gentleman on his bicycle.  And it only happens after about 20 min of running.

Dusterdude - I didn't think about condensation.  How do I tell if the white smoke is condensation or something else?  Do I really have to go and smell it...

Could it be burning excess oil?  Is white smoke a sign of running too rich?

[cafe75-550]

Black smoke is a sign of running rich, white smoke could be a couple of things. One being condensation, the other being oil... If its oil, the "white" smoke will probably be closer to grey/bluish, and will smell of course like burning oil.

If the exhaust gaskets were bad you probably would see this smoke fron the header connection.

Have you done a compression check recently? This could be a sign of your rings needing replacement...or perhaps your valve guides/seals...

[sparty]

The exhaust gaskets could be removed fairly easily even if they are 30 + years old with a flat head screw driver.  They are a soft copper.  Mine didn't want to come out, but I split them with the screw driver and they pealed out after some persuasion.  I agree that you might be burning some oil as the valves, over 30 year, do wear.  I would guess that you had smoke prior to the replacement of the exhaust, but it exited through your mufflers, now that you no longer have a tight seal at the header, you are seeing the smoke.

I would try to replace the exhaust gaskets again; they will cost you about 3 bucks a piece.  It is always recommended that you replace the exhaust gaskets even if you just pull of the headers to remove the engine, etc.  Once they “crush” you will never get a true seal again.

Did you re-torque the header after you brought it up to temperature?  It is recommended that after you install new headers, run the bike, get it up to operating temps, and then once cool again; re-torque the header bolts to recommended specks. 

I did this on my bike, and you could not believe how much more the bolts needed tightening.

I also recommend that you coat the bolt threads with anti-seize, it will make future removal a hassle free job.

Most of these sohc4s burn some oil, it shouldn’t be a big problem, but a valve job is in your future.  My current engine burned some oil prior to the new valves.

Sparty

[Andrew Foss]

if somehow you got water in your oil (possibly while cleaning up the engine) it could be coming from your crankcase breather. check to see that your oil is clean. if theres water, it will either look brown and sludgey, or will have white gunk on top.
-andrew

[78CB750F1]

Thanks guys - I will check all the areas you have suggested and try running it again.  If it is a more serious engine issue, is there anything I can do to reduce the problem without getting into an engine rebuild?  The engine has about 43,000 km on it.

[TLRam1]

Check your plugs to see what color they are. That may help determine what you are burning.

[Ibsen]

Also If the valve stem seals are leaking the engine will smoke at start up after it has been sitting for a while. And the smoke is usually white or whitish. And you can always see this from the colour of the spark plugs. One way to find out if these seals are leaking will be to ride the bike downhill and have a friend riding behind you. Close the trottle and brake the bike with the engine for a while, then open the trottle fully. If the valve stem seals are leaking, your friend will either see some white smoke or smell the smell of burning oil, or both, after you have opened the trottle.
If the smoke is condensation only, and you let the bike idle for a few minutes, you will see condensation water at the ends of, and on the ground just under the end of the mufflers.