Topic: Why is it blowing white smoke?

[78CB750F1]

I cannot figure out what the source of the excess smoke is - CB750 F1 stock motor with Keihin 29mm CRs and Kerker header/exhaust.  Runs perfectly but after I run it above about 5000 rpm, it starts blowing a LARGE amount of white smoke out of the exhaust.  After this point it keeps on smoking a lot, even at idle.  I think that the carbs are set too rich (there is an oily deposit at the end of the exhaust) and it uses a lot of oil - would this be the issue or is it something else?

[scondon]

Sounds more likely that your valves or rings are not sealing. Have you done a compression check?

[cafe75-550]

Sounds more likely that your valves or rings are not sealing. Have you done a compression check?

I'll second that. White smoke + using lots of oil usually means that its burning that oil, which would also account for the smoke. A compression check would help you narrow it down some, but if its burning that much oil its probably the rings.

[TwoTired]

What kind of oil are you using?  Synthetic?
Oil burners usually have smoke with a blue-ish tinge to it.  White smoke is usually water vapor.  Black smoke is unburned hydrocarbons/soot/fuel.

Your symptoms do sound like a broken piston ring (s), though, especially the oily deposits in the exhaust.

Compression check time.  Perhaps a bit of crying...

On second thought, check your engine breather system first.  It needs to vent to the atmosphere somewhere.  Over 5000 RPM is where blowby is likely to occur, which would pressurize the crankcase.  This would upset the piston ring seal, among other things.

Cheers,

[78CB750F1]

Thanks for the answers.  I haven't done a compression check.  What would low compression on a cylinder indicate?  Does that mean a broken piston ring?  TwoTired - I am using semi-synthetic oil and the breather line is directly to the air without a filter.

It's such a disappointment after all the work that I have put into the bike.  Can I run it like this for a while or will it get worse?  Anyone know someone in Barcelona, Spain who can rebuild my engine?   

[OldSchool_IsCool]

White smoke => water.  But where could such a large volumn of water come from to produce "a LAGRE amount of smoke"??

Contaminated oil, maybe? water vapor wafting up past the rings?

[morseji1]

I was behind a diesel truck one time when something in the engine went, and it started blowing a lot of dense white smoke out of the stack. However, by the smell it was obviously oil in spite of the fact it was very white. Steam is pretty easy to tell from smoke, so I imagine it is smoke. If you are burning a lot of oil and blowing white smoke, you need to find out what the problem is before you ride anymore. A compression test is the first step.

[bryanj]

You can get white smoke from oil burning if nearly all of it is burnt in a good running engine, usually means the oil scraper ring is tired

[78CB750F1]

Thanks for the replies - I will check the compression and see what that shows.

Strangely I rode it again yesterday and it hardly smoked at all - it was a good 40min ride and I was pushing it above 5000rpm many times.

[liaudio]

Sounds like your headgasket.  It's probably letting coolant into the combustion chamber.

[Steve F]

On second thought, check your engine breather system first.  It needs to vent to the atmosphere somewhere.  Over 5000 RPM is where blowby is likely to occur, which would pressurize the crankcase.  This would upset the piston ring seal, among other things.

I was thinking the same thing about the breather being clogged.  Check the canister under the battery where all the hoses come together, and remove, clean out and replace the "filter" element to see if that clears things up.  If not, then the compression check.

[OldSchool_IsCool]

Thanks for the replies - I will check the compression and see what that shows.

Strangely I rode it again yesterday and it hardly smoked at all - it was a good 40min ride and I was pushing it above 5000rpm many times.

If the smoke is gone AND the oil is gone, then there may be a direct relation between the two!  If the oil is still there, then maybe, just maybe, your exhaust was full of water and the "smoke" was actually steam.  Is it possible that your exhaust could have been flooded??  Seems like a long shot, but maybe.

Do the compression test anyway.  If nothing else, you will have a baseline to compare future compression tests to.