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Hey everyone, first post.

I have a problem that Ive been trying to diagnose for a couple weeks now.
I hope that theres someone here who can give me some solid, honest advice.

I recently obtained a 1984 Honda CB700sc Nighthawk S. It has 19000 on the clock. I replaced all the obvious components like battery, plugs, oil, filter, sealed tank, cleaned petcock. It starts up and runs smooth too.

My problem is this.....
... Whenever I start the bike, especially after its been sitting (say overnight) and even more so when Its colder outside, it releases a grayish white smoke from the exhaust. It almost smells like an old gokart or something. It will eventually dissipate, but i can still see faint smoke. Usually if its colder outside, it hardly goes away at all.
So what gives??

Is it bad valve stems? Bad head gasket? Cracked valve head? Bad piston rings?

Im really stumped, and I would like to fix this myself (even though ive never been inside of an engine but I learn fast! especially being a biochem major!). Id rather not get the shaft from the two local stealerships/mechanics.

IF anyone has any knowledge of or experience with this problem I would appreciate any input you may have. Thanks very much

It's normal for "white smoke" to appear out the exhaust in colder temps - there is a small amount of water present in gas that gets burned in the combustion process and is released as vapur, or white smoke, out the tailpipe. Other causes of white smoke are blown head gaskets on liquid cooled bikes - but as yours is air cooled, this obviously isn't the problem. Next time this happens, check the outlet of the muffler closely... if you see lots of water droplets coming out with the "smoke" then I'd suspect cheap too low octane of a fuel. Try a higher octane to see of the problem goes away.

White smoke can also be caused by unburnt fuel. If a spark plug is erratic in operation (ie: misfiring) it can fail to ignite the fuel/air mixture in the cylinder under compression forces and shoot it out the exhaust valve as white smoke. One other cause is the engine running too lean. As your bike is fairly old, I'd check the condition of the carb rubber intake boots. If they are cracked, this can allow air to get sucked in and lean out the mixture, causing white smoke. Another clue to a lean engine is that it will run very hot and the headers/exhaust pipes will turn a deep blue.

Hey thanks for the advice!

Yes, something i neglected to mention is that the tach will occasionally fluctuate 100-200rpm in a random fashion. And also the engine doesnt have a consistent "purr" it staggers sometimes (not drastically, but it is audible). I checked all 4 plugs and they seem to have consistent looking color to them which leads me to suspect this "misfiring" issue is caused either by bad wires, bad coils, or a bad CDI ignitor (i will narrow it down in that order.)

On the other point you made about the condensation...
I never see any vapor or condensation when i put my hand up to the exhaust, its actually pretty dry (from what i can tell that is) but im sure the increase in smoke during colder temps like you said is definetley condensation.

I will check the carb intake boots as well... The pipes also wont tell me anything about a lean mixture because they are blackish/grey and i dont think ill be able to see any blueish hue.

Thanks alot Mike, these are all factors i never even considered to be possibilities.

Deep down I just dont want it to be serious engine issues...

I appreciate the good advice! Thanks again.

Along Gummiente's train of thought, check your vacuum lines for cracks and leaks too. Usually a bike won't run smoothly if you have these kinds of leaks, but anyway. Did you see any oily residue or soot on the spark plugs, I mean the old ones you replaced? If not, then I doubt you're getting any oil blow-by or leaking past the valve guide seals.

Actually, the new plugs (and i think the replaced plugs were this way too) had some darkness to them. They were not completely black but there was some residue there.

how does it run at other rpm's/loads? smoke,bogs etc??? if its only at idle or just off idle. a carb sync may be just the cure.

at other rpms it runs great! not any smoke as far as i can tell/see and the engine isnt rough, just a rough idle with smoke coming out. I guess ill check all ignition components and carb components then take it from there. This bike rocks and I want it to be happy

Thanks for your advice.

fulcrum wrote:at other rpms it runs great! not any smoke as far as i can tell/see and the engine isnt rough, just a rough idle with smoke coming out. I guess ill check all ignition components and carb components then take it from there. This bike rocks and I want it to be happy

Thanks for your advice.

white smoke and rough idle is a good indication of out of sync carbs. the bike will run fine at mid to high rpms. so its a good,easy thing to check and rule out before you go attacking the ignition system.