Topic: Another dumb one from a relatively rookie owner

[OPKP]

Second year of ownership of CB750 K4.  As far as I know the engine has never been rebuilt.  25K miles.

I get smoke or vapor at idle after the bike is warmed up out of pipes 3 and 4.  Not when cold, when I'd expect vapor, but when warm.   And it's not always, but probably most rides.  It has occurred in warm weather and cooler weather--I'm in Chicago so you get both.

It's in tune and, other than the visible smoke/vapor at warmed up idle, no performance issues at idle or running; good smooth idle and pulls nicely in every gear across the RPM range.  I get a little of the gurgle-type backfire on decel in 2d and 3d gears, but thought that was pretty normal.


I think it's vapor because it's colorless (not black or blue) and I don't get anything when I'm actually running, just at warmed up idle (but I'm a rookie).  Pulled plug 4 and color is good (no obvious lean or rich condition), I pulled 3 a month or so ago and it was fine.  I do not appear to be losing any oil (level has remained stable) though I should take a 100 mile ride before the end of the season just to be sure!   

After I shut her down after a ride, I still get some vapor/smoke wafting out of the pipes for a few minutes (again would indicate vapor not combustion) and I get a nice burst of steam if I loosen and remove the oil cap.  Is this water condensation in the oil system that is steaming and if so, why is it going out 3 and 4 pipes?  I should mention that I'm not getting the white, kind of sludge on the oil cap, though I did in the past.  Any other troubleshooting steps??

Thanks for entertaining a dumb one, guys.

[BenelliSEI]

I assume you have a stock 4-4 exhaust? There are vent holes under each pipe, where the head pipe joins into the “muffler” part. Are they all clear? You could just be burning off condensation on short rides, that accumulates at that low spot. If in doubt, take an 1/8” bit and drill them right through into the main pipe. Mine usually spit water out of there on cold starts.

Do you get any of that vapour out of the cam cover vent pipe or the oil tank vent? Does the oil tank vent go back to the transmission vent pipe?

[seanbarney41]

Exhaust pipes gonna exhaust

[Don R]

 Getting it up to temperature helps evacuate the moisture from the crankcase and also keeps the exhaust drier. A lot of short rides make condensation.

[newday777]

How long of a ride are you doing when you get this vapor/smoke?

Do like BenelliSEI mentioned and clean out the drain holes of the mufflers(bottom of the mufflers near the weld to the header pipes). That is a key factor for the draining of engine moisture that happens during warm up, especially in cool weather of these original mufflers so they will last without rusting through from the inside out.
The other key factor of extending the life of the mufflers is to only start the bike if you are going to ride it for a minimum of an hour to burn out all the vapor moisture from the engine and exhaust. Short rides is the death of these exhaust.

As to why you have the smoking gun effect, you will need to check that the vent tubes, (back of the cam cover, lower back center off the back of the engine case near the swingarm and the oil tank vent behind the tank) are clear and the hoses are clear and not kinked, so they do their job of venting the moisture as designed. I've seen a vent spigot gummed/blocked with gunk, probably from lack of proper oil changes/maintenance.
As winter is settling in there in Chitown, it would be a good time to remove the mufflers, clean them up, patch any rust holes(stainless steel patches work tig welded with stainless rod and polished looks like chrome when done right), check the engine vent holes are clear and their hoses.

[MauiK3]

+1 no cold runs. I only start mine if I'm going out for a ride so it gets all heated up.

[OPKP]

Yeah, this typically shows up on a longer stop and go ride, when the bike is fully warmed up and idling.  Most recently a 45 minute slog through stop and go traffic.

The weep holes being clogged made sense, but they were ok.  I do notice water coming out of there early in a ride, particularly when it's cooler out.

I will check the breather lines next.  That makes sense.  No oil leaks though.

Thanks.

[Don R]

 If it had a thermostat I'd say put in a hotter one L0L!