Blue smoke, left exhaust.

[Magua]

I wheeled the cb out this morning and fired it up. Rather large plumes of blue smoke poured out of the left exhaust. A huge cloud of it actually :S It petered out somewhat, but not completely.

The bike has a minor head gasket leak and I figured it had just given way, but xerxesdaphat thinks otherwise.

I'm assuming that there's probably not much I can do diagnostically without removing the head. Thankfully I've got uni holidays from Friday and can get onto that fairly soon.

Ideas? (valve, piston ring?)

[xwhatsit]

Each pipe goes to one cylinder, there's no balance box any more, so that narrows it down easily to the left cylinder.

My theory was that as it's air-cooled, there's no reason for smoke to start pouring out the exhaust if the head gasket has let go. Blue smoke means oil. Where can oil come from? An internal oil passageway in the head/barrel? But why would the head gasket let go at such a point (cam-chain runs centre, along with the oil passageways).

My completely uneducated guess is that the engine stopped in a funny place it doesn't usually stop in, leaving the left hand cylinder in such a position that the leaky valve stem seals (I'm sure they're leaky by now) let some oil down from the top end into the cylinder.

[ManDownUnder]

See if it does it every time after sitting for a while - or not.

That'll test the valve being in a funny position theory (which I don't think is likely personally). Leaking head gasket? The oil obviously gets up to the valves etc therefore commutes through galleries up through junction from cylinder blocks to the head (and crosses the head hasket).

A leak in the gasket could allow oil into the cylinder, delivering blue smoke on startup, petering out as the initial quantity is burned off. Watch for a fouled plug too.. this would be a good time to start carrying a spare (no matter what the cause turns out to be...)

[xwhatsit]

The galleries run through the middle though, next to the centre cam-chain tunnel. Is it likely that a gasket would blow in such a place? Where is it blowing to? Surely it would blow to the outside atmosphere?

[Magua]

I just went out and started it, no smoke. Gave it a bit more gas, a few puffs. Even more gas and the smoke disappears. No smoke at all now.

[Ixion]

Forget it then The dynastic gentleman is right, just an aberration. A bit of smoke on starting is meaningless on a mature bike.

[Motu]

I presume it has a side stand on the left?......Try a side stand on the right,and see if the right hand cyl smokes.

[The Pastor]

I'd just leave it personally, my bike smokes on start up, my car smokes on start up. my old bikes have all smoked on start up.

If a head gasket isnt expensive i'd replace it tho (or at least start saving to replace it.

What condition are the piston rings in?

[xwhatsit]

Yep, I remember I had a similar problem (smoke was white, though) once. Very cold night and the bike caught a chill, methinks. A good strop and it disappeared never to be seen again.

Bike got left unused for a day a couple of weeks ago. Puff of blue/black smoke that had all but disappeared by the time the warm-up was finished.

It's a touch of personality coming out in its old age; takes a while in Hondas, but if left to mature long enough you'll see a bit of the Jim2 emerge.

Edit: you normally leave it on the centre stand, though? Also, yes, even though I'm not 100% the head gasket is blowing (oil leak could be coming from a few places, they're deceptive), it wouldn't be a silly idea to do a compression test at some stage. You've mentioned power is somewhat lacking in the past.

[98tls]

If its been sitting awhile between starts then rings (or a ring) would be my pick.

[Squiggles]

I wheeled the cb out this morning and fired it up. Rather large plumes of blue smoke poured out of the left exhaust. A huge cloud of it actually :S It petered out somewhat, but not completely.

Aint got nothin to worry about yet, my old man's CX looks like a 2 smoker at the moment

[Magua]

What condition are the piston rings in?

Don't know. Can I tell without removing the head? Compression gauge?

Edit: you normally leave it on the centre stand, though? Also, yes, even though I'm not 100% the head gasket is blowing (oil leak could be coming from a few places, they're deceptive), it wouldn't be a silly idea to do a compression test at some stage. You've mentioned power is somewhat lacking in the past.

Side stand, left hand side.

Got a compression gauge I can borrow?

[Bonez]

Theses things have oil ways with rubber seal/o-rings which get hard with age. Valve guide seals that do the same. When was the last time the head was retorqued?

It's probably prudent, given the age, to get a compression check and leak down test done.

What's a leak down test you ask-

http://en.wikipedia.org/wiki/Leak-down_tester

[Bonez]

Aint got nothin to worry about yet, my old man's CX looks like a 2 smoker at the moment

You old mans and my CX have something in common.

[Squiggles]

What? 5 years being stored outside? Started first press of the button and ran like a beaut after we put a new batt in. Rings/Bore probably got a bit rusty, or it was just that its getting on in the age/k's department