Looking across to the snow capped alpine mountains seen from the back seat of a motorcycle

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Smokey Startup CBF125

Idiot Date - 22 January 2021

Generally speaking, as a rule, usually, more often than not, the purpose of working on a motorcycle is to at least maintain the vehicle to a good standard and/or repair issues that have arisen. Generally speaking, as a rule, usually, more often than not, my work on motorcycles achieves these goals. Usually, Generally.

But there are times where the old adage "if it ain't broke, don't fix it" are wise words indeed. 

If you read "State Of The Engine Address" then you'll know I recently stripped down the top end of the long suffering CBF125. Nothing of great concern was discovered so I put it all back together with a fresh set of gaskets. All was well. For a while.

Then a couple of weeks back Sharon and I went out for a ride. On starting the CBF125 I noticed some smoke coming from the exhaust. Quite a bit of smoke actually. Hmmmm. This cleared up after a few miles and normal service was resumed.

Smoke on startup, particularly after an engine has been stood a while is, as I have oft been told, a sign that the valve stem seals have given up the ghost. Under normal running knackered stem seals will cause some oil consumption but not a drastic amount. However if the motor is stood a while the oil around the valve stems seeps down the valve stems and into the combustion chamber. On startup this oil is burned, causing the initial plume, then once burned the amount of oil gently leaking by the seals is small enough not to be noticed.

So, valve stem seals! Erm. I replaced them when I checked it over. Dagnammit! But, wait. Maybe one or both the seals have failed, come loose, were poorly manufactured or incorrectly fitted. The initial smoke that clears strongly indicates valve stem seal failure.

I resigned myself to stripping the head once more to figure out how I cocked up the simple task of fitting new valve stem seals. But, wait. I start it up and there's barely a whiff of smoke. It's healed itself, yay! But, wait. Another day I start it up in the back yard and fear the neighbours call the fire brigade for it looks like my house is burning down. 

You expect intermittent gremlins from electricals, this is what makes fixing electricals such exquisite agony. This and the way electricity can't be seen doing whatever it is the angry pixies are doing. We all know and understand electricity runs on smoke in the wires and if you're lucky you might see the smoke coming out, otherwise the mystery of what is in the smoke pipes (wires) might as well be happening inside a black hole. 

crusty old wiring in a block connector on a motorcycle
Corroded smoke pipes can be troublesome too.

But mechanical? Mechanical leaks?? Either you are leaking or you have run out of oil to leak. Simples. Not so with a 91,700 mile CBF125. Oh no. No. No, this oil is obviously subject to the randomness of the quantum uncertainty principle. This is chaos theory in action. Start it on Monday and save for a subtle puff of either smoke or condensate steam, all is good. Start it on Tuesday and and be reminded of "make smoke" in The Battle Of The River Plate. 

Warships making a thick black smoke screen
Tuesday morning in my back yard.

The only logical explanation my inconsequential brain can come up with is thus. For oil to dribble down the valve stem into the combustion chamber there needs to be oil around the stem. Maybe if the bike is leaned at large angle X when parked the oil in the head spills out of the head and away from the valve stems. If the bike is parked at slight angle x the oil remains in the head leading to, well, smoke. 

The valves in the cylinder head do no have a bath of oil for the camshaft
...Thing is no bath or well or dips for the oil to pool in...

Will I do the seals? Again? Depends if I can be bothered. The bike is running and if it ain't broke...

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Reader's Comments

Bob said :-
Have you checked the guides for wear?
I had an SLR650 that smoked on start up, I fitted new stem seals and it lasted less than 500 miles before the smoke came back. Second time I fitted genuine Honda stem seals as I thought maybe the generic ones I had used were no good, but the results were the same.
In the end it had to have new valve guides, it was only the exhaust ones that were worn but I did all four and that fixed it.
22/01/2021 11:08:13 UTC
Upt'North said :-
Ed, oh dear.
I suspect Bob is spot on.
You need a head rebuild, well not you, although yes probably you as well.
But I suspect the cost would be prohibitive on that poor old thing.
I had a CB750 sohc that did the exact same thing, 9 times out of 10 nothing, on the 10th occasion the street would be filled with a blue haze. It would stutter slightly for a couple of seconds too, I suspected a fouled plug.
Occasionally when you switched it off you could hear strange gurgling from within, which I suspected was oil running down the valve and burning within the cylinder. Either that or that Wright's Pie was a bit iffy.
I thought the intermittent nature of the problem probably related to the position of the suspect guide/ seal/valve on park up, if the valve was closed the blummin oil couldn't collect inside. Maybe?
The bike was 30 years old at the time and parts were made from unobtainium and short of a custom rebuild using race parts it had to go. So it went.
I think you need to shoot it Ed, its time is nigh. Depends how much you love it?

22/01/2021 13:35:26 UTC
Snod said :-
Shoot it? When there are cheap cylinder heads available that just need a bit of a scrub? Loser talk! The only down side is checking the guides means the cost of another exhaust and head gasket, assuming Ren isn't just smearing sealant over the old ones each time..
22/01/2021 16:07:29 UTC
Ren - The Ed¹ said :-
Love it? Shoot it? Blimey folks, if we can't agree what to do about the long suffering CBF125 we've no chance of world peace and goodwill to all peeps.

My thinking is this. I'm just going to ride it on the basis of curiosity. It's touring days are quite obviously in the past and it is definitely in the final stages of life. I'm not so sure it's "love" but I have gained a strong admiration of the bike. However there'll only a hint of sadness when it finally gives up the struggle and needs to be carried to a breakers yard. In the meantime the experiment will continue and I shall enjoy riding it.
22/01/2021 19:18:35 UTC
Bogger said :-
You need a scooter.

23/01/2021 16:43:33 UTC
nab301 said :-
I've an Enfield that doesn't smoke that much ( well not unless it wet sumps) and it doesn't even have valve stem seals.
Looking at the older linked thread it's a bit out of focus but in the background of the valve seat photo the valve guide looks suspiciously damp so maybe the guides are shot as already suggested . How much would it cost to do the valves and guides? It seems a shame not to get 100k miles.
My Bimmer in the last year has started doing something similar when parked on the side stand although I nearly always use the centre stand in which case it doesn't smoke . Has the CBF got a centre stand and can you use that ?
24/01/2021 19:29:01 UTC
Ian Soady said :-
Yes, my ES2 doesn't have valve seals and doesn't smoke at all. It does use a little bit of oil though and I'm at a loss to know where from as it doesn't leak.

My workshop manual suggests that anything up to 1600 miles per gallon (200 mile per pint!) is acceptable. Standards have changed......
25/01/2021 10:32:30 UTC
Ian Soady said :-
Of course I should mention that valve / rocker lubrication is somewhat minimal.
25/01/2021 10:33:24 UTC
pocketpete said :-
I can't believe you haven't cured that poor bike by now. Where's all that oil going?
They invented the covid vaccine in less time. Did you remember to put the oil drain plug back in.
25/01/2021 16:34:43 UTC
Ren - The Ed¹ said :-
HA! Maybe the drain plug has a leak Pocketpete. Valve guides? Cylinder head?? Sounds like work to me. At the present mileage during this uncomfortable period it's going to take 5 years to actually wear anything out anyhow.
25/01/2021 16:39:17 UTC
Upt'North. said :-
You're not wrong Ed, I say, you're not wrong.
I was planning to change the Cross Climates on Er'Indoors motor last year, together with a new set of Pirelli's x 4 on mine and then one or two sets of new Bridgestone's on the BeaST.
I reckon that would have set me back about £1500.00! Ouch.
In the end we didn't buy any, don't know if that's a good thing or a bad thing?
However, I'd be obliged if you could tell me where the money is.
I've looked in the old wallet, not a sign of it.
And I've only bought one Haggis and a wee dram for tonight.
25/01/2021 17:15:14 UTC
Ren - The Ed¹ said :-
You see the problem is that £1500 was absorbed into the vast wealth you have already acquired. You with your vast swathes of the Northumbrian countryside, your integrated garage and your high falutin' executive Pan Euro. I'm sure Mrs North has squandered a few pence here an there partaking of a snifter to ease the pain of you being under her feet all the time too. I can only recommend you ease your loss by sending a large contribution to "The Ren - The Ed Benevolent Fund" - a worthy cause if ever there was one.
26/01/2021 10:10:54 UTC

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