Classic cars forum & vehicle restoration.

[alecmcd]

I have a 1972 MGB, which +had a new goldseal block 17 years ago during the restoration, and was then gararged for 16 yeras. Since coming out of mothballs, the car has been performing very well, but every now and then it blows out the exhaust large amounts of grey smoke. I have checked the oil level and have topped it up recently, but the level is just on ful on the dipstick. The problem only occurs when the engine is hot, and has been running for a few mins. Last time it happened on a motorway, and so I pulled off, and after 200 yds the problem went away and it again ran as sweet as a nut. If the problem persisted then I would assume that it was a cracked block/head, or a broken oil ring, but the strange thing is that the problem only lasts for about 30 seconds and then rights itself. Has anyone ever come across the same problem before, especially on a 1800 cc MGB engine?, does anyone have any ideas

[Rick]

I don't suppose it could be something like a valve, or valve stem seal, sticking from time to time? perhaps a broken or damaged valve spring causing it??? I assume it has fresh oil, of the correct grade, since being laid up??

As it sounds mixture related, perhaps there is something amiss in the carb department?

Rick
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[Brian M]

Rick and Alec

I had a similar problem on one of my BMW 3 series - it blew out a smokescreen after a hard run when coming off the throttle. Especially when slowing down on a motorway run.

It was easily cured by replacing the valve stem seals. Somehow oil was being sucked down into the cylinders when there was "negative pressure" in the cylinders.

On an engine that has not been used for many years I would look at the seals first.

[Rick]

Agreed, if its only smoking on the overrun then the seals, or valve stems, are prime suspects.

If it does it at random times, at random throttle openings and engine load, then its a little more odd? You say the block is Goldseal, but do you know when the head was last overhauled?

Rick
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[UKdave2002]

Hi Alecmcd

Also....

I'd check any rubber breather / induction /brake servo hoses; you may find they are deteriorating from the inside, with bits finding their way into the engine and burning up, also check the fuel is new and, nice and clean, old petrol can do odd things, again check not dragging contaminants into the engine.

If the valve stem oil seals are failing you will get the smoke as others have said on over run (when you decelerate), but it would normaly be more consistent than you describe and more a short blast of smoke each gear change and a consistent blast when you decelerate .

If it was a broken ring you would expect it to be worse when the engine is cold.


Good luck

Dave

[62rebel]

are those skinner's union carbs, and mightn't the gaiters be perished by now? know my old spit and my triumph used to eat the darn things regularly. the B i had was never on the road while in my possesion, so if it's were bad i didn't know it.

[UKdave2002]

Hi 62 rebel, what do you mean by gaiters? ....there is nothing to perish on SU carbs? are you thinking of variable choke strombergs ?

Cheers Dave

[Rick]

I remember on my 67 Spit that it had a circular thing on top of the inlet manifold (standard twin SUs), and I think that had a diaphragm of some kind in it - it was donkeys ago so I can't remember much about it, so might be completely wrong

Rick
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[chimpchoker]

is it grey smoke or white smoke? I once had a servo put on my mini and when it knacked up it was sucking the brakefluid into the engine and burning off with a grey whiteish cloud of smoke every now and then but when the brakes were pushed very hard. Light pushing of the brake pedal was ok.

Also isn't burning oil a blue colour in smoke?

TC

Aaln