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MG Midget and Sprite Technical - oil smoke problems

hi all

I've a 1967 mg midget 1098cc. Its a new car to me and runs with an unleaded head. Recently its started smoking and burning some oil, but when idling at any rpm (e.g. 15/20/25 or 3000rpm) it does have any exhaust smoke. However, when i accelerate the car whilst driving, or in neutral whilst parked it does smoke. The car has a standard fit oil breather on the timing front cover connected to the carb.

I've looked at the archives but cannot see anything obvious so i'd appreciate any suggestions, before i start looking at a new engine.

cheers

Richard
rich auckland

Rich,
Smoking under acceleration, or if the car is left to idle for a while, is usually caused by leakage of oil down the valve guides. The manifold pressure drops in both of these conditions and creates a suction effect which draws oil down guides. This can happen if the guides or the valve guide seals are worn, or sometimes on unleaded converted heads which use a greater valve guide tolerance than normal.

The easiest solution is to renew the valve guide seals, and consider putting them on the inlet as well as the exhaust valves.

Another possibility is that the rocker shaft is excessively worn allowing oil to discharge from the rocker bushes faster than it will drain down the pushrod holes and back to the sump.

You could try a compression test, "wet and dry" which will give an indication of bore wear which could also be a source of oil burning.

Guy
Guy Weller

A 1098 engine wouldn't 'normally' have a breather on the timing cover. These were fitted to 1275 engines.

Is it definitely a 1098? Not that this would have an bearing on your smoke issue, just curious.
Dave O'Neill 2

Rich. As Guy says, do a compression test to check for ring/bore wear.
A 1967 car should have a 1275 engine so check your engine number. 1275s start with 12 and 1098s start with 10.

Bernie.
b higginson

Rich,

You made an intersting statement I want to follow up on for better clarification

you said the engine "does not" smoke when Ideling (Is that with the clutch depressed?)

then you said it smokes during acceleration and "in neutral whilst parked"


The part Im focased on is that it smokes while in neutral and does NOT smoke while idling....Am I understanding this correctly?

I dont have a clue at this point, but got my attention.

BTW... Are you SURE its oil smoke?

Oil...Burns Blue
anti-fresse.... burns White
to rich, over carbed.... Burns Black
If your enviorment is super cold...It can have the apperance of smoking alot, even after the engine is warmed up.

Prop
Prop

hi there

thanks for all your suggestions, The car is infact a 1275cc (my mistake on the earlier post stating it was a 1098cc).

Just to clarify - when the car is idling in neutral with the clutch depressed, then there is no smoke from the exhaust. If i increase the revs to 1500, then 2000 and then up to 3000rpm, with gentle use of the accelerator, then again there is no smoke from the exhaust.

However, when driving, if I accelerate, or if the car is stationary, and out of gear and i "rev" the engine, then in this case i do get smoke from the exhaust. It appears black/blueish, but there is no oil residue in the exhaust pipe, that appears dry and clean.

I have also checked the oil and water, and there isnt any cross contamination, so i guess the cylinder head gasket is ok at the moment.

Thanks for all the suggestion, I'll get a compression tester and give that a go in the next couple of weeks, then if that appears ok, i'll give a go at renewing the valve guide seals.

cheers again and thanks very much for the suggestions

rich
rich auckland

Hmmm...My guess would have to be a worn valve guild as stated above...and My 2nd guess would be a broken/worn oil compression ring


prop
Prop

Rich,

I had the exact same thing until recently.

When a head is refurbished, most engine builders will fit 4 stem seals. I renewed the existing seals and added seals to the valves which didn't have them. I used the high-spec viton ones from Minispares, as fitted to Mini MPIs.

Following the exact same advice from Guy as he's giving you above, I also renewed the rocker shaft with a thick-wall shaft also from Minispares. The old one was definitely worn - I could feel slack in the rockers.

Was a good day's work to remove the head, remove the valves, renew the seals and rebuild the rocker assembly, but my oil consumption has been cut in half, for a total outlay of about £50 including a copper head gasket set. Oil pressure is running maybe 3 or 4psi higher than it was at idle, too.

In my experience, it's the way to go - it works!

Mark
MarkH1

see if there is any change when you remove the oilfille cap while idling
a.o. arnold

I recently rebuilt my 1275. I was informed by the machinist that he had put valve seals on the intake valves only. There was a comment about interference with valve springs.

The car will puff a bit of smoke at idle and when the throttle eased back after reving. I need to replace the head gasket (due to slight oil leak to the outside) so maybe I will have the valve seals reworked to include 8 and to make sure they are viton.
Glenn Mallory

I'm just re-reading Vizard at the moment and he doesn't recommend the use of seals on the exhaust valve stems (p231):-
(a) because the exhaust valve being subject to pressure,tends not to draw oil vapour in and
(b) he thinks any oil on the stem helps the transfer of heat from the exhaust valve into the head thus reducing the risk of hotspots.
M J Chapman

I know that vizard suggests only using 4 seals as do many race engine builders but I have found the same results as Guy and others that when 8 are fitted smoke is a thing of the past and oil consumption is reduced.
Robert (Bob) Midget Turbo

I think there are two other factors that come in to play regarding the amount of oil that may be drawn down the valve stems. One is the type of cam - I suspect that the overlap on some cams may have an impact by creating an amount of vacuum in the exhaust port. Possibly some sort of venturi effect from exhaust gasses passing over the end of the valve guide.
The second is I understand that the guides used when a head is converted for lead-free fuel have greater clearance than standard. I think that on my engine the combination of these two factors was what caused oil to be drawn down the exhaust stems until I decided to put seals on these too. Since then the blue oil smoke problem has disappeared.

That is my experience but of course others may have different results.

Guy
Guy Weller

Well, I must admit, I replaced all 8 seals about 10k ago and everything is fine, even though I used the apparently useless-looking miniature rubber doughnut jobbies that came with the gasket kit.
M J Chapman

Guy that is exactly as I see it.
The exhaust gases passing by the nose of the valve guide will have the same affect as any other venturi action. Thus it will cause a vacuum in the guide that will cause the pressure above the guide to force oil down the valve stem.
If someone would care to state why this would NOT be the case then we are all ears I am sure.
Bob Turbo Midget England

My rebuild last year puffs a little in the same way. High lift cam (Morspeed PH3), fresh unleaded head and I only put seals on the inlets. One of the jobs this winter is to get seals on the exhausts and see if that stops it.
Dean Smith ('73 RWA)

The venturi effect occurs only when a stream of gas is forced to speed up when flowing through a restriction, causing a local drop in pressure. For the sake of arguement then, let's assume that the protrusion of the valve guide boss into the exhaust port does actually produce such an effect. In order to draw oil down the exhaust valve stem, the pressure around the valve guide boss must be less than that in the rocker cover i.e. atmospheric pressure. Now it would be nice to know what the average pressure is in the exhaust port but let's look at it another way - at the tail pipe, the pressure of the exhaust gas must be greater than atmospheric otherwise it wouldn't come out, added to which are the pressure drops associated with the silencer, exhaust pipe and manifold. So all I'm saying is that the pressure in the exhaust port must be a fair bit higher than atmospheric and I'm wondering if any venturi effect would be of such a magninitude as to overcome this ?
M J Chapman

Since the exhaust gas is pulsing, rather than a steady flow, then there must be variations in the pressure around the valve stem. As the exhaust gas goes down the manifold pipe it has significant velocity. At this point the valve snaps shut behind it, resulting in a vacuum, or at least a sudden reduction in pressure behind the speeding "slug" of gas. I think that the idea that the exhaust port on a running engine is always at a pressure above ambient is inaccurate.

Guy
Guy Weller

It seems to me that all of the valves should have stem seals.

Just to enter the venturi fray, what would be the effect of running the air pump as I do? This provides a constant, low pressure stream of air into the exhaust port so that unburned fuel will continue to burn.

About 4,000 miles into the rebuild, the car is burning a quart about every 1,200 miles or so (burning less and less). This is about what it consumed after the first rebuild 75,000 miles ago. If I remember correctly, the car used a quart every 3 - 4,000 miles when it was new.
Glenn Mallory


My thinking in adding the additional 4 seals was basically 'why not'?

I know Vizard's word is gospel and the guy obviously has forgotten more than I'll ever know, but the two points from Page 231 seem to be contradictory - you're not going to get any oil down the stem, and oil down the stem helps heat transfer. Can't have it both ways, surely?

My engine is not highly tuned and I try to drive it sympathetically as it's getting a bit worn, really.

So I don't see what the negative side-effect of halving my oil consumption is? Seems that plenty of folks running much hotter-spec engines don't have any negative effect either?

So, for my information when I think about building a new engine next year - anyone out there able to convince me there's a good reason for not fitting seals to all 8 stems?
MarkH1

my wife bought an Astra years ago (auction, cos I'm dead cheap me) we took it for a run the next day and although it was fine, starting and low speed running, as we got onto the A5 near Lichfield and stepped on it we left a humungeous cloud of smoke behind us

Daylight dimmed all over Staffordshire so we slowed way down and limped off to the local Vauxhall dealer for spares

New valve seals on all eight cured it to perfection

Mark, I'd fit all eight to any A series
bill sdgpm

This thread was discussed between 17/01/2010 and 27/01/2010

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