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MG MGB Technical - MGB puffing smoke

I own a 1973 MGBGT, and have recently had the head modified for unleaded fuel. I had it gas flowed at the same time. Since re-fitting the head I notice that I get a white (could be light grey) puff of smoke from the exhaust when changing up gears, specifically 1 to 2 and 2 to 3.
In my attempts to find the reason for this, I have done the following:
• Replaced the head gasket and checked the head for cracks (possible water vapour)
• Replaced the valve seals. (Just in case I got it wrong first time)
• Checked compression (125, 130, 130, 125 front to back – book says 130)
• Isolated the brake booster. (could be brake fluid)
• Isolated the engine breather to carburetor connection.
None of the above make any difference. Has anyone else had this problem?
C.S. Barrow.

Puffing oil on gear changes, or when accelerating after a long down hill on the overrun long idle, is usually valve seals/stems/guides. However that would be blue smoke, white smoke is usually water. Is this when fully hot? Are you losing coolant?
PaulH Solihull

When you had the head work done, did you make the necessary improvements to the fuel supply? i.e., K & N's, rich needles. How a weak mixture would cause your problem, I'm still pondering!! Have you examined the condition of the plugs? Was the head crack tested before the work was done? Changing up involves closing the throttle and hence creates a high manifold vacuum. So if something can be pulled into the combustion chambers it will happen then and also on over-run. Water, oil and "maybe" neat fuel.
Allan Reeling

Hi Paul, It is difficult to tell the color in the rear view mirror. It looks white but I guess that it could have a blue-grey colour, it is definitely not black. It only happens on gear change, not when accelerating after a long downhill on the overrun, although it is worse after a long idle. As you can see above, I have replaced the head gasket and checked for cracks. I also visited my local radiator repair shop and checked for combustion products in the water. I am not losing coolant, and I have even gone to the extent of fitting both “O” ring and Umbrella oil seals on the valve stems. I also do not appear to be using any oil. The plugs are burning a light chocolate colour. It seems to diminish as the engine warms up, but this could be my imagination. I am fast running out of ideas!

Hi Allan, I did not have the head crack tested prior to modification, but I have had it tested subsequently and it came out OK. I also believe that checking for combustion products in the water would show up a crack. I run this car at an altitude of around 6000 feet with standard needles in the carbs, so it has always run a bit rich, so maybe at last I am compensating for this. The plug insulators tell me that the mixture is probably OK.
I agree with you completely about closed throttle conditions producing a high manifold vacuum and that is why I have even gone as far as disconnecting the brake booster in case there was a small brake fluid leak there. I have even theorized about the possibility that although the compression figures look fine, the high vacuum may be sucking oil vapour UPWARDS past the rings from the crank case.
C.S. Barrow.

Diminishing as it warms up sounds more like condensation, which is only visible until the engine and exhaust pretty-much gets up to temperature. It would be continually visible during idle though, gradually reducing.
PaulH Solihull

Condensation is only there when you are burning petrol, every gallon produces more than a gallon of water. It exits the cylinders as water vapour and turns to water when the engine/exhaust is cold as its latent heat is lost to the exhaust. When the car warms up it remains as vapour and you dont see it. This has to be valves or rings you have eliminated everything else, I cant account for why it doesnt happen after a spell on the overun though. When my car was leaking a bit through the valve seals it gave a blue puff on start up, a pretty serious puff TBH
:-) but it still did not need topping up between changes sovery little oil is burned in these puffs. I had the great pleasure of driving the MGB GT in this picture in SA in 2008. I'm sure you recognise it. The altitude isnt kind to the performance!

Stan Best

How many miles are on the lower end of your engine? Whenever you do work on the head, you tend to magnify the weaknesses in the lower end. Even if you have good compression, you may be pulling oil past the oil control rings. When you had the head off did you ream the ridge, at the top of the cylinders? If so, this will cause the engine to smoke on deceleration. RAY
rjm RAY

Many thanks for the input! I think that Ray is probably right. When I had the head off I checked and there was virtually no ridge on the cylinder bores so I did nothing. However, at the time I noted that the cylinder bore honing marks from a previous re-build were still very visible. There were two things that bothered me about this, the first was that the marks were so prominent even after a fair mileage and the other was that the bores had not been correctly honed as the marks were circumferential rather than criss-cross which seemed to indicate that whoever had done the job had kept the honing tool stationary rather than moving it up and down. However, because the compression figures were good and the oil pressure is very good I decided to leave the bottom end alone. Maybe I should have had the bores re-honed and fitted new rings. I will have to have a think about this.
C.S. Barrow.

It shouldn't be a major concern, if the engine is running strong enough for your tastes. This is a common problem, when less than experienced people work on engines and wonder why they still have problems. After working on cars, boats and motorcycles for 45 years, almost nothing surprises me anymore. Enjoy driving your car, a little oil consumption won't bother it.RAY
rjm RAY

"Checked compression (125, 130, 130, 125 front to back – book says 130)"
This is quite low in my experience, and my book says 160psi. If I get readings under 150 I start looking for problems. A quick look through some old records shows less than 150 only on known worn engines, usually a couple of cylinders at 140-150 and a couple 130, even on US LC cars. Compression test readings can be greatly influenced by electrical problems in the heavy current path to the starter. And compression tests must be done with throttle wide open.

What was done to the head when reworked - chamber cleanup, skimming etc? Were any volume measurements done? Is it the correct head for the engine? Is it supposed to be an HC or LC engine? What pistons?

If all that is in order, I suggest checking cam timing. Late cam riming will cause both low compression and oil burning, not to mention lazy performance and poor fuel economy. If the cam timing is right, then check lift on all valves, since worn cam can cause similar.

FR Millmore

This is probably an export model with LC pistons to cope with varaiable quality petrol. This car is in SA. and the 130 PSI could well be the factory figures for that market.
Stan Best

From the Bentley reprint of the WSM:
All 18G* are shown as 130 for 8.0:1 and 160 for 8.8:1.
I've never seen an 18G with 8.0, as we didn't get any, but I think the 160 is a sort of "low limit - adequate for further service" designation. These engines generally are on the high side of 160 if all is good.
18V 72-on US cars are 8.0, shown as 160, but 18V are also shown at "170psi nominal" for both 8.0 & 9.0 versions, except 18V to ECE specs shown as 9.0 with 170-190psi nominal. No specs for the 18V US catalytic converter engines, which had different cam timing.

These specs are clearly rough, since they do not match known changes that might be expected to change things and show differences without apparent cause. As I stated, my experience of US 8.0 engines is that they are generally around 160-170 if good, and below 150 indicates some problem. I would not expect 130 or lower unless it had 100,000 miles without rebuild.

Since the engine in question has been rebuilt to some degree, everything is open to question & review. Unless the fuel there is really awful, I would certainly be looking at more compression, especially at that altitude.
Another point is that if it has a performance cam fitted, low cranking compression could be a result; a somewhat more comprehensive cam check might be in order.

FR Millmore

Thanks again for all of the input! This car was originally purchased in Blantyre, Malawi so I imagine that it was equipped for fairly basic fuel. Thinking back on the history, the problem only seems to have surfaced since I had the head re-worked. I keep on wondering why……..?
C.S. Barrow.

This thread was discussed between 19/07/2011 and 21/07/2011

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