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MG MGB Technical - Smoking 71

This is sort of water under the bridge, but just the other day I was looking to purchase a VERY nice 71 MGB that was perfect in all but two areas. The one that had me the most concerned was not apparent until I followed the owner to my home to look at a possible trade. I followed the B about 5-7 miles on all sorts of streets and highways. Whenever the owner would take off, change gears or rev the engine at a red light, lots of smoke would appear from the exhaust. Being at night, I couldn't tell if it was from being too rich, or oil smoke, but it really seemed like the later, and for sure it wasn't from condensation and cold weather as the owner suggested.
A few days later, the owner says he took the car to a shop and the mechanic checked the compression and said it was great, and said the smoke must have come from the oil seals on the valves. He also added that it had quit smoking all together.
Does anyone have an idea of what this could have been and what was done to stop it short of replacing the seals, which wasn't done? For a car that was NEW in every way, it seems strange that a person would not change the valve seals, if that were the problem, before selling it.
Like I said, this is water under the bridge, because the car was sold Monday morning as I was calling the seller.
I was curious to know what the majority thought about the smoke, for my future reference.
Cleve

Cleve-
If it smokes upon closing the throttle, then its definitely a case of bad valve stem seals. If it smoke upon acceleration, its the rings. From what you describe, I suspect that in this case its both. Neither of these things stop of their own accord.
Steve S.

I've always understood that if it smokes when accelerating, i.e. taking off from a standstill or coming off the overrun after a long downhill stretch could be caused by either valve stems/guides/seals *or* rings/bores, it all depends on how the oil gets into the engine. If it comes in via the crankcase breathing system it is rings, if not is is the valve stems etc. Alternatively, if it continues to smoke all the time it is accelerating or being driven hard it is rings, if only briefly at the beginning of acceleration again it is the valve stems etc. The difference being that when the engine is on the overrun or at idle the high vacuum is tending to pull oil in through the valve guides directly into the cylinders which is only fully burnt on acceleration, even one might think it should smoke all the time at idle or on the overrun, whereas with rings oil is being burnt in the crankcase and the smoke pulled in, or oil itself is being sucked in via the breather due to excessive crankcase pressure. There again it is not unknown for oil to be pulled in via the breather when the rings and bores are fine, and it is a problem with the breather system.
Paul Hunt 2

In the sellers defense, I believe he may have good compression results as I have a similar problem with my '74 although the compression checks fine...I suspect the valve stem seals. I didnt notice how bad the problem was until driving at night with a small car close behind me with very bright headlights. Since I plan on updating the head for unleaded fuel soon it should take care of this problem. If it doesnt Ill re-post.
jjralston

JJ,
It sounds like the same story. I followed the MG home after dark, and with all the cars on the road, only the MG had big puffs of smoke in my headlights.
I parked my car behind it in my driveway and it was doing it on rev-ups also.
I guess it's just the seals, but at least now I know for future reference when looking at LBCs.
Cleve

What did the smoke smell like? If it smells like oil, the answers are above. A rich running engine will show some smoke in headlights, and smell different than an oil burner. Did you ask what the oil consumption of the car is? Just trying to cover other bases here.
Tom

Tom,
You sound just like the seller!

We have all driven close behind cars that are running too rich, and you can smell them in your car even with the windows rolled up. This was definitely not running too rich. With this much smoke, it may have been a contributor, but not the source.

Yes, I did look at the oil and it was a half quart low, which is odd with fresh looking oil, that it would have used half a quart before getting dirty. Before I checked it, the seller said he drove the car over a hundred miles and it didn't use any oil. Then when we checked it was a half quart low.

Hey, like I said, I wanted to have the knowledge for future reference, not to be critical of the car or the seller. :)
Cleve

This thread was discussed between 02/01/2008 and 03/01/2008

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