MGB: Engine trouble...

Hello.
Having owned my ´71 MGB for quite a while now, I have always been very pleased with it and ejoy driving it in the summer, here in Denmark.
Lately, when driving for longer periods a time at 60 -70 mph, the car suddently seems to burn a lot of oil for appx. 10 sec. at a time, engulfing the entire road in a smelly blue smoke. It will do this every 10 to 15 min. or so.
I am thinking that it there must be some kind of pressure build up somewhere in the engine but I haven't been able to determine where. Has anyone had similar experiences or any idea what may be the cause of this problem?
Any help will be appriciated.

Safety Fast

Mikkel

are you hitting your smokescreen switch by accident?

If it is pressure buildup, and its getting released by blowback through your rings and to combustion->exhaust orhwhatever...

could the oil pressure relief-valve dumped on you?
whats yer oil pressure guage readin when you're driving? does it build up them simmer down after you burn up oil?

I'm just thinkg up stuff in my head... take with a grain of salt!

Just a guess, but bad rings can allow blowby of combustion gases. This in turn causes a buildup of pressure in the sump. When the pressure reaches a sufficiently high level, it pushes oil through the breather and into the air/fuel intake stream.

Alternately, the pressure buildup can force oil up through the oil passages into the valve train area. If you have leaky valve seals, the excess oil can leak down into the cylinder where it is burned.

I would suggest a compression test, both wet and dry. The compression should be within 10 percent on all four cylinders. If one or more cylinders show a low compression, put a few cc's of oil in that cylinder and try again. If the compression goes up significantly, then the rings are probably bad. The oil forms a temporary seal, allowing a higher pressure reading. If the compression remains low, then it may be the valve seals that are leaking.

if it is Paul's Idea #1, you could check your breather setup where it enters the induction system (dunno whwere that is on a 71) if its really oilfouled, there might be a big clue...

compression test still probably the best way to test for stuff tho

This is the first time I've ever heard of this problem.
When it has this oil spasm, does the power drop off? Are there other noises associated with this phenomenon? What does the oil pressure gage do? Ditto tach.
Is it burning oil or leaking oil?
Could your pushrod side covers be loosening up & splashing oil out onto the exhaust manifold?

The second run of Geo 3-bangers would do that with the head gasket for one of the first of the run head gaskets on it. It was an oil drain from the head problelm, but it would take an enormous amount of sludge to make a B engine do it for that same reason. It could be the breather in the front lifter cover is clogged, slowly filling the tube wit oil, then the intake is sucking it out. It could be a faulty PCV valve which is easy to take apart on your model. In either case you'll probably find excessive oil in the PCV valve.

Take the breather hose off the PCV valve and look for signs of oil in it. You should be able to blow, (like a big huff of air), through that hose into the fron lifter cover with little or no resistance. If there is excessive resistance something, most likely the breather medium, is clogged. If you find that get back with us, there's a cheap perminate fix for it.

PCV valves are made ot operate in a wide open state at idle, but close off to a trickle of air above idle. If it is sticking open that alone could be the problem.

Given that it was intermittent, that was my guess too.

Thanks to you all for all of your good advice.
Today I got around to doing a compression test, and I got the following results: (cyl. no/dry/wet), 1/130/144, 2/135/146, 3/129/145, 4/146/156.
In each try it took 7-8 revs on the starter to build up max. press.
I am not entirely sure what this means, so any suggestions are welcome.
To answer some of your questions, the pressure is steady around 70 psi both before, during and after it burns the oil, and power is equally unaffected. There are no signs of any major oil leaks around the manifold so I assume the oil is being burned, not spilled.
John, I assume the PCV valve you're talking about is the flat topped one positioned on the left side of the head. My car doesn't have one of these, but only two hoses going directly to the carbs. I haven't gotten around to checking the crankcase vent though, so it may well be a clogged filter in the side cover.
I hope to get around to checking the side cover filter and lines, as well as the pressure relief valve one of the next few days.

Your engine looks to have good compression, within 10% low to high.
Two tubes, no pvc valve? I wonder if that by itself could be your problem?

I belive the setup without the PVC valve (if it is the one I am thinking about) is an original factory fit, it is shown in Original MGB and Moss also sells this setup.