MGB Engine problem

I've got that at the moment on a car.

The crankcase pressure builds up due to blowby gas escaping past the pistons during combustion. This happens on a new engine, but can become significantly worse when the bores are worn or when the piston rings are worn or broken.

There is a system to allow the gases to escape or to be actively extracted, e.g. by sucking air from the crankcase into the carb intakes and by letting fresh air into the other end of the crankcase to replace it.

If the crankcase breather system is blocked up, e.g. by old oil sludge, then the gases will build up and escape somewhere eventually. Similarly, if a ring breaks and the blowby gases are worse than the breather system can deal with, the same thing happens.

When the engine is under load, it will develop more blowby so you know this is probably happening when the car smokes badly while accelerating / driving at speed on the motorway, for example. Accelerating from rest to some modest speed usually takes less time and the pressure doesn't have time to develop.

Smoking when the throttle is lifted off, usually comes about when the valve guides are worn and you are under strong vacuum, sucking air (or oil) through wherever it can come when the inlet can't deliver enough. This tends to happen worse when the crankcase is pumped up because the air in the rocker cover above the inlet valve guide openings will usually be at crankcase pressure to make the guides route even more desirable.