Exhaust Smoke

[september2014]

When I drive my 1967 MGB for 20-30 minutes at 50 to 60 mph then go down a street with stop and go traffic the car will emit a blast of smoke when I take off after sitting at a light.  Any ideas on what might be causing this?

[Art]

Have you checked your PCV system?  On the 67, there's a valve on the intake manifold with a hose connected to the forward lifter gallery to draw-off gases from the crankcase.  At higher speed, the intake generates more draw (vacuum) and with a weak valve, it can pull oil in with the vapors directly to intake manifold.  Wouldn't show as much under continuous running, but stop & go might cause the oil to collect and cause the 'puff' on acceleration.

Checking the valve is easy, if not always conclusive.  With the car warm, but off, take off the retainer and top cap to see it is wet with oil, particularly on the down (manifold) side.  If it is, you may have a culprit, but some oil will always migrate there and at these car's age, unless fully rebuilt, some evidence of oil from the gallery is to be expected. 

Also, these valves do not last forever and there are springs to weaken and gaskets to fail or harden with age and exposure to oil, so if there is a real presence of wet oil, I would change the valve or at least the internal components.   

There are other potential reasons, but this is the easiest to check (and cheapest to fix), so I'd start there. 

Art