Classic/Antique Car Repair: 1938 Pontiac Opera Coupe 6 cylinder, white smoke, compression test

Question
Having the air cleaner off I started the car and what looks like white smoke was coming from the carburetor I could only keep the engine running for short time, about 30 seconds, before it stopped running. What was the white smoke?

Answer
There are three explanations that come to mind. All three are exhaust. First thought is that there is a tube in the intake manifold that separates the intake from the exhaust. This is to heat the air and fuel coming through the carburetor to prevent carburetor icing. If you remove the carburetor you can look down into the bore with a flash light and check the walls of the bore for burn through. The second thought is a jumped timing chain that is throwing the sequence or valve opening and closing off so that the exhaust valve is open too far into the intake cycle. And the third is burned valves allowing exhaust to be pumped into the intake manifold. The later two can be check with a compression test of the engine.