I was driving my 66 1800 last week when, stopped at a light, it began to smoke really badly. It hadn't been driven for months. I shut it off and it dieseled for a long time then quit. I let it sit for a while and then it started right up and drove fine. Any ideas? It always ran good before with no problems.

Sounds to me like it was overheating, and that the 'smoke' was probably steam. Thermostat stuck after sitting for a spell, perhaps.

Of course, if you know it was not steam and was indeed smoke, first check for oil leaks (on the exhaust). That it was while stopped at a light, you might notice smoke that while driving gets blown under the car...
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-Matt I ♥ my ♂

It was gray smoke and not steam. It runs fine now.

I think the respondents missed where the original poster said it was "gray smoke". I didn't see it either. Actually, I still don't.

To me, it sounded like the OP was concerned even though it ran ok afterward.

How about the possibility that a float valve got a bit of crud in it and hung open, allowing the float bowl to dump over into the intake? I'm a D-key guy so I don't know if it would spill over into the intake or overboard.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)

Like others have noted, the color of the smoke will be important. White exhaust smoke can be a sign of coolant leaking into the combustion chamber. The exhaust will typically have a 'sweet' smell to it. The fast test is check the coolant level to see if it has changed after the car has cooled down. Also, pull the dip stick and look for traces of coolant and do a smell test. Neither of those checks will provide definitive negatives. A compression test and cooling system pressure test may be in order to rule out head gasket damage.

Pulling the spark plugs may provide some evidence of what is going on. Particularly if the problem is oil related smoke the plugs would show up as oil fouled.

Dieseling is caused by something hot in the combustion chamber acting as a source of ignition after the ignition is shut off. It can typically be a hot piece of carbon. If your car has been running rich, carbon can build up in the combustion chamber can result in dieseling. An abnormally high idle speed will also support extended run-on. The dieseling may or may not be related to your smoking issue.

What color was the smoke?
How was the car running while it was smoking?
SUs? Perhaps a jet didn't return after "choking".

More information would be needed to make an informed assessment.

Hal;

That happened to my 122 also a long time ago...spooked me pretty good... it turned out to be the Brake Assist unit...that's kindof what they're known for...failing...so if it was white smoke, and your vehicle has a vacuum brake assist unit, it might have been the failure of the membrane in your unit, which allows Brake Fluid to be sucked into the intake and burned by the engine. In that case, disconnecting Vacuum line should prevent it, but be aware that once vacuum is disconnected, brake pedal will need more pressure from you for the same amount of braking...

Good Hunting!