Dieseling. Yeah, it's embarrassing. The cause is simple. There is something in the combustion chamber that is hot enough to act like a spark plug, igniting fuel even with no spark. Usually, carbon in the combustion chamber does it, but sometimes it's something like the spark plugs' ground electrode. Especially if they are worn and the electrode is real thin. Without going to the root problem, you might be able to keep it down by changing the plugs (if it hasn't been done in a while), or by running a top cleaner. You can use sea foam, I know that GM makes a superb top cleaner. There are other aftermarket brands. Basically, you take the air cleaner off, pour this stuff in slowly while keeping the engine running, and then just dumping it in until the engine dies. Let it sit for 15 min. and start her up. This dissolves most of the carbon.
As for root causes, the most common are 1. Idle speed set too high, 2. Engine running too hot, 3. poor fuel quality 4. It's actually a ford 300 six-cylinder you are working on
Having the ignition timing advanced too far will also raise combustion temps, and will get that carbon glowing.
Well, that's all I can think of right now.