One nice thing about old cars is that they are simple to understand. This means that sometimes you can trace the exact reason for a problem you have with the car.
The problem with my MG TD was that the engine suddenly started racing. If I hadn't known enough to switch off the juice quickly, we might have hit the pickup in front of us and I wouldn't be around to write this.
I suspected a carburetor linkage problem, but nothing looked out of place. While tinkering, I noticed the air cleaner wing nut was loose. I removed it and saw that the tiny hole for the rod had become a jagged crater the size of a half dollar. Could a piece of my air cleaner have broken off and got stuck in the throttle plate?
That is exactly what happened. Actually, there were two pieces and I'm happy they didn't go into the engine. But in the big picture, "death by air cleaner" is even worse than a ruined motor, isn't it?