It's typically caused by a lean condition and or retarded timing. Check for vacuum leaks around the carb, especially if it's a bbd. Now, why it would just start doing this??? who knows? could be summer heat...
I've always found the cause to be a high idle. Whether it's from a vacuum leak or an improper tune. Whatever the reason, it's usually because of a high idle.
Lean mixture/vacuum leak, improper ignition timing, restricted exhaust, carboned-up combustion chambers and/or spark plugs, improper spark plugs, proper spark plugs but someone forgot to remove the metal ring washers from them before installing them so they're overheating, severely crudded-up cylinder head coolant passages, improper idle speed, improper cam timing…
What's the status of the car's EGR system?
Tune-up parts and technique suggestions in this thread. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download.
I've always found the cause to be a high idle. Whether it's from a vacuum leak or an improper tune. Whatever the reason, it's usually because of a high idle.