Real carburetors fail over decades.
Rochester B's fail over several years.
The fact that your issue was almost overnight almost certainly points to an issue other than carburetion.
Like Jon G., I did read that your engine was rebuilt; however, I would still do a compression test.
Jon G.'s comment about stopped up exhaust sounds reasonable. I don't know an easy way to test this.
If the compression is good, and there is no blockage in the exhaust, then it is ignition, ignition, ignition. If you installed ANY NEW (not new old stock) ignition components, they are definitely suspect.
The poor old Rochester B deserves lots of criticism, but don't think it is the culprit this time.
Jon.
