Did u possibly cross a plug wire during vacuum work did some plug wires come offI once accidentally disconnected one of the short U-shaped vacuum hoses from the carb and the engine would race on start-up and continue to rev above normal RPM and misfire or backfire if I pressed the accelerator pedal. Once the engine warmed up it would settle down to normal RPM and the misfiring ceased. I removed a couple of spark plugs and noticed that they had white deposits signifying a very lean mixture and that's when I started to look for vacuum leaks and discovered the disconnected vacuum hose. In my case it was one of the short U-shaped hoses between the weakening device and the carbs. Hopefully you have a similar issue which could be easily rectified.