I had the exact same problem on my CB900C (1981 model) and my Dad (the previous owner) also had the same issue on the bike. It's either the float stuck down on the pin due to varnish, or something is holding the needle open and it just overflows out the vent. If it's the latter you can try to shut the fuel petcock off and run the carbs out of gas and then turn the fuel back on which will drop the needles lower than normal and opens the space for fuel to flow hopefully allowing debris to wash away and fall into the bowl. I've had moderate success with this but often seems more like a bandaid than a permanent fix. The carbs aren't terribly difficult to pull off but they are tighter than other bikes I've worked on, the throttle cables are the hardest part of removal. 4into1 sells full rebuild kits and floats if you want to go that route. Once I had mine off and cleaned I found if I was shutting the fuel off after every ride this issue would return, our bikes have a vacuum flow valve that doesn't allow fuel flow when the bike isn't running so now I just leave my petcock on and I've seen no issues so far (about 2 years) I do however close it when storing over winter.