The late CB750 is very susceptible to dirty carbs. I don’t know if it’s more sensitive than, say, Yamahas, but the impression I get (confirmed by my limited experience) is that the EPA was all over Honda in the late 1970s to improve idling emissions, so the fuel delivery is on the lean side.
I just recently enlarged the slow jets on my 78 750K, changing from #35s to #40s. Best thing I ever did.
Also, old gas tanks rust, and a rusty fuel tank will dribble grit into the carbs. The float and valve system is so finicky that any contamination in there will keep the valves from closing properly. That makes the bowls overflow. The 1978s are actually better in dealing with this, because our float valves have Viton tips, so they seal better. The older metal-on-metal float valves sound like they were a constant problem.
Also, the air flow through these carbs is highly sensitive to air leaks. Any one of the 12 clamps on the air/fuel system can throw things off. The rubber insulators and intake boots get hard and inflexible from heat and age.
The acceleration pump on Carb #2 also degrades over time, because it’s made of some fabric or rubber material, and it sits immersed in old gas all the time. You can get a replacement kit for very little money. It’s usually a good idea. You should also check to see that the pump mechanism works. There’s a spring-loaded ball bearing in there that gets corroded. It took me hours to spray and soak mine to free it up. Others drill out the plug that holds the ball bearing and clean it manually. I didn’t have to resort to that, fortunately.
