Motorcycle Repair Questions? Ask a Mechanic ASAP
I am an expert on vintage motorcycles so I can assist you.
It's very possible your fuel delivery is not consistent and it's running out of gas. The best way to check it would be to remove a drain screw from the bottom of one of the float valves and attach a fuel fitting (just a hollow bolt you can attach a hose to). Attach a clear line and run it up alongside of the carb body. Your fuel level should be about 2-4mm below the top of the float valve and should remain consistent while running. If it drops way down while running, the problem is a plugged tank vent or bad petcock.
You can also remove the fuel line and turn the petcock on and observe the fuel flow, but this is less accurate and won't show what's going on inside of the carbs themselves.
If you are getting a loss of fuel flow the longer it runs, open the gas cap. If that cures the problem, then repair or replace the cap so the vent works again. If that doesn't help then the problem is in the petcock. Remove it and repair or replace it as needed.
A bad air leak can also cause the engine to rev up when warm. Check your intake manifolds for cracks or leaks.
Let me know what you find.
It's a lean condition so the answer will be in either a fuel flow problem or an air leak problem.
The only other issue I have seen is if the slides are sticking when hot. If the carbs were properly services, that shouldn't be an issue.
When did the problem first start? What was your fuel level reading on the 4 carbs while running?