Sounds like it is running very lean. I suspect
there must be an air leak somewhere around
the carb or intake manifold.
Check it with some carb spray or starting fluid.
See if the engine speed changes.
Check if the fuel petcock has a vacuum line,
the petcock diaphragm might be torn allowing
fuel to pass by. Maybe the float bowl is not
receiving sufficient fuel to keep it full.
Check tank or gas cap for plugged air vent.
Also check the ignition with a timing light if possible
to see if timing is stable.
Let me know any new info.