An engine running worse after driving it awhile is normally either the fuel delivery system or heat breaking down something electrical.
Fuel delivery could be a plugged fuel tank vent, plugged fuel filter, etc.
Electrical could be electronic ignition conversion, or a coil where the secondary resistance is going non-linear with heat.
The Zenith carb has no personality. If it works cold, it will work hot (providing of course that fuel is coming in from the fuel delivery system).
Jon.
