bike stops running after an hour ride consistently

hi-new to cycling-so i really know nothing. bought 1980 kawasaki  440 ltd. dies after 1 hour of straight riding. will not start but still has "light" spark. leave over night and will run for another hour. o.k. if just 1/2 hour spin?? replaced spark plug coil and same thing. points and condensor look brand new. what does anyone think; rectifier or mag coil? 

 

Perhaps the tank vent is plugged? Try opening the fill cap after 45 minutes and see if you get past the 1 hour mark.

Fuel line might be kinked or partly clogged. As I recall, the 440 has a vacuum operated petcock. If so, it may be malfunctioning, indicating it is time to clean and rebuild it. Or the vacuum line to it is leaking, or both.
Have you performed all of the routine maintenance listed in the factory service manual and the owner’s manual?

Here is a downloadable pdf copy of the factory service manual.
www.kzrider.com/modules/ServiceManuals/kz440.pdf

...maintenance like valve adjustment?  Tight valves often cause problems after the bike is hot.

Kinda need more info.

When you try to start it after it dies does the starter turn quickly?
Do you always have to wait overnight before it will start again?
What do you mean by "light spark"?