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Running that long indicates the engine is running out of fuel. You will probably need to go ahead and replace the fuel pump with a new one,(###) ###-#### ***** can test this by gently squeezing the primer bulb while running, if the engine stays running then the fuel pump has failed.
cleaning is not good enough, the diaphrams and gaskets get old, and harden, no longer pumping fuel. In 1979 Evinrude did not supply parts to rebuild the pump. You can get a new pump at discount from boats.net.
This should take care of your problem.