I have a murray ride mower that will not fire. I replaced the magneto. If I un plug the plug the safety plug it have fire.

Hi,
Not quite sure which connector you are saying you are unplugging though on this machine if you unplug the wire the connects to the harness and grounds the ignition to shut it off, you are also removing the power from the backfire solenoid which means it will not get fuel. Suggest it is probably the key switch, my guess.
Here is a link to the owners manual:
http://bsintek.basco.com/BriggsDocumentDisplay/default.aspx?filename=UemmACTLnfBhU7y
And I will post the schematic separately though it is in the back of the manual along with the rest of the parts breakdown.
Peace,
Clarke