[roadhog]

I'm not sure if the ignition switch problem so common to many 911s extends back to 1970 but if so the switch can fail to engage the start contacts. If you pay close attention you may notice the lack of a spring return as you turn clock wise to the start position. It's common to replace the electrical portion of the ignition switch. I have had good results by simply cleaning the tumblers of the ignition switch with WD-40 you may want to try this since it is so easy.