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Is this the first time you have tried to start it? What wiring harness are you running? What type of ignition system?
I had a similar problem when I went to start the 400 in my '53 for the first time. I had a new wiring harness and had never fired the engine. The engine would crank, but I just didn't have spark. I called around MSD and Painless Wiring as I was running their products and it ended up that I needed a jumper wire on my ignition switch. I installed that, and it fired right up. Good luck, you'll get it figured out!
Ok if it is wired like a "normal" Ford electronic ignition.....you are missing the 12 volts to the coil + for starting. Usually this comes from a wire running from the starter relay "I" terminal to the coil +. This circuit bypasses the "run" ciruit during starting and sends 12 volts direct to the coil for faster starts.
Since when you let off the key it fires the "run" circuit is fine, including the igntion switch, the ballast resistor (if used), wiring, coil etc etc.
So make sure there is a good wire from the starter relay "I" terminal to the coil + and it should fire right up.
Hope this makes sense.
The truck has been wired up for almost 2 years now and it just started doing this about a month ago. It is a centech harness and MSD ignition w/out the boxes. okay i will see if the wire is hooked up on the relay, I had a starter relay get all messed up and so i replaced it so i will check