I have replaced the ignition coil, the modulator, the key switch, and the ignition. Every time I turn the key to start it it just turns over and when I go to let off the key you can hear it get the fire. I don't know why it's not firing like it's supposed to I've replaced everything that starts it except the starter and my starter is nice and strong. Any helpful ideas?

 

Yeah. This is a DuraSpark II ignition system, right? With the distributor cap off, you would see a 8-toothed gear-looking thing, the Reluctor. Each "tooth" on it passed a pickup coil that has a magnet as part of it.

Check This:
The pickup assembly has 3 wires: two wires for the pickup coil, they may be orange and purple, and a ground shield wire. Disconnect the plug outside of the distributor to the pickup. Then using a DVM/ohmeter, read the resistance between the two pickup coil wires. If you read something way high, like greater than 10,000 ohms or infinite, then there is a break in the wire inside the pickup. To replace the pickup, you have to get the Reluctor wheel out of there. Drive the roll pin out of the side of the Reluctor wheel, then carefully pry the Reluctor up and out with two screwdrivers, one on each side of it underneath it. It's brittle. If you break it, you CAN get another.

The DuraSpark II system fires the coil ONCE when you let off the ignition. I had a pickup coil open up on me with heat. It would crank and crank, not a pop. But when I let off the key, it would start!
It turned out that when hot, the pickup coil winding would open up. If the engine was running, the break in the wire dropped the pickups output, but with engine running it still generated a high enough AC voltage for the ignition module to make a go of it. The problem was trying to start it if it was open... the AC output voltage of the pickup was too low at cranking speeds due to the break. But most of the time, the one spark from the releasing key kicked the engine just right to get enough output to run. Till it didn't...

Test yours, it's easy to do.