Ok, I did a L28et swap on my 240Z, and i am not sure if that is how it start up. I turn the key to start and I hear the engine cranking, but won't start... and as soon as I release the key from start... it's started. Is that how it suppose to work on the L28et???? or I am missing a few wiring??

Your L24 had two ignition circuits, represented by two wires connected to one post on the coil.

One is the Start circuit. This runs 12 volts directly from the ignition switch to the coil, Only when your key is in the start (cranking) position.

The other is the Run circuit. This one runs from the ignition switch to the coil, but first, through the ballast resistor, witch drops the voltage to around 6 volts. This operates after you release the key after starting.

This arrangement was used on points style cars, to preserve the contact points and extending their service periods.

From what you have described, something is afoul with your Start circuit. This has been found as faults in the ignition switch or errors when reconnecting the coil.