having problems starting my vn

[smoke-me]

hay everyone i own a 1989 vn commodore and it wont start i have replaced the crank angel censor and the coil pack and i have checked the wires leading to the coil pack and its all good i have checked all the fuses and relays and there no problem with them, some times it will start for about 30 sec but then there wont be any spark if any one has any hints or tips on what the problem is could you help me out. thanks

[DrJeckyl]

edited post

[Zuicider]

is your harmonic balancer on all the way? also next time u have the problem put a screw driver onto the starter motor and see if she cranks