Well , although the rotor phasing was off i'm not 100% sure this was the problem that caused the nitrous backfire , could be down to too many variables , one being i might of let off the throttle before shutting the gas down , who knows.
Also thinking the cam grind may of played a role too , due to the late opening of the exhaust valve (cam retarded) , coupled with a tight LSA (large overlap) , the exhaust did'nt have a chance to scavenge (positive pressure) , hence not drawing in the intake charge , causing the nitrous/fuel to stack up in the runners/intake manifold. 
 
 
I realise the cam grind was totally wrong for a nitrous motor but @ the time i could'nt be arsed changing it out , now the motor is stripped i'm gonna swap the cam out , the replacement will have better suited valve timing events (valve events BBDC) due to an increased LSA , helping to eliminate pumping losses , will also advance the cam , really need that extra 100/10ths.  
 
   
 
Probably run like Bananarama! off the bottle. 
 
 
 