Hi Fellas, i am having trouble with my AT 3 specifically when trying to start it is hissing and popping sounds to be firing in the exhaust ?  .thanks for any help  . Jez...

Jez if you have recently had the top end off retorque your head that could be the hissing it’s happened to me before 

If it's a magneto model with flywheel ignition i'd say the crank key has broken & flywheel moved putting timing "anywhere it wants".
But if yours is electric start then i guess similar could happen but a far less common fault with 'that' system.
Crank nut would feel less than tight on the flywheel. But on electric the center long bolt would also feel loose.
Check you're points are opening or retaining screw not loose.
All a guess but gotta start somewhere for spark to be happening with exhaust port open.

Hi fellas, so i finally got some time to have a proper look at the bike today. i was intending to replace the crank oil seal on the ignition side as i didn't fit the right one from the rebuild . i fitted a single lip one by mistake. when i removed the alternator i noticed the woodruff key was in the bottom of the casing, so the timing would not be right . hence the back firing etc. i changed the seal and replaced everything resetting the timing and she runs fine. school boy error. thanks for your help. Jez..

Congrats. 

Something satisfying about finding the cause by yourself & getting it running as it should. Top Marks!!.