Its a points ignition system is it?
Tell you what my train if thought is.
With a points system you may still get a good spark even if the timing is way way out, you could perhaps get a spark that will burn fuel/air in the crancase...perhaps giving a loud sharp 'pop'...maybe (just guessing here)
how could it be that the ignition is so far out that this would happen you may ask yourself?
Simple...you dont torque up the flywheel tight to within specifications, then you try starting the bike...there is a backfire...this shears the flywheel key and the crank rotates some inside the taper
now your timing is way out and all your efforts to start it are in vain!!!
Check your ignition timing...if its good then all well and good, if you have torqued up the flywheel to spec...ditto
but if you didnt?!!!!