Big thank you to John!!  Timing 180 degrees out and it's now a runner again.

Were both the valves closed,ie on compression stroke, when you timed it.
You've described exactly how to set up the timing.
Have you rechecked that the points still open at 1/2" btdc cos sometimes the mag shaft can slip when tightening the nut..I find that tightening the nut finger tight and then putting a larger socket over it so that the socket sits on the mag pinion and then tap the socket onto the pinion with a hammer ( not big thumps just a tap) will normally lock the pinion onto the taper and you get less slipping when the nut is fully tightened.  John

Trying to start my 1938 sv 600. I recently took of the oil pump side cover (internal pump) and have put it all back together and have timed it up as 1/2" before top dead centre, fully advanced, points just opening. It backfires!! and won't start. Have I done it wrong or are the settings different. It worked ok before work but it was such a long time ago that my father and I first did the timing. Any recommendations?