Neil wrote:

Yes, I had it in my mind that it was a single throw crank but it's a double throw so no need to check the mag rotor for the second cylinder. Overthought that one! : )

Neil you have been hanging around those Harleys too long!! At least you have the capacity for constructive thought but the thought process left some of us years ago! On the serious side I really enjoy your comments!

Did you ohm the wires ? I had new wires on my pony with backfiring ..... even though it showed spark at plug it was still a bad wire.....replaced it and sheathed them with heat shield. Ran perfectly for 9 years now. .....knock on wood.
Have a good day.......Bill

What Cysco said about checking ohms in the wires got me to thinking, are your wires copper core or the more common carbon core wires?

Cysco & Ol Gump - wires are new and copper core.

I followed the advice of Neil and others and pulled the #1 head and checked the sequence. I was using the wire on the left side of the mag for cylinder #1, and when the points were opening on that port, cylinder #1 was on the intake stroke instead of the power stroke. This explains the backfiring through the carb. I put the head and mag cover back on and swapped the wires and after a couple of cranks it fired up. I shut it down because there was no coolant. Filled the coolant and called it a day. It appears that the right port on the mag is #1 and the left is #2. I had the wires backwards. I did swap the wires once earlier, but it's possible there was too much fuel in the cylinders to fire when I did that, and I didn't try very long when I swapped them. Thanks to everyone who had suggestions.

here is something to remember

just because a plug has spark outside the cyl DOESN'T mean it will work when under compression.

i have been caught with this myself with stationary engines. fitted a new plug and away it went.

any moisture dirt or carbon on the plug can stop it working.
a little bit of 2B pencil can help a sad spark. clean well with metho or rubbing achohol before applying only to the mating surfaces of the magneto and the plug body.

cat magnetos can be wierd ours was the plug leads didnt go how logic suggested i.e. left to left right to right they were the opposite spent half an hr before we figured it out.