I'm hoping the timing marks on that flywheel are the same as on a Harley motor, but I don't know if they are. If not, you might have to call S&S and ask them what sort of marks you have. Anyway, I would go back over my installation and check each detail, like so:
First, gap your points. .018 to start, unless you have something that specifies a different gap.
Rotate the engine until you see the static timing mark line up with the timing hole. Straight up and down. Your timing mark should be a straight line, (I hope,) that follows a dot that indicates top dead center.
Rotate the mechanical advance until it is at full advance.
Loosen the points plate hold down screws just slightly and rotate the points plate until it just starts to open. Barely starts to open. If you're using a test light that is until the light goes out. Tighten down the screws and that should be close enough to get you running decently. You can then rotate the points plate very slightly in either direction to fine tune the ignition, but usually that's not necessary.
If you're still back firing and running rough, check your advance unit to make sure the weights move freely and nothing is binding up. You can try experimenting with your points gap too. Anything from, say, .012 to .020. Obviously make sure everything else on the engine, fuel and intake and so forth, are right too.
