Saving Old Seagulls

Howdi
I have theis LLS century
It runs and starts but it seems to miss quite dramtically every 4 or 5 seconds, or backfires
Should I be leaning it out
Or is this a timing thing
\Is it possible to have the timing wrong on a century

I have a silver century too , it runs quite well but I noticed the alignment of the spark plug and the scrrew to hold the magneto plate (term) in place is aligned differently
on the missfiring Century the tightening screw isn't dead center on the right hand side(facing flywheel side of engine)?

Are they different between these models
As they are the same engine basically one would assume they're the same timing

the LLS should have a different magneto than the later one but the position of the sparkplug lead and the baseplate screw should be the same in each case. looking down at the flywheel from above (and from the front) the plug lead should be at 4 o' clock if the screw is in the correct position