SS is right. A bad timing chain can be so loose that it presents a double image. When I had this problem I did the timing chain before I tackled the cam tip for just this reason.
There is also the issue of the large gear on the cam end of the chain. These are not supposed to go bad. I don't like that. They don't want me to change the large gear? But they do not have a reason I can accept for the uneven firing. And the cam bearing, nobody ever says to change the cam bearing. Why not?
There's a lot of stuff on this issue. But nobody seems to know why it happens.
It is most important what it does at full advance. I rode mine with the double image for quit awhile.
I also like the idea of a crank mounted distributor. I had one. I got rid of it. I decided I didn't like the electronic advance idea. There may be nothing wrong with it but it wasn't for me.
This was also several years ago and the units available were rather flimsy. I think they are better constructed now.
Never memorize something you can look up.