My 81 has points ignition instead of the stock electronic ignition. Based on that, should I use a plug like the 4-5 for earlier sportsters vs. the 4R5? Also, should I be using a different coil than what came stock? Still troubleshooting a hesitation problem referenced in an earlier post.
The '71 - '78 XL with points called for a 4R plug, so I doubt using or not using an R plug is your problem.
The '71 - '78 used a 31609-65A coil, and when they went to electronic they went to a 31609-80 coil. I imagine the ohms were different there for electronic vs. points, but usually when you use the wrong ohm coil, it just burns up and fails, doesn't cause hesitation problems.
I don't really think either of these things are causing your problem, but I will look for your other thread...