I run 100LL as well and I did check the plug. Looked ok, but I replaced it anyway. It runs better after replacing the plug, but still the starting can be somewhat difficult. (I run Avgas since there is MUCH less smell, considering I keep my planes in a bedroom.)

With the last ignition, I did adjust the timing and believe it is set where it should be.

It will take several flips after priming to pop. Then it will pop for 4-6 times, then generally start. Not terrible, but not great either.