I replaced the “suspect” plug with a new one (top plug on number 1) and went flying. With that one change all cylinders had an increase in EGT instead of only 3 cylinders last when on the left mag.
The first table is the previous flight where the engine was very rough on left mag.
The 2nd table is after replacing the top plug on #1 cyl. The right mag was perfectly smooth running. The left was slightly rougher but no missing or bad vibration.
The next thing I may do is change the wires as clam suggested. At this point it is running good enough on the left mag.
RV-10 N331JH 3rd owner (First flew in 2005, #15 flying)
Northwest Regional 52F