Ignition break up.
 

Have an FD with a T62, PFC, 850/1300 inj, walbro pump FMIC and other supporting mods. Well I've been trying to tune it richer for safety purposes and was getting ignition breakup any time the AFRs went under 11.4...I stopped being lazy and installed MSD box and FC leading coils with 10.5 plugs and it got way better but it doesn't let me get in the 10s 11.00 without some break up...if I take out fuel it gets better but I really wanted to stay away from 11.5s-11.7s... Any tips... My timing is really conservative and split is 15 across the board.

Running 8psi at this time.

The poor thing is telling you exactly what it wants to run right, and you are going nah nah I know what you want.....

A factory FC3S ignition system with factory plugs BUR7EQ/BUR9EQ plugs can light 11.5:1 mixture at 18psi just fine.

Well I had an FC with an E11 and never had these ignition issues.

Don't tune it richer, it is worse for your engine, keep it in the 11s. Your timing is conservative so then you are plenty safe with 11 - 11.5 afr. Richer is not better, it can build more carbon and cause more knock. No point in tuning for tens which will make it use more gas, loose power, more carbon and detonation, ignition break up, etc.

@8psi just give the engine what it wants...

Wideband might be not reading correctly. Had a similar issue a couple years ago. Did an install and was trying to rough a tune in on the street before heading to the dyno. Wideband said something like 11.5:1 and I was getting bad breakup. Fitted my own wideband and with the same map it bottomed out at 10:1. Did some back-to-back testing with a 3rd wideband on the dyno to confirm that the one in the car was the odd man out. It read the same as the other two around stoichiometric, but read much lower numerically as the mixtures fattened.