"Run on" "Dieseling" when shut off
I have a small block chevy with a Fitech #30002. It runs perfectly in every sense. But when I turn it off, it "Runs on" briefly. The initial timing is a 10 degrees btdc, idle at 825rpm. I have tried octane boost in increase fuel quality. I do not have any external spark boost accessories like a MSD 6AL. The distributor is a vac advance. I have had a similar issue with older vehicles with warning lights on the dash causing a power bleed after shut off but this vehicle does not have any lights. Could this be a injection tuning issue?
You may be a little rich at idle. Mine does it sometimes when I stop and immediately turn it off, but if I give it a few seconds to idle in park there is no issue.
Try adjusting your idle AFR. You can use max vacuum, idle quality and plug read methods, much like carb tuning.
This sounds like pre ignition caused by carbon build up in cylinder head try lowering idle speed and use a decarbon additive