#1: DONT RUN THE N2O UNTIL YOU GET THIS CORRECTED. Just friendly advice
#2: I would say take the timing back to 10 degrees(stock), and see if its still happening. I would also re-check the timing after you tighten the bolt to make sure it doesent "wander" while tightening. (You can also try a higher grade with the timing at stock, and see if it make a difference)
What do the plugs look like? Lots of ash, or oil fouling?
Some other q's would be, when did this start/did it come on gradually? Last general tuneup?
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'66 Mustang Coupe- AKA "Lenore" *on disability for a (long) bit*
'93 GT Mostly stock AKA "Fawkes"
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