nitrous spark plug

I would back the timing up a LOT until you get the bugs worked out (like 28°). For NGK plugs I would use R5671A-8 for a 150-180 shot nitrous, -9's for your 250 shot. My nitrous motor used those plugs for 8 years, sprayed all the time, no problems....but ALWAYS keep and eye on your plugs. Proper plug reading should warn you if something is going south before you destroy your engine.