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Hi.
I,d say- in a 500Hp+ 347, it sounds like a colder plug is the right thing. (Generally , higher performance, compression etc- a colder plug). NGK BCR7ES (?- am not 100% on the part number) is what I'm running. 2 grades colder than "standard" NGK5.
If you have the Webers well tuned, you should have very little fouling. I finally found a setting letting me cruise at 2000 to the track and 7500+ on the track without any fouling and good A/F- numbers. If you do some jetting changes, it will probably get better.
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