With that compression ratio you could run just about any octane. When you say it is back firing then you are talking timing. It won't matter how rich or lean it is, you are getting the fuel to ignite at the wrong time. Check on where your timing is set, and you can't always believe your timing mark cause the dampner can move if the rubber is getting old. Try checking this out and moving it till the engine runs smooth.
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69 351W Sportsroof deluxe
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