I'm having trouble with an older gm motor

I have a carbureted intake 355 midly built, adapted to an older gm two barrel tbi. For some reason it has to be timed extremely advanced to run correctly and if you hit the gass to quickly it backfires through the throttle body. What could cause this?

You're saying that the spark is firing while the intake valve is open (backfire). I would check the valves, lifters etc. fix it then re-time it.

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