That’s not a power valve issue. The only way fuel can blow out of the vent is the needle and seat aren’t shutting off the fuel.
Why it’s ok on the bench and not on the car I can’t say. If the float level is correct, the floats aren’t sunk and the fuel pressure is 7ish pounds or only way fuel can get out of the vent is the needle and seat isn’t doing it’s job.
When you were going through the carb, did you happen to remove the floats? Just curious. And IIRC those carbs had brass floats. At least they did in the 1980’s. I haven’t been into a newer one.
