360 back firing bad

My 360 in my 75 dart backfires bad on the top end, it runs ok idleing and through mid range but when i stomp on it it back fires like crazy out the exhaust and sometimes out of the carb. The timing is set to 10° before. I took the carb apart and there was some junk at the bottom but after i cleaned it out it still does the same thing.
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This little gear in the distributor can move up and down like an inch, is that normal or is my distributor messed up?