There is no such thing as a "bad carb."
There is a dirty carb and there is a broken carb and there is a carb that has the wrong pieces in it, or a carb that is tuned incorrectly.....
I have a 1409 on my 454 and it performs flawlessly. The previous owner is the one that bolted it on (to save gas) and I have no idea if it was straight out of the box or if he messed with it. I've never done anything to it except run gas through it. The edelbrock is easy to rebuild with a kit, but I'm not convinced that your problem is carb related.
Backfiring and paffffff sounds (to me at least) sound like timing problems. Edelbrock have good documents on their website as far as tuning -- but if the boat won't even run, you aren't quite to the tuning stage yet. Are you sure your distributor is on the right tooth on the oil pump? Are you sure that your spark plug wires are in the correct order? Are you sure your distributor is in good shape?
Former owner of a 1987 Supra Saltare. Current owner of a Malibu 23LSV.