Hmmm...I think there are a few possibilities.
1. You still have an air leak.
2. Your feed rate is higher than you think. (count how many times the check pawl clicks during the downstroke)
3. Your coal is undersized (or contains a lot of small pieces ) and is restrictive to air flowing up through it.
It sounds like you painstakingly assembled the stoker with new gaskets, so let's assume there are no big air leaks. If the boiler produces enough btu's to meet your needs and doesn't overheat, leave the feed rate alone and bump the air up to ~5.5, then repeat your experiment. You want about 1.5-2" of ash around the fire.