Is your keystoker equiped with the full time combustion blower?? I believe the newer stokers come with the additional combustion blower, the for the older stokers this is a retrofit kit available from Keystoker..
If you DO have the full time combustion fan and it is running full time,, then in my opinion the coal is either not very good, or needs to burn a lot hotter than it is..
Is this ash from the stoker running 90% of the time just on the timer? How wide is the idling fire,, 1" or 2"? once the boiler is calling for heat, you may get a more complete burn..with the much longer firebed. But I really think you should not see as much unburnt and partially burnt coal
What breaker or coal supplier is this coal from??