How much of a fire do you have at idle? 190 degrees at idle sounds like a very high pilot flame. 4 minutes on out of 19 minutes total is about 21%, a very high rate for pilot mode. I run about 6% at idle, my boiler water runs at about 140 to 145 when there is no call for heat. The fire at this setting is pretty small, maybe about an inch or so wide and all the way towards the back of the grate.
I'm not sure why you experience outfires as you pilot flame is fairly large. Do you have the high temp control wired to cut power to the stoker or blower, or does it just turn on a dump valve for the excess heat.
You can try using a shorter on time and dialing back the off time, say 2 min on, and 8 min off, this will have the stoker running more often, but for shorter times, it might help somewhat.
The stroker length should be set for the maximum amount of red coal on the grate without pushing burning coal into the ash pan. After this is adjusted, you then set the on / off times for the desired pilot flame.
Member Coalmeister has built the variable speed control for the combustion blower using the Harman stoker control, hopefully he will chime in here with his thoughts about combustion air.