Thanks everyone for the suggestions.
1. I will reduce the exposed length of the auger in the bin; I'll try three full flights.
2. The gap in the flights in the coupler is indeed close to 2.25"
3. Will try it with pins pressed in the coupler, not bolts.
4. I have a new, thicker, smooth auger extension pipe, will try that.
5. I have carefully inspected the stoker auger and it is in similar excellent shape to the extension auger.
6. the angle on the joint is only a few degrees; I will try it with the joint straight.
The measurements I posted initially were entirely wrong
. I have a number of different bit coal augers/auger tube measurements written down and somehow posted those NOT the measurements for the GJ stoker that I had questions about. The correct auger/tube measurements for THIS stoker are as follows:
stoker pipe - ID-2 1/4" auger - 2 1/8"
feed pipe (extension) 2 1/4" auger (extension) - 1 15/16"
If this doesn't work out, I may just get rid of this stoker and buy one that is a similar straight -through design with a larger (as close to 3" as possible) auger/auger tube. I would just use one of my bit stokers, but I'm trying to do a bin-feed setup.