OK, I have solved that mystery but now I've got a new one. After a couple of hours of trying different things like putting spacers under the stoker mounting plate to lift it up ( that increased the travel backwards but reduced it forwards) and adding spacers between the pusher rod and the yoke paddle (couldn't get that to work any better), it finally became apparent to me that the distance between the yoke arms was too great. The cam rides in those yoke arms and the contact with the cam bearing on the inside of the yoke arms is the action that moves the pusher back and forth. Some how, the yoke arms had spread apart and that was the slop in the travel. I removed the yoke, placed it on the work bench and "adjusted" it with a hammer, closing the distance between the arms. Now I've got full travel and the buck coal feeds fine. The new mystery is how did the yoke arms get spread apart? Anyway, the boiler is working fine but I'm getting cold feet about buying 24 tons of buck since I couldn't test it under real world load conditions. I'm leaning toward getting rice and a small quantity of buck to try when it gets cold.
Thanks everyone for your help. It's really great to have a bunch of guys available so quickly to bounce things off of. My wife will listen , but she's not much help otherwise on stoker problems.