Learned more about this from Avstar
After installing the straight arm, I still have the problem for the first half inch of throttle travel when the engine is hot, as during a refueling stop. When cold, it works fine.
I called Avstar and found something I did not know. When an engine with their servo is shut down and there is no fuel return line, pressure builds up between the servo and mechanical fuel pump. On my Dynon system, it got to about 150 PSI and then a red X appeared. Avstar says on their system, this high a pressure results in a side load which ultimately causes the problem I have -- hard throttle for the first half inch or so. They recommended that prior to start, after the engine instruments are working, to move the mixture forward to drop that pressure to 60 or below. This releases the side load and lets the throttle run smoothly again. I tried it today and it works.
As an FYI, I have 890 hours now on the experimental IO-390 Lyc sells and this problem did not occur until recently.