I was finally able to get everything cleaned off and defrosted and upon examination see that on the left bottom track )slider track) the plastic bullet shaped end cover has been wearing where the cogs contact it on the outboard (left side). The track looks centered all along both sliders but the top looks too far to the right and is rubbing on that plastic piece.
That would be Part #77 in the diagram
It appears that either the driver sprocket is too far to the right, or the track end is too far to the left, or possibly bent outward a quarter inch ir less. The machine runs well, but sometimes when the cog is in contact with the wearing part (77) it starts off hard with a lot of throttle necessary. Once it is moving it goes fine.
I'll pull it all apart this summer.