Thanks all for your input. I've hoisted the old 1229 up and crawled underneath. No immediately recognizable adjustments for the walking foot and feed dogs seem ok if perhaps a hair below the face plate.
With the outside off and innards exposed, there is what appears to be a cam adjustment for the feed dog height and I'm summoning the courage to tease it. Then I'll probably be off to the sewing shop for the 50k tuneup.
One of the first projects I ever made with this machine, way way back, was a mylar vest that makes me look like michael jackson on a bad hair day (but very warm at 2 oz). That stuff, some kind of sandwiched aluminized mylar scrim, was very thin and I had no problems handling it. My guess is machine adjustment will solve the silnylon problem.
Again, thanks all.