serging on the bias

The manual and VCR tape for the 936 both provide good instructions for serging curves, which should be of help learning to serge bias (since at least part of most curves are on the bias).

Practice on lots of scraps until you discover how to hold the fabric at just the right angle, try altering the cutting depth, experiment with the differential feed settings and different stitch lengths, etc.

Bias is not for the faint-of-heart under any circumstances.

My new Viking 905 will cut the fabric but does

Whoopsie, my mind translated "905" to be "936". However, similar advice applies, unless the 905 does not have differential feed and adjustable cutting depth.

It does. I had a 910 for years: same machine, but with added memory and advisor... With practice, it will sew bias rolled hems very nicely. BUT! Make the cutting width quite a bit wider than the recommended cutting width.

The 905 does have differential feed and adjustable cutting depth. I have one. If it's fine on the straight grain then it's probably a need to adjust the differential feed more than a need to adjust the cutting depth.