I have 2 ST51 and they are great. I got a used ST90 and after 2 months gave up and bought a new one. It would just cut out mid flight. Also, I found that to prevent mid-range from being too rich, I was always too lean on the low end which also caused problems in-flight.

The new one is better but still not great. As others have said, you need to have a rich low-end and run a high idle. I still find the mid-range rich and sloppy. I will try a OS #7 (vs #8) glow plug as some have suggested that helps.