Initially, my first OS.91fx ran erratically because it came from the factory with a loose "banjo" fitting on the needle valve. If you can rotate that fitting (see picture) using moderate pressure with your finger tips it will be a source of air leakage. To stop the leak just tighten the nut against the fitting. The nut only has two flats on it and may not look like a nut at first glance.

All of my OS 50sx's, 91fx's and 160fx's have back-plate mounted needle valves and all run perfectly.
No dead sticks, no over-heating, no finicky needle valve adjustments, no anything except strong reliable operation. "Lucky me". I wish everyone the same luck.