My ST 90 did this. Man it was frustrating! I was thinking a brass tube was cracked and it was sucking air in front of the tank. Every time, no matter if the tank was completely full, if I pointed the nose down past a certain angle, it died.

What is wierd is that my engine runs amazingly, as long as it didn't point down. One person mentioned that the crank case could be filling up with fuel and this fix requires a minor carb adjustment.

I purposely ran the ST rich, being a ringed engine. Is the fix to lean both top and bottom ends up?