I have a ST-90 side mounted (and its about 25 years old)

Tuned high end no prob; set low end; it runs fine on the stand at full throttle and hums along at low throttle; good transition back up again.

At full throttle, lift and point plane up in the air, and everything is fine. Start to point it down and it cuts out right away;maybe 2 secs even before it is fully down (maybe 45 deg incline down). The engine just dies like you hit the kill switch; it does not race up in rmp; just straight died.

Tank was probably at 3/4 full at this stage.

What is wrong; What do I change/adjust?