Please see my reply to winston1.Depending on the Yanmar model and the hydraulic system under the seat, you may find a tiny lever to adjust in small 10 degree increments to slow the drop and lift speed. This is what one looks like on a YM2500, YM2610, YM3000, YM3110, YM4300 and possibly others.
Flip the seat forwards and look for this tiny lever to make adjustments.
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Eric