rmacAU may be correct but I would go slow with that adjustment. Those older models can be dead headed if not careful and destroy the pump. I believe you have a T handle in the same location as rmacAU's round handle. I also believe it is to be used only for locking the 3 point in the up position and not for speed adjustment. It may be possible your 3 point control valve has a little lever on the side of it for adjusting down speed. I do not know this for sure, just putting something out there for you to look for. Note this control valve (probably not just like yours) and the little T handle on the side. See if you have anything that looks like that. Yanmar Tractor Parts: HYD VALVE _
Explain "control lifting capacity", I am slow. If I am understanding the question right it is controlled by a system relief valve.