In your picture you posted as "valve" The silver hex fitting opposite the inlet hole looks like the pressure relief. Try to take that cover off and see if there is a set screw in there that you can back off some. That will lower your max system pressure. I would recommend you have a gauge installed before you try to adjust it.
Like said above the engine is fine, just idle it down if you want to run slower. Force of things is only the pressure setting of that valve. Engine size has nothing to do with it as long as its not stalling out.
