Z650 decompression lever adjustment???

Hi Folks,
I'm trying to find info on decompression lever adjustment on a Z650 engine.
I recently installed the Kubota on my woodmizer bandsaw mill but it is very hard to turn over cold or hot even with good batteries and decent cold cranking amp ratings on a warm summers day (all battery connections are clean and solid). Nothing happens when the levers are actuated in terms of speed of engine turning over. Once the engine has warmed up it starts every time on 2nd revolution, but very slow to turn over.
The attached pics show the 2 levers that go down through the valve cover, and the inside of the valve cover and it's related parts for decompression.
I can't find any info anywhere on this engine and even Kubota America doesn't have much info.
Any help would be hugely appreciated!
Cheers

Thanks for the replies guys.
The valve train looks just like any other (cam,rockers,springs etc). The threaded post goes through the valve cover and pushes down on the valve stem. It has a lock nut once proper tolerances are met. I didn't want to start threading the rod downwards to make contact with the exhaust valve in case once the lever was released that the valve wouldn't be seating properly and get burnt or worse yet make contact with the piston.
I think I'll get a Japanese friend to contact Kubota Japan and see if she can translate info for me.
I was hoping there may have been a north american version of this set up somewhere that people knew how to adjust.
This was an engine brought back from Japan so I have no idea what tractor it came out of.
Cheers