Serial number 5u17156 
After letting the big motor turn for a couple minutes I  push the decompression lever to run the pinion drive immediately disengages and the big motor stops turning.
if I’m not mistaken there’s a oval-shaped inspection plate with two bolts to be able to look at that pinion drive.

May be main is kicking over and releasing pinion larches. Inspection port left side pinion housing. Adjust latch screws after removing Cotter pins. Try tightening 1/2 revolution on both latches. Only adjust 1/2 revolution of crew at a time.

Do you have manual(s)?

Yes correct, on my D2 3J it is under the fire wall/rear panel, you can see the end of pinion thru hole but not much more, Sounds like an adjustment could be required to hold pinion locked in. Thru the hole you can adjust the pawls, adjust both equal amount and not more than half a turn each time as once to far it will stay locked in to fly wheel ring gear. I hope this helps.

Shut the diesel engine throttle lever off! let it turn over on compression a few minutes then open the throttle.

Screw 3B5131 Latch 3B5441. This is a D4 parts book but I think adjustment is the same.

you can hold the clutch engaged till it starts but yes i would say the pinion latches are out of adjustment and its throwing out too early. did you have the main throttle open or off?

It sure didn’t appear that the main was trying to start, fuel Lever definitely in the off position.
And just what kind of a magician do you have to be to get to them cotter pins out and then turn the adjusting screws on the latches.

And yes I did hold up the engagement lever ,decompression to run ,hit the throttle fired right up purrs like a kitten a big one. Now more problems I think.
First went to put it in gear and it was grinding, hand clutch definitely disengaged , Then pushed the lever forward to try to stop the rotation after about Many attempts of exercising clutch lever back-and-forth I was able to put it in gear and move the machine forward and backwards maybe did this six times moving 6 feet in either direction.
Maybe sat there for a minute went to put it in gear ,hand clutch disengaged , grinding gears again push lever forward pull it back push it forward finally rotation Stopped, I was able to get it in gear again
any thoughts and suggestions thanks
 

To Neil thanks Just short of me leaving through all this material than dad had
On your fly wheel removal, socket and impact nut off then did you need or make some kind of puller to remove fly wheel
How many d2 s. do you have