D6 9U starting issue after rebuild

Are you sure you're getting fuel....takes a good yank on the throttle to get it to open past detent. Next would be to take the side cover off the fuel injection pump and check to see your getting full throttle rack movement and no stuck lifters.

What is the fuel pressure reading when cranking?

Have you run through a complete bleeding process to get air out of the system?

Line leaks need to get fixed but that is probably not the root of your problem. Check for split ferrules on the ends of the fuel lines. No amount of tightening will fix that.

Since you have electric start, you may need to use air pressure in the fuel tank to get the fuel system bled out completely. A small piece of an old tire inner tube which has the valve stem in the center of it can be clamped over the fuel tank filller opening with a large hose clamp. Apply air pressure until it bulges out tight and then go through cracking bleed screws and work your way to the injection pumps or the fuel lines at the injectors if you don' thave one of the little tools to reach in behind the lines and bleed the pumps. Once you get clear fuel there, go for a start. If it's cold, you may need quite a spray of ether. I would use about 2 seconds worth into the oil bath air filter.

You can also pull it to start it. If you need, you can pull backwards in 4th gear, but not in 5th. Fifth gear is forward only, no matter which way you put the reverser. Remember, the reverser goes forward to travel backwards, and back to travel forwards. Get it wrong and you'll get oil on your pants and shoes from the air filter.

Thanks for all the advice. After 6 months of rebuilding, I got her to fire up today. The air pressure to the tank using an old inner tube was a great little trick. I got all the lines bled and it managed to fire. Unfortunately, it was running a little rough, so I still got to play with the injectors and fuel lines to get it right. A couple lines from the pump to the injectors seem loose no matter how tight you make them. They started spraying fuel after a few minutes of running, so I'm guess those lines need replaced. I'll have to post some pics once she is 100%. Thanks again!