jegssr,
Pretty sure your model has 4 cap screws (10 MM head) that bolt through and cover the inspection port on the side of the injection pump. Remove the cover. With a flashlight you should be able to see a small metering rod...the same rod that won't allow you to remove the pump unless it's disconnected first. IF the rod has come loose you'll have problems like the throttle not responding in any direction....and the tractor will either not start, run at a very low idle or run nearly wide open. If the rod is broken or somehow disconnected you will have to remove the four nuts/washers that fasten the pump down to the timing case cover, reconnect the rod and you should be good to go.
If the rod is not the problem, you might want to remove, clean and inspect the fuel pipe connections at the top of the pump. The "O" rings can split, dirt can trap there, but you would still get fuel to the injectors.
Since you've opened the lines at the injectors and verified you're getting fuel....this can sometimes be a bit deceiving as you need 2175 #'s of fuel pressure (cracking pressures) to open/inject the fuel. I would first check every possible problem related to the fuel, filters and delivery system (all the way back to the tank) before dismantling the pump. The pumps are a very complicated piece of engineering, more moving parts with close tolerances than your entire engine. The hemispherical beveled bears that run on a tiny rack gear are as intricate and a Swiss watch and timing is critical. I have, twice, removed and attempted repairs and both times surrendered them to the specialists at a fuel injection pump rebuilder. ($300. X 2).