This machine has a different pump than the one you describe. There is no 3/4" plug on the top. I have good fuel flow to the transfer pump. I took the fuel line off the elbow on the hex (transfer pump screen) and then removed the elbow and screen at the same time. The screen had what looked like brass slivers sticking out of it. Further inspection revealed them to be some type of strong fibrous material. I pulled out as many as I could and then sprayed some cleaner in the elbow on the other end. I blew through the elbow and it seemed to have a restriction like there's a smaller orifice in the hex part or the fibers are a finer screen. It wasn't plugged solid though.
I put it back together and tried it out. I didn't have to bleed it cause I already had it running. I had the fuel line loose and turned the machine over just to lift the boom a little and it fired so I tightened the fuel line and loosened the injector lines to bleed it. It started right up so I lifted the loader all the way up and put an angle iron on the cylinder. It ran good for 45 minutes this time but then started acting up again. I limped it back to where I could work on it, took an hour to move 1/4 mile! When it would die or almost die, it would have puffs of black smoke for a few seconds then clear up and rev faster. When under a load it would just bog right down. Could it be a check valve problem or in need of a new transfer pump? Never had any problems like this before.