Deere 5020 that is used 50 hours/year as a PTO mule. NA 531 inline 6. Roosa Master injection pump. Got it out of the shed and hooked it up and she is short of power. Every other year by the time you pull it down 200 rpm it is rolling enough smoke to make a 16 year old jealous. This year it pulls down easier, and only has a little smoke. I changed fuel filters, no help. The fuel line to the injection pump is seized to the flare nut, so I can't unscrew the fuel line. I will cut and patch the fuel line if necessary. My question is "what else do I need to check on the injection pump before I cut the fuel line, if anything?"
fuel line is also a good start. The suction side lines can collapse internally and make a metered restriction that will start and run OK, but limit the top end flow.
Screen on the pickup in the tank? Any rubber suction hoses, check for cracks, or just replace. For the fuel line, you could try two hammers, one medium held behind the nut, and a tiny one to smack the opposite flat, work around the nut and see if it pops loose off the line.
This is what your shut off valve looks like in the fuel tank. Probably needs to be removed and checked for a blockage.