There's a difference between and 'injector' and an 'injection pump.'
I'm having a hard time visualizing what you are doing and what you've changed on your mower.
First, you've replaced the factory fuel filter arrangement, which was a primary/secondary filter system, with a spin-on filter like the one shown in the picture of the L-3000 I posted? Is it located between your lift pump and the injection pump? If so, once you see fuel come out the bleed screw on that filter housing, the bleed screw needs to remain shut from that point on.
Then you open the bleed screw on the injection pump. You may also have to 'crack' an injector line open and have someone crank the engine until fuel appears at this loose fitting. Tighten it. You may have to repeat this with a second injector. At some point the engine will start and clear the remaining air out of the system and you can shut the bleed screw on the injection pump. It will run rough until all the air is gone.
If that doesn't work, I'm not sure what to try next.