Installing rebuilt MFI pump
 

I just got my rebuilt MFI pump back for my 71 911E. I did the procedure to of setting the FE mark on the pulley and aligning the marks on the pump pulley before installing the belt. I saw some posts regarding the need to purge air out of the system. Is this something that always needs to be done? The reason I ask is, my car won’t start after repeated attempts
Thanks for any help

It's been a long time since I had my pump done, but I don't recall doing anything special to purge it. The fuel delivery to the pump is recirculating, so that should take care of itself by running the electric pump, and I'd think the flow to the injectors would as well given the high pressure.

Just to confirm, you let the electric fuel pump run for 10-20 seconds to pressurize the system?
How long did you crank? It may take a few attempts to pressurize the injector circuits.

Assuming the above, I'd check a few basics:
Is fuel getting to the MFI pump?
No blockages anywhere? Make sure you didn't put a plug, rag, etc. in any of the fuel lines (or the pump) and forget to remove.
Are the injectors spraying fuel? You should be able to smell it through the intake/filter (though might be hard to distinguish between fuel from cold start).
Cold start is operational?
If the above check out and it's still not starting, make sure you're getting spark, since that's the other main thing you need to get an engine to fire.

I recall I also had an issue getting mine to start initially. I believe I'd set the FE mark incorrectly.

Find #4 TDC and continue clockwise to the FE mark.

Running your electric fuel pump will only circulate fuel around the plungers/cylinders in the fuel head of the MFI pump. No fuel will get past the pressure valves on top of the plungers with just the electric fuel pump. Cranking the engine over with the starter motor @ 200 rpm will take some time before fuel gets to the injectors. Keep a battery charger connected to your battery. This will take a few minutes to bleed out all the air in the fuel lines and injectors.

I’ve always primed the air stacks with fuel from a squirter or bottle on initial start up. Cold start will work once or twice, just not enough. Be careful not burn your car down.

Yup. I use a rubber squeeze bulb full of gas and shoot it right down the stacks after removing the air cleaner. The thermal time switch, which fires the bypass solenoid on top of the fuel filter console, will heat up under electrical current and time out too soon, and not allow the bypass solenoid to supply enough fuel for that initial start.

Kinda like this. You can pretend you are a Top Fuel crew chief. That's gasoline in the squeeze bottle that he's shooting right into the butterflies. Like Gordon says, though, be careful not to burn the car down. Or the house. Do it out in the driveway, fire extinguisher on hand.

<iframe width="560" height="315" src="https://www.youtube.com/embed/2x2SBXEDpg4" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" allowfullscreen></iframe>

Keep a garden hose handy. Unlimited supply of fire retardant. Less damaging than many fire extinguisher chemicals.