If you indeed have all the air out of the system as mentioned by UncleD, then perhaps it is a problem of not enough fuel. On the right hand side of the pump, above where the lines go out is a screw with a locknut on it. This is commonly referred to as the fuel screw. Loosen the locknut and turn the screw in about a half a turn as the car is idling.
Do you notice that it gains RPM? Good, now adjust the idle back down some by loosening the locknut on the screw that the throttle lever is touching. You have increased the amount of fuel going in at a given position of the throttle lever and need to back the lever off the screw stop to reduce idle RPM.
Tighten the locknuts on both screws and give it a test drive. Repeat process until you get some speed out of it or you start noticing a bunch of black smoke out the tailpipe. At that point you have screwed around with it enough that you are wasting fuel and it is just plain silly to do that. Back it off some if you want to have any amount of miles per gallon.
You don't need to move that fuel screw much. I said turn it in one half of a turn but on second and third turns it might be better to only go a quarter. It is very sensitive and a little bit goes a long way on that one.
Other things might be need as well but start there and tell us what it does. Having an old unknown pump working is a good thing but not knowing settings on it is sort of a problem.
I hope they sent his old pump back to him. He could likely sell it and recoup some of the 500 he spent. Seems a bit steep for an old IDI pump. 250 is more like it.