Jim, I would guess that what is going on is that you are having a valve related issue.  Either your valves aren't adjusted properly, you have a push rod that is not centered in a lifter (my bet), or that your lifters aren't pumping up and causing a valve issue that way.

Pull the valve covers off.  Look down each push rod tube with a flashlight.  You should see the push rod centered in each lifter.  If that is true then doublecheck your valve adjustment by turning the engine to TDC on each cylinder in turn and setting the valve lash to zero (just touching).  If your lifters are soft you should adjust the valve screw until you can just feel the lifter start to compress as this will allow a spongy lifter to pump up properly.

If all this checks out then you at least know that your valve timing and adjustments are correct and you can move on.

Let me know if I can help you further,
Ken Wilford
John 3:16
www.vanagain.com

-------------- Original message --------------
From: Jim Felder <jim.felder@GMAIL.COM>

> Got the right head on this morning, hooked everything up and it started
> right up, at least from the sounds of the initial idle.
> It is powerless, though, and it feels as though the throttle is connected to
> the accelerator by a long, soft rubber band, meaning that there is a strong
> lag behind pressing the accelerator and the rev of the engine.
>
> The engine may (or may not) idle nicely for a while, and then the rpms fall
> and the engine quits.
>
> I know the spark plugs are going to the right place. Could I have the
> injectors swapped? Could the new, unprimed lifters be doing it? My
> experience with setting valves on cars tells me that it couldn't be this far
> out by making a mistake in the valve timing, or could I be wrong about that?
>
> Thanks,
>
> Jim