backfire after injector R&R

[dallas]

My son and I just changed out the injectors, cleaned out a little gunk, and checked all the hoses and wires under the plenum. I started and ran the car for about 20 minutes. It started right up, but did seem to have a slight miss at idle. You know...the little "pup-pup-pup" that comes out of the exhaust even though the engine sounds smooth up front. We figured it might need a little "relearning" from the computers since it was down and disconnected for several months.

I started it again today after a week of sitting. It started right up, but still has the "pup-pup" at the exhaust. This time, however, when I made some quick stabs on the throttle from idle speed I got a very sharp backfire Not a low "pop", but more of a higher pitched "crack" or "snap" sound. When punched from approx. 2000 rpms it responded very well and was smooth till I let off around 5K.

I checked all the hoses that would normally blow off or have a problem and they all seem to be attached and ok.

SOoooooo...what would be causing this backfire?

OH...the TB is open in front and I have not put the air filter tube back on yet.