"94 Geo Tracker. Runs fine, when 1st started. Begins to require high revvs to run, especially on take-off.

If turned off for a short time, gas pedal has to be floored. Runs very "doggily" until revvs built up & belches black smoke. If turned off longer time (hr+) starts normally & runs o.k.for a while. Tried replacing ECM. Ran fine for a couple of dsys, then back to problem. ECM has been tested by 2 re-builders who say it tests o.k.
"What's up w/ that?"
Thanx, Mack