I've got an odd one, Got around to buttoning up a project vehicle i recently got (99' Trailblazer with an '01 Block). threw a battery in and went to crank it, has fuel, spark, 150+PSI Compression, It caught, ran like hell for a few seconds bouncing between 1000 and 1500 RPM while holding the starter on, it backfired a few times out the exhaust and made a fairly loud high pitched droning noise, shut it off to prevent damage.
To me it screams ignition issue, but i don't know what the noise it made was and I'm not up for a bottom end rebuild. Any ideas?, It had good oil pressure (>40 PSI on the gauge and seemed to be accurate), I didnt hear any knocking. it managed to shake the dash enough to force the temp and fuel gauges to droop to pointing straight down and peg the tach at 6k, it didnt get anywhere near that so im (hoping) its just a connector issue.

 

If it runs rough after rebuilding/swapping the motor, you might want to double check that you don't have some ignition wires crossed, or maybe your distributor off by a tooth or two?

 

I think the plugs are undergapped, for the life of me i cannot find the vinplate on the motor and i didn't fully trust that it was a VIN W Block as the previous owner had said, as they also said it was a 2000's when it was an 01 Casting. Today im going to tear the entire front end apart, recheck the valve timing and fix the balance shaft, and while the cover is off do the Distributor, it was at TDC #1 on compression with the rotor pointing at the #6 Stamped on the distributor housing, And then regap the plugs to .060. I laid the wires under the assumption that the cylinder order is 1,3,5 and 2,4,6 Front to back Driver and Passenger, Though i will recheck them anyways.

 

And just like that she runs, has one HELL of a vacuum leak (missing hose from EGR acuator) but thats this solved, just undergapped plugs.