I just got the motor installed and started. It's a 4.3 TBI from an 89 s10 blazer. It's only got around 3000 miles on the motor so I'm ruling out internal mechanical problems for now. When I start it I have to hold the gas down a little to keep it running and ease it off until the IAC adjusts to maintain the idle, which it tries to idle at 750 when cold. It idles a little rough but not too bad. The engine temp sender is new and the IAC is only a year old. Spark plugs, wires, cap and rotor were new two years ago. I had the plugs out and they looked good.

When I drive, it feels a bit slow and like it's missing especially at 1800 RPM. I checked the headers and they all get hot imediately after starting so I think it's fireing on all cylinders at idle and it runs smooth until I hit 1800. I considering that I may have to much oil in it but I think it would be more consistant throughout the RPMs. I'm at a loss at the moment so any help is greatly appreciated.

 

I really don't think it's a miss at this point. It feels like the torque converter is unbalanced. I used the flex plate from the blazer which was balanced well but I used the TC and transmission that was in the Camaro which shifted good and may have had that vibration before but the 305 that was in there was about to blow when I bought it so it may have been vibrating before anyway. Any else ever have a TC vibrate like this?