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Have you verified the plug wires are installed correctly?
Did you remove the upper intake and fuel injectors??
How did you set the timing??
Timing is set with a scan tool using the cam offset parameter. should be 0 deg cam offset.
Was the distributor cap in good shape?? no corrosion on terminals?? These caps can jump between two terminals inside. Pull the cap off and see if the terminals are running close together, and possibly jump inside the cap. These terminal may hook up on one side but will cross the cap inside, I don't have one in front of me know, but i remember this as being a problem with these caps.
How did you mark the distributor when you removed it??
And are you 100% positive it went back in in the exact location??
It sounds like you are one tooth off on the distributor timing.
Do you have a scan tool to check the cam offset? It will read 45deg if the distributor is off by one tooth.
This is very easy to do on this engine if the distributor is not marked good before removal. You can not go by if it drops back into the oil pump driveshaft , that it is in the correct location.
Ok it is out of time slightly but not one tooth off. you have to move the distributor to the drivers side a little, then rev the engine over 1100 rpm and the reading will change.
Get it as close to 0 deg as you can.
Note the reading will not change until the engine is reved up over 1100 rpm.
Set the timing, correctly first. Verify by tracing from the plug to the cap all the wires are in the correct position.
-11 deg will cause a lean running condition, and will cause a miss fire/back fire while driving.
The first one I worked on , some tech had swapped distributors, and did not time either one. The engine would misfire , and we worked for two weeks trying to fix it. had GM send an engineer out, and the only thing we saw abnormal was the cam offset was at -11 deg. no one new how to set it, or what it was to be set to. once we found out from engineering how to set it and the spec. we set it to 0 deg and the miss went away.
three weeks later the other truck came in with the same complaints. We knew how to fix it by then.
Unless you have the gaskets on wrong or are not sealing, and the wires are on correctly , new cap and rotor, and this was not there before. Then set the timing back to where it was before you started and see if the miss goes away.
Timing must be at 0 deg, the computer uses this as base timing, and will advance from there. It doesn't know the exact timing, only what the timing is from the base setting.