It sounds like either, the air/fuel ratio control valve is incorrectly adjusted (way too rich) or if you have the earlier style of air/fuel ratio control valve it may have a blown rubber diaphragm, inside of it.

To test for a blown diaphragm, just remove that little steel line that drops down from the intake manifold, and goes to the air/fuel ratio control valve (above shutoff solenoid). Then rig something up to add about 15 psi of air pressure to that air/fuel ratio control valve and it should hold that pressure inside of it.

 If it won't hold any pressure, at all, then the rubber diaphragm is blown, most likely. (#23)

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3406B_left side_in paint program.jpg (71KB - 926 downloads)

3406B_AFRC.jpg (198KB - 913 downloads)

3406 _AFRC parts photo.jpg (108KB - 924 downloads)

3406 _AFRC parts photo_in paint program.jpg (71KB - 1249 downloads)

3406B_scratch marks.jpg (99KB - 900 downloads)