F/A ok at idle,, but leans out under load
 

I have a SSI heat exchangers with a O2 sensor installed to gauge... on my 76 2.7.. Anytime under load,, cruising at 60- 70 mph and especially accelerating the gauge goes from 15 ish to 17 ish, the leanest number on the gauge.. F/A is fine at idle.. I'm at a loss... thanks, g rad

Assuming you have an accurate wide band, you are lean.

You might have a leak on the intake side. Maybe a screw or plug fell out.

When I had the engine rebuilt, I used, can't remember why, a different throttle plate body... I have a 76 (my year) vacuum diagram,, so I have been changing them around,, to good effect,, with the intent of returning my original first opportunity.. at least the F/A gauge is nonfluctuating every time I accelerate or under load... Thanks for the reply......... g rad