black smoke ??

[martiniqueln]

89 3.2 186K miles , smoke at start up 5% of time , lasts 3-4 seconds then gone .. 1 Qt every 700-800 miles

Still pulls stong

My question is that when I apply acceration say from 5k-6k in 3rd gear I can see a puff of black smoke for 2 seconds from the rear then clean again . No smoke on decel
If I do the same thing again I see no black smoke .

valve guides? Too Rich ?, Timing off ?
or just tired engine

Thanks in advance

[KCinBR]

Black smoke usually indicates a rich condition......that's all my knowledge