Is it a stock motor or modified for mild performance. Needles and jets are set for certain performance specifications. It could be lean with the a leaky shaft and stock set-up.
** I put a newer set of heads on one of my SBC 350's. I had to go up a number on the jets to get the Air/Fuel mixture right.
Let me know what you the numbers are on your jets and needles and I can tell you if they have been changed from stock.
If interested:
http://67-72chevytrucks.com/vboard/s...d.php?t=751483