1971 Pontiac Lemans cannot find problem

Engine Performance problem
1971 Pontiac Lemans V8 Two Wheel Drive Automatic? Miles

3 days ago my engine started running poorly I cannot find the problem i'll try my best to explain, it starts fine but from idle to maybe 30% load the motor sputters/stumbles/backfires if you get past 30% load it clears up and runs OK the ignition parts are all newer the compression is fine (all over 130 psi) the vacuum is steady at idle if u push throttle it drops and quickly returnes higher than at idle then goes back to normal. The carb setting have not been changed and are where they should be. I dont have many vacuum hoses but I have checked them. The plugs read a little rich but I have not been on a long enough drive to get a good read. I don't want to get lazy and just think this is a fuel quality issue, any thoughts would help.