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.
Probable causes of an engine running rich are as follows: 1. Excessive fuel pressure can be cause by restricted fuel return line or defective fuel pressure regulator.
2. Defective Oxygen Sensors.
3. Leaking injectors and fuel pressure regulator.
4. Coolant temperature sensor out of calibration.
5. Restricted air intake system.