390BigBlock,
If you rebuilt the engine, then what I’ll explain shouldn’t be a big deal…
- Check the fuel pump (Assuming its mechanical) with a fuel pressure guage. Look for a minimum of 7 PSI
- Check the float level of the carb (read the climber, chilton, or whatever manual you have for that…)
- Check the engagement and disengagement of the choke on the carb.
- After checking all this, read the plugs (one on each bank) to determine if the fuel mixture is good-- a nice carmel brown color is the “ideal”. A black color indicates rich conditions (suspect your float is bad) and a white color indicates lean. (Suspect jetting, or an intake leak.)
Has it always driven poorly? Did the “bad gas” you suspect start this condition? Is it a Holley 2BBL or Rochester?
Best,
Matt