The thing is, it should not be doing what you describe. It is an air valve secondary carb like a Q-Jet and the back barrels will not pull open without enough vacuum pulling on them. GM used Q-Jets on 231's back in the day, so it is not that out of the question to make it work properly. If it is really that rich, you probably need to replace the step up springs with the weakest pair. This will maximize the time the power piston stays in the down position and lean out the part throttle circuit. If it is still too rich, you can change the metering rods to a leaner set, or even go to leaner jets as a last resort. A spring assortment is around $10 and available from Edlebrock ( it's pretty much the same carb as a Performer 600), so try that first. I would also look at the booster venturis at idle. If it is really screwed up, they will be dripping fuel, a sign that the primary circuit needs some attention. Remember that a 231 will need a leaner part throttle calibration than a 350 simply because the fuel needs of a larger engine are greater. Most universal 600 cfm carbs are calibrated to run well enough on a mild 350, not an engine 120 ci smaller. You may also need to figure out how to lean out the idle feed restriction like people do on a Holley 3310, but I tend to think it will not be necessary. Also, what is the model number of this carb? Are you sure it is a 600 and not a 525? There was even a 400-450cfm variant available back in the day for dual 4 apps, IIRC.
