Ron - and others.
First, you are not a "pest". The only really dumb question is the one left unasked.
Something which many fail to realize is: THERE ARE NO PERFECT "OUT OF THE BOX" AFTERMARKET CARBURETORS.
Your Carter 600 is actually a Carter 625 that was renumbered to compete with another brand's 600. Yours was originally calibrated for an early 70's 400 CID small block V-8.
AS A GENERAL RULE, the Chevy small block likes a huge differential between the high vacuum and the low vacuum step on the metering rods. The Chevy 6 cylinder likes proportionally more fuel at the lower RPM spectrum, thus generally requiring a smaller differential between the high vacuum and low vacuum step. You may also need to change the step-up piston springs if you have a wild cam. The stock springs are probably OK if you have at least 14 inches of vacuum at idle. Less than that, and you should experiment with the springs.
You may see no improvement at all, or even see a HIGHER E.T. if the carburetor is not properly dialed in.
The Carter AFB (MOO) is one of the most tuneable (and responsive to tuning) carburetors ever built; BUT IT MUST BE TUNED!
The general procedure with the AFB is to install the carb in stock form.
Then verify that the step-up pistons are functioning as they should. If not, the springs should be changed to allow the step-up pistons to function with the vacuum available.
The next step would involve the adjustment for steady-state or high vacuum. This should be done using an O2 sensor (or some other form or exhaust gas analyzer). This adjustment normally involves changing the primary jets.
Once the high vacuum adjustment has been made, one needs to adjust the low vacuum on the primary side. This is done, again with the O2 sensor and the secondary side disabled. This adjustment will use the same primary jet, and vary the high vacuum step of the metering rod (changing rods).
Once the primary has been adjusted, re-enable the secondary side and again adjust with the O2 sensor. This adjustment is made by changing the secondary jets.
Each style of carburetor will benefit from a step-by-step adjustment; the AFB should be adjusting in the steps above.
More than likely, the Holley 475 which you had is a more ideal size; but I can't help you with that, as we do not do Holleys, nor will we ever. However, there are plenty of people that do Holleys.
The biggest difference between dialed-in Holleys and dialed in Carters of the proper size (again MOO) is fuel economy. No one ever accused a Holley of getting good fuel economy.
And check my signature about the "wrong" carburetor.
Jon.
