I've run a number of Edelbrock and Weber built Carter carbs for lots of years never had much trouble with them until recently. So I guess I'm somewhat familiar with making these work, but lately I've had some problems that have me stumped.

Problem 1; Edelbrok 750. Idles fine, accelerates fine, got decent mileage and over all worked great for many years. Over the last summer it started to develop a part throttle, cruise rpm surge. This was at a somewhat fixed throttle position, high vacuum, cruise. It would happen in town at low mph , but steady throttle, or at highway speeds at steady throttle. I tried a friend's 650 and the problem went away, so it is something in the carb. I rebuilt it and the surge was not as bad, but still there. Finally got tired of it on a recent long trip and decided it was time to ditch it.

Problem #2 Carter 750. Tiring of the above conditions and its annoying cruising manners and increasingly bad highway mileage, I dug up an old AFB Comp series 750 I had. Rebuilt it, installed it and I can't get the thing to idle under 2000 rpm. Double checked throttle blades to ensure they are closed, check all possible vacuum ports to ensure they are closed, backed of idle rpm screw until it is not touching the lever and it won't idle down. It idles so high, I can screw the idle emixture screws all the way in and it doesn't change anything. Disconnected the power brakes to verify I don't have a massive leak in the diaphragm. I tried spraying starter fluid around potential vacuum leak sources and got no change in rpm, however, if I cover the primaries with my hand it will idle down where it should be.

So, thoughts on which carb to pursue a fix on and what fixes would you suggest. Or should I pitch them both and put one of my older 600s in the carb cleaning bucket and rebuild it instead. Oh, not interested in swapping to a Holley, but thanks for asking.

I had a problem setting idle speed when I had the vacuum advance hooked to manifold vacuum. Not sure if you had all vacuum line disconnected and capped the ports, but it wouldn't hurt to check this if not. I also had the floats on my Ebrock develop pinholes and start sitting lower and lower. Can't remember the symptoms now, but if you pulled the float and shook it you'd hear the gas inside. Also, since it happens at cruise, you might want to make sure the bores for the metering rod pistons are smooth and clean, I've had them get get gunky and bind up the piston.

Just a couple of ideas off the top of my head.

Carb #2 has rust/ junk in the idle passage.

Try removing the idle screws completely, spray carb cleaner in the hole and blow air into the idle screw hole with compressed air. replace screw and set idle.

You might also check for a fuel feed problem, sounds like the problems I have when the fuel pressure gets to high.

Get a holley and don't look back.

The edelbrock needs new springs or heavier ones. The springs are probably causing the metering rod to flux like a jackhammer. Do you have a strip kit for that carb? Well worth a couple extra bucks, and order the pump nozzels if you haven't tried fine tuning with those.