I have a 1963 Riviera and have had nothing but problems with the carb. The original 4 Jet was in pretty bad shape so I replaced it with an Edelbrock (AFB style). The problem is I have a real bad off idle stumble, and I don't know what to do about it. It is a pain to drive it. Help!

Read the replies to post about 5-6 below this one titled,"Which Edelbrock Carb? for 401." Edelbrocks are probably good on some GM car but not Buicks. They prefer OEM.

A serious off-idle stumble, or flat spot, can have two significant causes. One is that the primary throttle blades are cracked too far open at base idle, which causes an over-rich mixture situation as both the idle and transition ports are feeding fuel to the engine. The other is that the primary throttle blades are closed too far at base idle, which keeps the transition fuel from working as it should.

First thing would be to make sure the idle mixture screws are about 1.5 turns out from being lightly seated (turned in all of the way, gently).

Second thing would be to take the carb off of the engine, with the existing idle speed adjustment, empty the fuel out of the fuel inlet into a suitable container, and then see how much of the transition slot is below the throttle blades. Seems like the spec is something like .040" exposed below the throttle blades for the transition slot fuel to be where it needs to be? Then carefully turn the idle speed screw to get the slot exposure as it needs to be. Reinstall with NEW AND CORRECT gaskets for the manifold you're using.

Now . . . while the carbs might look to be the same, the air flow though the AFB will be greater than what it was through the prior OEM carb--even on the primary side of things, at idle. So with the throttle blades set correctly on the AFB, the idle speed could still be too high.

The "easy" reaction would be to turn the base idle speed screw "down" to slow the idle speed down to the desired factory specs. BUT the factory idle speed specs won't work any more! Different carb and all that that might mean. So you'll probably need to look at a newer model year that used the AFB as OEM and use THOSE specs to get into the ballpark.

The "tweaking" measures could include getting the idle speed where it's high enough to stay running smoooothly in gear with the a/c running wide open. To get there, you might need to tweak the base ignition timing a few degrees and also lean out the mixture screws a little too (evenly with respect to each other), which would help slow things down to where they might be. With less base timing and a leaned idle mixture, you might be able to open up the throttle blades a little (IF they were closed too far to start with).

Key thing is that when you finish, there is a decent amount of sensitivity to the adjustment of the idle mixture screws. Meaning, when you turn them either in or out, the idle speed reacts immediately rather than being "soggy" in response. And, in order to get things to work right with the AFB, the base idle speed (in Park or Neutral) might end up higher than what it used to be with the orig carb.

An example . . . years ago I swapped the QJet on my father's 1969 Chevy pickup 350 for a Holley 4360 (for valid reasons). With the orig QJet, I had to idle it at 780rpm in Park to get the idle speed in gear with the a/c running to be high enough that it ran smooothly in that operational mode. To me, that seemed a little high, but it worked. The Holley was the same way. With it running that fast, the idle screws were highly sensitive, with a very small turn in or out having an immediate effect on idle speed. Below 750rpm, you could turn them about 1/2 turn before any change was made in idle speed. In other words, there has to be a certain air flow through the carburetor for the sensitivity to be what it needs to be and for things to work right. If the idle speed and air flow is too low, the adjustment will be "soggy", but when it's just past the threshold of being what it needs to be, the idle mixture screw adjustment will be highly sensitive. Until you see that happen, you might not believe it, with all due respect. I never figured that 50rpm would make THAT much difference.

With ANY aftermarket carburetor, especially a "universal fit" one as all of the Edelbrock AFBs and the old Holley 1850 now is (after being an OEM Ford carb number for ages), they might bolt on and work decently well, but they WILL take some rigging and tweaking and even some finageling to get them to work right with any engine they might be on outside of a Chevy 350 V-8.

You might end up getting your orig carb refurbished by one of the accomplished carb remanufacturers which others might recommend for your type of carb. It might sound like a lot of money to do that, but not in comparison to the investment (in the AFB and time you've spent on it so far) that otherwise be made.

Now, IF you might find an OEM Buick AFB for your general model year and engine size, you might swap out the venturi clusters with yours, plus the metering rods and jets and power piston springs. I suspect they should all interchange, but then you'll have the pieces to get the fuel curve calibration closer to what it should be for a Buick V-8 . . . IF things work like I suspect they might. And then you can see the fuel pattern from the holes and slots in the throttle body, beneath the throttle plates, to see what they might need to be on your AFB.

Just some thougths,
NTX5467

Thanks NTX for the analysis. I have three carbs to work with, 1) Rochester 4 Jet (OEM for my Riv), 2) Carter AFB vintage 1964 from a Riv and 3) new Edelbrock AFB.

The question is, which do I pursue putting more time and money into to get the car running right?

I think I'd use the production Rochester to get refurbished as "the regular" carb. We know it worked well at some point in its prior life, so getting it refurbished by a competent operative might be a good investment.

Personally, I always thought the Carter AFBs were good carbs, as were the later AVSs. Simple designs that worked well. In that orientation, using the '64 vintage AFB would be an option, too. The one thing about a higher-mileage AFB (or AVS) would be how well the power pistons fit in their bores as both wear surfaces seem to be aluminum and I don't know what clearances they are supposed to have between them. They don't get lubricated (any more) with TetraEthylLed as they did when they were new, but it might be that a little richer jet might compensate for the additional air bleed around the power piston might be all that's needed. Bushing the throttle blade shaft is one thing, but I don't recall anybody worrying about the clearance/wear in the area of the primary power pistons on the AFBs and AVSs. And, these might be the justification to see if the new carb will readily accept the old carb's primary jets, metering rods, venturi clusters, and power piston springs--plus whatever applicable secondary main jets and venturi cluster items for the secondaries might also work from the old AFB in the new Edelbrock AFB.

One thing you can do to any of the carbs is take a small round file and smooth out any casting flash edges you might find in the throttle bores and on the venturi clusters. Just dress them down so everything's smooth again. Kind of like "blueprinting" that area of the carb. NOT to change the shape, just get things smooooothed out with respect to casting flash "lines".

Just some thoughts,
NTX5467