Some carb advice for a new guy.

[tim_mc]

Originally Posted by Steeveedee

Not sure what you mean by 3-stage metering rods. I bought an AVS2 and a calibration kit (slight surging at part throttle). The rods in the kit are small at the tip, taper, and further up have a step. I suspect that that largest diameter doesn't actually enter the jet, given the carburetor configuration? I haven't changed anything yet, as it's a real mother female dog to get up there when I'm using a cane. At least I got the carb on and the truck running.

Brings back my own experience tuning my 1905 AVS2. A nice feature of the Edelbrock is the ability to swap metering rods and/or springs in a minute, adjust the idle mixture screws (if needed) and take another test drive. My neighbors were probably wondering what the hell I was doing driving in and out of the subdivision a bunch of times when I was tuning it. I printed out the calibration chart and kept track of the combinations tried and what I'm currently using. Also, it's a MUST to limit fuel pressure to Edelbrock carbs to no more than 4-6 PSI, so I added a fuel regulator and VDO #152002 gauge. That needs to be factored in on the carb decision as well.

I've uploaded a copy of the 500CFM 1901-1904 chart for any that may be interested.

[MikeB]

Originally Posted by Steeveedee

Not sure what you mean by 3-stage metering rods. I bought an AVS2 and a calibration kit (slight surging at part throttle). The rods in the kit are small at the tip, taper, and further up have a step. I suspect that that largest diameter doesn't actually enter the jet, given the carburetor configuration?

Yes, the rod's main body doesn't go gown into the jet. So, there are only two stages, for example a .073" and .047". These rods would be stamped 073x047, but sometimes the numbers are almost impossible to read. So what you have to do is measure them with a micrometer or dial caliper.

At high engine vacuum (when cruising) we have the .073" diameter in, let's say, a .100" jet. But, when vacuum drops during acceleration, the rod lifts up and now its small .047" diameter is in the jet for a richer mixture. The rods work in conjunction with step-up springs to determine the vacuum at which they operate.

On at least some of the original equipment AFB carbs used on AMC, GM, and Chrysler performance engines, the rods had 3 stages, which would have made for ultra-fine tuning. I don't think the aftermarket universal versions of AFB carbs had 3 stages.

The metering rod equivalent on a Holley is the power valve, which is designed to open when manifold vacuum drops to a certain level adding more fuel for acceleration. The most common size being 6.5"hg for a mild performance engine.

Page 3 here is a better explanation of how the metering rods and springs work together. https://www.edelbrock.com/media/wysi...ers-manual.pdf

[Steeveedee]

I know how a carb works. I also have the guide that came with the cal kit.

[MikeB]

Originally Posted by Steeveedee

I know how a carb works. I also have the guide that came with the cal kit.

You asked a couple questions, but I guess I went overboard!

[Steeveedee]

No biggee. Someone will get some use from it.

['68OrangeSunshine]

Originally Posted by Steeveedee

Not sure what you mean by 3-stage metering rods. I bought an AVS2 and a calibration kit (slight surging at part throttle). The rods in the kit are small at the tip, taper, and further up have a step. I suspect that that largest diameter doesn't actually enter the jet, given the carburetor configuration? I haven't changed anything yet, as it's a real mother female dog to get up there when I'm using a cane. At least I got the carb on and the truck running.

Way back when Carter still built carburetors they built the AFB -- Aluminum Four Barrel and the AVS -- Adjustable Vacuum Secondary.
When they quit the carb business [in the Mid '80s -- due to factory fuel injection] Federal Mogul took over building AFBs for the aftermarket. Edelbrock also got some AFBs from the same Weber factory still making them, and marketed them with a higher polish and emphasis on high performance parts.
To my knowledge, no one still built the AVS. [Until Edelbrock revived it fairly recently.] The original AVS carbs had three stage Metering Rods and Jets. The middle apertures were to dial in midrange performance. I have an old Carter AVS i found discarded as a prop after a movie wrapped. Instead of annular rings in the secondaries, it has 1/4'' x 2''[~] probes with many small holes. I bought Edelbrock AVS2 Rods and Jets in the hope of rebuilding it, but found they were only two-step. Edelbrock consolidated logistics by converting the AVS2 to use 2-step parts like their Performer [AFB]
Federal Mogul quit making AFBs in the early 2000s.
Guys at the local Hot Rod Shop said my rebuilt AVS would ''Probably run like ass anyway'' so I dropped the project.

[MikeB]

Originally Posted by '68OrangeSunshine

Way back when Carter still built carburetors they built the AFB -- Aluminum Four Barrel and the AVS -- Adjustable Vacuum Secondary.

That's some good AFB history there! Back in the late 60s, I bought an AFB that was OE on a 300hp/327 engine. The owner had decided to go with a Holley that was used on the solid lifter 327 (365hp?). I installed the AFB on a 292 (283 +.030") in a 57 Chevy. Don't remember which manifold I used.

That is still one of my all time favorite carbs. Every one of them was configured by the factory (GM, AMC, Chrysler) for a specific engine. I never even re-jetted mine, because it worked so well (and I wasn't very knowledgeable about carbs back then). Tried to find that exact model/part number many years later for a 327 that I had built. There were a few available on eBay and elsewhere, but prices were stratospheric.

[MikeB]

I want to add that all Edelbrock Performer-series carbs use Air Valve Secondaries, not just the ones ones branded AVS and AVS2, which are adjustable. I copied this explanation from a Jeff Smith article:

"The AVS name is an acronym that stands for Air Valve Secondary. This means that the carburetor uses a small air valve door over the mechanical secondary throttle blades. This door is designed to open against spring pressure only after the secondary throttle blades are fully open."

What he didn't say is the air valve on the earlier Edelbrock carbs used a fixed weight instead of an adjustable spring. So instead of overcoming spring pressure, the air valve opened when air flow overcame the counterweight. Sometimes guys would lighten the weights, but that could introduce a bog.

Also, the AVS2 has a major improvement, and that's the annular boosters. Plenty of info about them in the Interment.