Quote:
Originally Posted by tekuhn
I hope you don't mind me jumping in here with what I think are questions related to the subject being discussed. I do not have a bathtub intake, but I am building a pair of SD clone carbs from a pair of 2840S AFB's ('59 Buick 401). My biggest concern is regarding the weight of the secondary air flapper valves. Seems to me that with half the volume of air passing through each carb at any given RPM (compared to a single carb), it's going to require considerably more RPM (twice?) to fully open the air valves.
Any thoughts on how to go about scientifically and methodically fine-tuning the air valves? Is the shape of them all the same and the only changes are to the counter-weights? Perhaps you do indeed want to delay the secondary air valve opening somewhat in a dual carb setup to avoid low velocities and poor atomization?
The jetting can be worked out in a straight-forward conventional manner, but the transition of the primary circuit enrichment for decent street manners is another matter. I think getting the spring just right is going to be hard.
I'd appreciate any and all thoughts and comments.
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I can address your question slightly. I ran all my dyno pulls with the supplied air valve. There is no spring involved with the air valve on an AFB. Just the valve, shaft and the two counterweights. My original plan was to mount a GoPro camera above the carbs and take some video of a pull. With all the Chaos I had with the dyno, air hat, tuning, O2 sensors, and printers, it just never happened. I thought about just using my phone and filming holding the camera. As much as I trust my ability to put an engine together, standing right next to it holding a camera at 100% load at 6000 RPM's didn't seem smart. I am confident in my case, the air valves open completely and quickly when the throttle is opened. You can see it by a dip and recovery in the torque curve. I can also say now that AFB's are the most sensitive carburetor I have ever worked with concerning the air flow through the carburetor. My AFB clone SD carbs are not identical. The rear carb has the choke shaft and butterfly. The front, no choke. The front carb flows about 8-10% more air than the rear. I found that interesting. So I swapped the air horns front to rear. Then the rear carb flowed 8-10% more air than the front. The choke shaft which is right above the primary venturi cluster, blocks 10% of the air flow through the primary along with the butterfly. Much more than I expected. A real learning experience. I think anyone able to extract maximum performance from them in a nostalgia super stock car is some kind of magician. I talked to so many people over the last several months about tuning dual AFB's for racing, Mopar, Chevy and Pontiac. Every single person I talked to said it took a ton of work and many, many runs on the drag strip to optimize them. Dyno tuning would only get you in the ballpark. Tuning for the street was pretty straightforward. For the street, they would set them up a little rich and enjoy them.