[cobraviper_99]

In my experience, it's just getting several contributing elements to work simultaneously. Obviously, it's an idle circuit issue. That means idle jet size, jet holders, idle mixture adjustment, spark plug condition, even fuel pressure can come into play. As I mentioned earlier, it's real easy to over-fuel the engine with Webers and run a too-rich mixture while chasing that stumble. An overly rich setup can camouflage a host of sins.

I start with a baseline setup I know is lean: 55 idle jets, mixture screws turned out half to three-quarters of a turn. I make sure the plugs are clean and fuel pressure is no more than 3.5 pounds. Some guys run as much as 4 pounds but again, I'm trying the leanest setup I can so I run the lower fuel pressure.
I also check the synchrometer reading to be absolutely sure the carbs are synched. Also, linkage geometry is key. Carbs which are synched at idle can be way off as the linkage opens due to a bogus hex-link configuration. All eight throats should be pulling equally at all RPM readings.

Now, I take a test drive, expecting to find that stumble. Yup, it's there. It's gone by 3000 RPM's because the main jets are now taking over. Before switching any components, I gradually turn out the idle mixture screws no more than an eighth of a turn and drive the car again. If there's still a stumble, I'll stop and turn the mixture screws out a little more, but no more than another eighth turn. Now, if the engine starts backfiring under acceleration, I notice what side it's happening on and that tells me I've got too rich a mixture in a cylinder on that bank. I may also hear a backfire when lifting and that tells me I'm too lean in a cylinder. It could be on the same side as the backfire when accelerating in another cylinder or on the other side of the engine. Here's where reading the plugs is so important.

I've seen sooty plugs and whitish plugs on the same side of the engine during this dialing in process. You've now got a reliable roadmap for what each cylinder needs. Make the cylinders happy, so that your plugs are reddish brown after a test drive and there's a good chance the stumble will be corrected, or at the very least, much better. One cylinder may want a bit more fuel, another may want less. The backfiring evidence (and/or the carbs "sneezing" under acceleration or at idle) is how your engine tells you that a cylinder isn't happy. Remember, unlike a plenum-type intake which delivers fuel to every cylinder from a common vapor bank, each throat of the Webers has to supply its assigned cylinder with the proper air-fuel mix. Now, there is a chance that your cylinder head choice, cam profile, or type of ignition can complicate the tuning, but Webers are so infinitely adjustable, I'm of the opinion that you can get them dialed in regardless of what other goodies you're running.

Don't rush the process. My Cobra spent most of the summer in my garage last year as I methodically used the Braden book and some other materials to understand what was happening in my engine, rather than just changing this and that, hoping to hit the setup. And don't be afraid to treat each cylinder as a separate patient. As I've suggested, you may think using the very same jets, holders, idle mixture settings, etc. in all four Webers is an unbreakable rule, but if you had eight kids, each one would probably have different personalities, wants, dislikes, etc. Your Webers are no different. The differences may be slight or they may be significant. But you would be amazed how your engine will respond if you step outside conventional wisdom when tuning these carburetors until each cylinder is getting what it wants.

One other thing: it's not a lock that a 2000-2800 RPM stumble is a lean stumble. If you're running too rich a mixture in a cylinder or two, you'll get a spark plug misfire and it feels just like a lean stumble. Again, I ask my spark plugs to give me the inside scoop.