I presume that you are talking about 48IDA's on an original individual runner manifold, like the Ford or the Cobra script?
First off before you even run the engine, get a camshaft designed for this setup. If you do not, the way high performance camshafts are designed, they have "overlap" built into them.
The overlap is there to help the exhaust scavenge, or push out the exhaust gases into a header, and pressure the intake manifold plenum under a four barrel carburetor to push in mixture into the cylinder.
It does this by providing overlap. Overlap being the time in engine degreess that the intake valve and exhaust valve are both open.
A normal high performance cam will have something like 72 to 75 degrees of overlap. Race cams, more.
The problem with webers is that they work on an independent runner manifold, giving every cylinder it's own carb, and have no combined plenum.
What this 72 degree overlap cam will do will be to push back through the carb and blow raw unburned fuel out of the carbs at speed.
You need a cam that has something like 26 degrees of overlap to limit this effect to a safe level. Those cams are now referred to as Weber cams.
Next, what you need to do with IDAs is determine the f/a ratio in the mains for your engine. It is more related to engine size then performance ad ons.
The ratio as I remember with a 289 is a 125f/215a on an f5 tube.
A 351 is something like 135-140f/160-165a.
I never did a 427 but I think they are 165f/130a. All these engines run best on an F5 tube and can get by very nicely on an F7.
Since this is strictly a mechanical system, you are not going to be able to tune to every rpm power range, but these jets will more then get you in the ball park.
As far as the "auxiliary chokes" go, 37mm are what is stock in a 48ida-4. That is simply because they are built for VW dunebuggies and the VW's like 37.
Gary Hall (when he was alive) was selling street Webers for Panteras with 28mmchokes (machined down from billet). I personally have run as large as 44mm in my Pantera. I have settled on 40.5mm (from billit tube) as a compromise.
Changing sizes of chokes won't drastically effect the total hp that the engine makes, just move around the rpm range where it makes the best power for your application.
Forget about getting the engine on the dyno to idle at stoiochemic, 14.6. What you are doing with this system is getting it to run the best that it can and be acceptable and that number is going to be around 12:1 all the time.
Remember this is an archaic race design that has the ability to be streetable, if you aren't too picky. Don't expect it to reinvent the wheel, because it won't and if you want it too you're dreaming and best look elesewhere.
Don't expect anything under 100mph with these things except a lot of belching, and black smoke clouds from the exhaust. Regarless of what you do they have a flat spot in the 2800-3500rpm range. Some people swear a third transition hole fixes that but if you believe any of these lies you might want to look into buying a bridge with a toll booth from that guy in the street with the trench coat hiding from the light. He'll give you a good deal.
They do pull like a freight train at over 100mph AND have throttle response through the turns unlike anything a holly carb can provide, but other then that they were meant to run flat out at LeManns and when they do they make power like a FI system does. On the bottom end, they loose quite a bit of it unfortunately.
Oh, and forget about air cleaners on them. Put screens on them to keep from swallowing small animals and mechanics that are built like jockeys and get sucked in. The engine doesn't like that.
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