I got this response to a inquiry I sent Carroll Shelby some years ago. I thought I'd post it FWIW for anyone running 48IDAs on a 289/302.
John,
Carroll Shelby sent your question to the Shelby American Automobile Club
and I contacted THE expert in the Webers for Fords field; this is his
answer:
Howard C. Pardee
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The 289's run best with 37mm chokes in the 48 IDA's, but if it's strictly a track car, put the 42mm chokes in. Run a 155 main jet, and a 215 air corrector. If running open headers, however, drop the air corrector back to about a 170. The idle jet is going to be a 60 f10, and the idle jet holder should have a 1.10 mm air bleed. If it won't run well on that, go up to a 65 f10 idle jet and a 1.20 air bleed in the idle jet holder. Set the idle mixture screws by seating them, and then open each one exactly 3/4 of a turn. Do not exceed a full turn open, and if you have to, then go to the bigger idle jet, then turn the screws back down. The correct setting on the mixture screws should not vary more than 1/8 of a turn from the 3/4 open position. The emulsion tube is usually an F5, and try a #50 bypass valve in the bottom of the fuel bowls. Set the floats exactly to factory specifications, and install a Grose Jet in place of the Weber needle and seat. The stock needle and seat is not at all tolerant of pressure rises, and will leak easily. The D&G valve will handle up to about 6 lbs....cheap insurance. They'll cost about $30. These are sold by D & G Valve Co. in Massachussettes; they advertise in Hemmings (no, I'm sorry I do not have their phone number). Fuel pressure is very critical, and should be strictly regulated not to exceed 3 lbs. Fuel lines must be a minimum I.D. of 3/8". A mechanical, block-mounted pump should be used in conjunction with a Holley low-pressure regulator...the red one...not the high pressure blue one. I've set up a couple of these; this info should get you out of the woods.
Good luck.
John,
Carroll Shelby sent your question to the Shelby American Automobile Club
and I contacted THE expert in the Webers for Fords field; this is his
answer:
Howard C. Pardee
************************************************************************
The 289's run best with 37mm chokes in the 48 IDA's, but if it's strictly a track car, put the 42mm chokes in. Run a 155 main jet, and a 215 air corrector. If running open headers, however, drop the air corrector back to about a 170. The idle jet is going to be a 60 f10, and the idle jet holder should have a 1.10 mm air bleed. If it won't run well on that, go up to a 65 f10 idle jet and a 1.20 air bleed in the idle jet holder. Set the idle mixture screws by seating them, and then open each one exactly 3/4 of a turn. Do not exceed a full turn open, and if you have to, then go to the bigger idle jet, then turn the screws back down. The correct setting on the mixture screws should not vary more than 1/8 of a turn from the 3/4 open position. The emulsion tube is usually an F5, and try a #50 bypass valve in the bottom of the fuel bowls. Set the floats exactly to factory specifications, and install a Grose Jet in place of the Weber needle and seat. The stock needle and seat is not at all tolerant of pressure rises, and will leak easily. The D&G valve will handle up to about 6 lbs....cheap insurance. They'll cost about $30. These are sold by D & G Valve Co. in Massachussettes; they advertise in Hemmings (no, I'm sorry I do not have their phone number). Fuel pressure is very critical, and should be strictly regulated not to exceed 3 lbs. Fuel lines must be a minimum I.D. of 3/8". A mechanical, block-mounted pump should be used in conjunction with a Holley low-pressure regulator...the red one...not the high pressure blue one. I've set up a couple of these; this info should get you out of the woods.
Good luck.