New Guy Here; Hopeful for a Weber 289 48 IDA Baseline Recommendation.


I’ve been lurking on these forums for a while trying to learn all I can about Weber’s. I’ve got a 289, currently in a Mustang but will hopefully be making its way into either an FIA Cobra or a RCR GT40 in the next couple of years. In the meantime, I want to put 48 IDA’s on it, and I’m at a bit of a loss on where to start.

First, specs:
1965 289 bored 4.030”, stock stroke (292cid), 9.6:1 compression
AFR 165cc Heads 1.90/1.60 valves
220° @ 0.050” duration, 0.512” lift 110° LSA hydraulic roller cam
Headers into race muffler, full exhaust
3.89:1 rear axle ratio
Lives near sea level

So, I’ve got two calibrations that I’ve stumbled upon for combinations very similar to my own, but they’re vastly different. Which one should I go with?

The first is from a 289 in GT350 Mustang with very similar compression, cam, and cylinder head specs. It’s from a 10 year old Mustang Monthly magazine article, but it seems to jive with what I’ve read so far in the books:
37mm Venturis
F7 Emulsion Tubes
155 Mains
210 Air Correctors
60 Idle Jets
120 Idle Jet Holders

The second calibration is what Redline/Weber (who I’m buying my carbs from) recommends as a baseline for a 289/302 V8:
40mm Venturis
F7 Emulsion Tubes
170 Mains
170 Air Correctors
65F10 Idle Jets
100 Idle Jet Holders

When I asked about the magazine baseline, the fellow at Redline said that a lot of the difference was based on how much gasoline has changed, and how much that effects tuning. This WILL be a pump gas motor. Is there anybody out there in the GT40 collective that would be willing to weigh in on these two vastly different approaches?

Thanks for reading!


Thanks Brian,

I’m part way through the series now. Great resource. The part where I wanted to come on here and try to get some specific information is because the vendor selling me my carbs is giving me some information that goes against a lot of what’s in the books, but they’re saying that gasoline has changed so much that what’s in the books doesn’t work anymore.

Also, the modern carbs have a different progression circuit, which changes a lot of the baseline settings.

Brian Stewart
May be of no use to you, but the attached is a record of my experience. 351W with AFR 185 heads and moderate cam.

May be of no use to you, but the attached is a record of my experience. 351W with AFR 185 heads and moderate cam.

View attachment 103871

Thanks Brian,

It’s all helpful. What kind of compression are you running, and what is your peak power RPM?

Assuming (6,000 rpm) the formula puts you closer to 44 venturis than the 42’s you’re running. The formula puts me at 40’s, but I’m thinking that with 9.6:1 compression I may be better off with the 37’s. But it would be a bummer to run out of air before my 6,300rpm rev-limiter kicks in.

AND, I don’t know why, being a total newb who’s only read, like one Weber book ever, that I am second-guessing the recommendations of one of the largest Weber dealers out there...
Running 9.6:1, using 95 octane pump gas. Peak power around 6000 rpm.

Thank you. Seems like applying the formula to people’s various combinations, that most folks err a bit on the small side. I think I’m going to go with that 37mm setup as a baseline (less parts to buy vs what comes “stock” in a 48 IDA). I can always go bigger later if I feel like the motor isn’t getting enough air...