Confusing 48 IDA jetting comparison on stock 12A
06-13-12, 12:56 PM
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Wankeled 86
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Confusing 48 IDA jetting comparison on stock 12A
I'm about to pull my street port 12A and run a stock 12A in the interim.
I've been referencing these 2 threads for baseline jetting the 48 IDA for the stock 12A.
Weber 48IDA on a stockport 12A
and
Weber 48 IDA on Stock 12A
(the similar titles are rather telling on what they're about, huh)
Wacky is making 2 different recommendations but they're separated by give or take a couple of years.
I'm comparing those jettings to that of RB's baseline 48 IDA jettings for a Street Port 12A.
For starters, I understand that locale, elevation, ambient temp, etc...play a factor in tunes but these 3 jettings are noticeably different.
Why would a stock 12A respond "better" to larger chokes, bigger mains & airs and a larger needle valve? Whereas a street ported 12A starts out at 37mm chokes and smaller mains, airs and needle valve?
Then in the 2009 thread it goes down from 40 chokes to 38 (still larger than the RB recommended 37mm for a street port), back to the 170 mains in a street port but smaller airs: 130. The needle stays at 300 (again larger than RB's recommendation for a street port).
I'd like to minimize expenses to re-jet the IDA for the stock 12A. Anyone have any feedback on this?
I've been referencing these 2 threads for baseline jetting the 48 IDA for the stock 12A.
Weber 48IDA on a stockport 12A
and
Weber 48 IDA on Stock 12A
(the similar titles are rather telling on what they're about, huh)
Wacky is making 2 different recommendations but they're separated by give or take a couple of years.
I'm comparing those jettings to that of RB's baseline 48 IDA jettings for a Street Port 12A.
For starters, I understand that locale, elevation, ambient temp, etc...play a factor in tunes but these 3 jettings are noticeably different.
Why would a stock 12A respond "better" to larger chokes, bigger mains & airs and a larger needle valve? Whereas a street ported 12A starts out at 37mm chokes and smaller mains, airs and needle valve?
Then in the 2009 thread it goes down from 40 chokes to 38 (still larger than the RB recommended 37mm for a street port), back to the 170 mains in a street port but smaller airs: 130. The needle stays at 300 (again larger than RB's recommendation for a street port).
I'd like to minimize expenses to re-jet the IDA for the stock 12A. Anyone have any feedback on this?
06-13-12, 01:07 PM
#2
Old [Sch|F]ool
If the choke size is different, then the jetting will be different as well, because changing the choke size will change the airspeed through it and therefore the depression.
Don't forget the big difference - exhaust system. Exhaust changes more about carb tuning than any other factor, in my experience.
Don't forget the big difference - exhaust system. Exhaust changes more about carb tuning than any other factor, in my experience.
06-13-12, 02:09 PM
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Wankeled 86
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Thanks peejay. I agree that changing choke sizes will affect jetting because of air velocity change. I guess I was wondering why the actual need to change to even larger chokes on a stock 12A over those used for a street port.
Jim Merder from Racing Beat got back to me. Here's my initial correspondence with them followed by Jim's response.
Jim Merder from Racing Beat got back to me. Here's my initial correspondence with them followed by Jim's response.
Originally Posted by J Velasco
"I'm currently pulling my street port 12A for a rebuild. In the meantime I'll be using a stock 12A. I don't have any of the factory intake or nikki components and only have my 48IDA and manifold. Does Racing Beat have any jetting & venturi recommendations or kits for the 48IDA for a stock 12A motor? If not, would one of your experienced tech representatives be so kind as to advise me on a ballpark jetting configuration to start with? I look forward to hearing from you.
Regards,
J. Velasco
Houston, TX"
Regards,
J. Velasco
Houston, TX"
Originally Posted by Jim Merder
Your message was forwarded to Jim Mederer, Racing Beat’s co-founder and chief engineer, for review; his reply is as follows:
The jetting difference between stock and street port is not large. I suggest you start with leaving everything the same except lowering the fuel jets two steps - as a start. That should be Venturi -37, Air - 150, fuel - 160.
Jim Mederer"
The jetting difference between stock and street port is not large. I suggest you start with leaving everything the same except lowering the fuel jets two steps - as a start. That should be Venturi -37, Air - 150, fuel - 160.
Jim Mederer"
06-15-12, 10:25 AM
#4
38 venturies are for track use. For street use, it will give lots of low end but not enough top end for freeway speed, kinda driving a small street port baed on racing beat template I use 40 and up but you need to experiment with different sizes and jetting. Feel free to ask me via PM or phone call. Ty
06-15-12, 02:22 PM
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the other thing is that the different engines might be tuned to a different AFR..
ideally you have a big pile of jets and start rich
ideally you have a big pile of jets and start rich
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