I just bought an Offy DP for my truck, and I'm trying to decide wheather to run a squarebore or a spreadbore. I know the DP was designed to run a squarebore, but I can get an adapter to run a spreadbore. The DP is designed with small primary runners, and larger secondary runners kind of like a spreadbore. I have access to both types of carbs and they both work well, Im just trying to get some opinons on this.
Thanks
Bob

 

I like spread bore carbs on the street. The smaller primaries offer better mileage and response when properly tuned. That is where the problem lies. There are only a handful of spreadbore carb out there, and many are not easily tuned. My personal favorite was the Holley model 4011, but I believe it is discontinued.

 

The spreadbore carb I have is an old Carter AFB. Thats a pretty easy carb to tune isn't it?

 

Yes that's a great carb and easy to calibrate and tune. But it's not a spreadbore. An AFB is still considered a squarebore. Carter's spreadbore was their 'Thermoquad', which is easily spotted by it's main body being made of phenolic (brown looking plastic). The main body had a nasty habit of warping over time, so don't use a Thermoquad.

Easiest spreadbore to find is a Rochester Quadrajet. Great when in tune, but plan on kitting them regularly to keep them that way.

Good Luck!
Brett

 

But I do notice on the AFB that the secondaries are larger then the primaries. Not a huge difference like the Q-jet, but there definitly is a size difference.?

 

I believe you. AFB's ranged from 400 cfm to 800 clm, with models in between. Not each carb is completely different from the next in line, though. For example, the 500 and 625 AFB's have the exact same primaries. But the secondaries are larger on the 625. I suppose this was to cut down on costs.
Anyway, flip a 500 AFB and a 750 AFB both upside down and compare the size of each's butterflies. Big difference, but both still considered 'squarebore' carbs because they'll both bolt down to an intake designed to take 'squarebore' carbs.
Clear as mud? Flip a Q-jet, and notice that the butterflies would be nowhere close to lining up on a 'squarebore' intake.
Adios,
Brett

 

I see Now do you know if there is a website or a thread that will help me figure out what carb I have. I think its an OEM carb off of something, cause none of the local parts shops can match up the numbers I give them. I cant tell you for sure off the top of my head what the part number is but I do know it starts with 4B4 then a space then some numbers.

 

Post the numbers you have and I'll try and look it up. It'll probably be 5 digits or so and end in a 'S', like 9933S . Here's a website with a picture that shows where the number is stamped. Scroll down to the picture that says 'Model AFB':
"http://www.cfiworld.com/everythingcarbs/antique/carb-id-num/carter.htm"
Adios,
Brett

 

These are the numbers off of my carb 4B4 9637S
Thanks alot
Bob

 

Bob,
Good carb. Aftermarket 625 cfm, Ford linkage, w/ emission and EGR hookup.
9637 is the model number.
Uses Federal Mogul carb kit 902-317A.
If you need an assortment of jets and rods to calibrate it order the Federal Mogul Strip Kit 10-201.
Good Luck!
Brett

 

Brett,
Thanks alot man you are a great help in my quest for 6-cylinder dominance.