Cliff's Quadrajet Parts and Rebuild Kits

Quadrajet Problem Solving => Dialing in your rebuilt Quadrajet carburetor => Topic started by: DJD OLDS 4MC on April 30, 2026, 05:38:55 PM

Title: OLDS 4MC Quadrajet 7040251 for a Big Block Olds - Advice Needed
Post by: DJD OLDS 4MC on April 30, 2026, 05:38:55 PM

Hi Cliff.

I placed an order last month, and received it within a week. Thanks for shipping it out so fast!

The parts received have been installed, and my son and I are just about to start road-testing.

We’re looking for suggestions for improving a 4MC 7040251 Quadrajet installed on a 463ci (10.4:1) Olds V8 engine in a 1980 Olds Cutlass Supreme that we’ve been building for the past few years.

We’re hoping you have time to look at the included attachments...

Attachment #1: - lists the specifications for the engine, induction, exhaust and driveline for the car.

Attachment #2: - shows the original factory specifications for the 7040251 before the carburetor
                          was modified following suggestions from your book. The current modified
                          specifications are also shown.

The carburetor has a 14" to 4" power piston spring installed. 

A pair of Innovate O2 sensors installed on the engine last year show that with #69 main jets and #39B primary rods, the BB Olds will cruise at 18" vacuum with about 13:1 AFR. With the power system engaged at 5" vacuum, the sensors show almost the same 13:1.

When you have time, please let us know of any changes you feel will improve the Quadrajet before we start road testing.

Thanks very much.

DJ and Jed in NIAGARA FALLS NY

Title: Re: OLDS 4MC Quadrajet 7040251 for a Big Block Olds - Advice Needed
Post by: Kenth on May 01, 2026, 12:06:13 AM

When measuring the primary pump and secondary POE discharge orifices you´ll have to look further into the holes were the restrictions are located.

I have found the primary pump orifices in a 7040251 are .026" and the secondary POE orifices are .029".

HTH

Title: Re: OLDS 4MC Quadrajet 7040251 for a Big Block Olds - Advice Needed
Post by: DJD OLDS 4MC on May 01, 2026, 09:49:07 AM

Thanks for getting back to me.

Besides the 2 items you mention, how does the rest of the list of specs look?

Anything stand out as needing adjustment?

Thanks, again.

DJ

Title: Re: OLDS 4MC Quadrajet 7040251 for a Big Block Olds - Advice Needed
Post by: Kenth on May 01, 2026, 11:25:05 AM

I would open the sec. POE .054" all the way in.
You may find the 39B pri rods somewhat rich, "tip-in" test will tell.

Other than this i think you´re good to go.

HTH

Title: Re: OLDS 4MC Quadrajet 7040251 for a Big Block Olds - Advice Needed
Post by: novadude on May 05, 2026, 07:34:41 AM

Fatten up the primary rods a bit.  13:1 is too rich for steady state
, light throttle cruise.  I would target 15:1-16:1 range for max efficiency, longer oil life, less ring wear, etc.