Quadrajet Throttle Body Swap - Will it work?

[DJD OLDS 4MC]

Hey, ya’ll

In the process of converting a 1970 4MC Olds Quadrajet 7040251 to accept a vapor cannister purge line.

Have a 1972 4MC Olds Quadrajet 7042250 throttle body with a vapor cannister purge nipple.

Queston is, can the throttle bodies be swapped out between the two Quadrajets? Are there any changes that need to be made to the 7040251 (such as bypass air), for this swap to work?

Please see attachments.

Thanks     DJD

QUADRAJET #1:   1970 OLDS 4MC #7040251

                             THROTTLE BODY CASTING #7038294
         
                             DOES NOT HAVE A VAP CAN PURGE NIPPLE

                             DOES NOT HAVE - 2 SQUARE CHAMBERS LOCATED NEXT TO
                             THE IDLE DOWN CHANNEL HOLES THAT FEED INTO THE IDLE FUEL DISCHARGE
                             NEEDLES

QUADRAJET #2:   1972 OLDS 4MC #7042250

                             THROTTLE BODY CASTING #7024285

                             HAS A VAP CAN PURGE NIPPLE

                             HAS - 2 SQUARE CHAMBERS LOCATED NEXT TO THE IDLE
                             DOWN CHANNEL HOLES THAT FEED INTO THE IDLE FUEL DISCHARGE NEEDLES
                                           
COMPARING THROTTLE BODIES:

   - PASSAGES & HOLES LOCATIONS IN TOPSIDE OF BOTH THROTTLE BODIES APPEAR TO BE   
      IDENTICAL EXCEPT SQUARE CHAMBERS

COMPARING FLOAT BOWLS:

   7040251 - FLOAT BOWL - CASTING #7038266
   7042250 - FLOAT BOWL - CASTING #7044997

   - UNDERSIDE OF BOTH FLOAT BOWLS APPEAR TO HAVE IDENTICAL PASSAGES & HOLES
   - PASSAGES & HOLES LINE UP WITH THE SAME PASSAGES & HOLES IN THE THROTTLE         
      BODIES

   QUESTIONS:   Can the throttle bodies be swapped out between the two Quadrajets?

                         Are there any changes that need to be made to the 7040251 (such as
                         bypass air) for this swap to work?

Thanks again.

[Kenth]

Yes, the throttle bodies can be swapped with no ill effects.
The throttle body gasket will cover the square holes to not interfere with the idle channels.
The idle bypass restrictions are located in the throttle plate and in some units is extra bypass air added in the throttle blades, you may need to copy the idle bypass air to the new throttle plate/blades.

HTH

[DJD OLDS 4MC]

Thanks, appreciate your reply.

Not clear on one thing.

After the swap, with the hose from the canister connected to the throttle body nipple, will the vapor canister purge circuit be functional on the 7040251?

Thanks again.

DJD

[Kenth]

Yes, vacuum will be pulled thru the small holes next to the idle needle holes to the tube via the channel ahead of the throttle blades.

[DJD OLDS 4MC]

Thanks again, Kenth

Currently soaking the APT screw in the 7042250 throttle base in PB Blaster and adding some (but not a lot) heat using a butane torch to get it to loosen up. So far, it's still seized.

Do you have any suggestions for getting the screw to release for the casting threads?

I know I have to be extra patient, but is the heat from a small butane torch enough?

Thanks
DJD

[Kenth]

I have managed to loosen of maybe 1 out of 10 APT screws in the throttle plates with the help of various sprays and heating with, among other things, a butane torch.
Nowadays I don't even bother trying.
If the carburetor needs jets or main air bleeds replaced, it will show up during the tip-in test.

[DJD OLDS 4MC]

Hi Kenth

We appreciate the advice you shared a couple of months back about swapping throttle bodies on a Quadrajet to accommodate vapor cannister purge. As a follow up to you APT screw removal suggestion, today we successfully drilled out an APT screw without ruining the threads in the throttle body after spending weeks without any luck soaking, heating and trying to gently break the little screw threads free. So far we’re 1:1 in the APT screw removal challenge - two more throttle bodies to go!

My son and I are hoping you have a few moments to spare to look at the attachments we’ve included.

We’re looking for any suggestions you may have for improving a 4MC Quadrajet before we install it and road test it in a 1980 Olds Cutlass Supreme that my son and I have been building for the past few years.

The first attachment lists the specifications for the engine, induction, exhaust and driveline for the car.

The second attachment lists the current and original specifications for the 7040251 Quadrajet we plan to install on the engine next month.

The Quadrajet was modified following suggestions from Cliff Ruggles book a few years back. The current carb specs, as well as the original specs before it was modified are listed.

Based on an Innovate O2 sensor setup we installed on the engine last year, it appears the BB Olds will run at about 13:1 with  #69 main jets and #39B primary rods cruising at 18" vacuum, and almost the same 13:1 with 7" vacuum and the power system engaged. With #70 jets installed, it ran a tad richer into the mid-12s, especially with the power system engaged.

When you have time, could you please take a look at those carb specs and advise us as to any changes you feel will improve the Quadrajet before we install it and begin road testing.

Thanks very much.

DJ and Jed