Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Quadrajet Throttle Body Swap - Will it work?

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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

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