'73 SD-455 versus 74 SD-455 Quadrajet Main Air Bleeds

[Brian B.]

Could someone please confirm a detail regarding SD-455 carb MAB's?  According to Delco service bulletins I've read carb #7043270 ('73 SD Auto) utilizes 76 primary jets while carb #7044270 ( '74 SD Auto) utilizes 70 primary jets.  Is this large jet size difference due to the '73 SD carb having larger main air bleeds?  Thank you.

[Cliff Ruggles]

No, the difference in jet size has nothing to do with MAB's.  If you look closely they also used different primary metering rods to keep the metering area pretty close when the PP is down.

Most likely just attempts to get the big Super Duty engines thru every tightening emission standards so they took some fuel away from it on the primary side......Cliff

[Brian B.]

Ah!!! Thank you  Cliff!!! This solves a dilemma for me.

[Cliff Ruggles]

I just restored a 7044270 in the shop a few weeks ago.  Pretty sure it had .070" main jets with .036" metering rods, or something close to that.  I did NOT use that calibration as they tend to be a tad lean on this new fuel and the engine was getting a roller cam upgrade and slight increase in compression.......Cliff

[Brian B.]

I've also added a mild roller cam, went with larger jets, and gotten the best street and track results in my 3:08 geared car.   13.115/102.65  mph in 50 degree weather with 78 jets and DG secondary rods.  Hoping to find 2 more tenths next season.