Restamp 7040256 ID

[4speed455]

I bought a beautifully restored restamp carb. It is stamped 7040256 which is a 1970 W30 4 speed carb, but is an obvious restamp. The pad shows no casting detail that would normally be present under the stamp, so it was machined off and restamped. Looks like it was setup close to the specs of a true 0256 carb with 58 jets and no power piston or primary rods and BE rods(should be AU). I want to add a power piston and primary rods and change the secondary rods and jets so I can use it on a stock cam 1968 Oldsmobile 455 with performer manifold, headers and HEI. I bought the book and some micro drill bits so I can measure everything. This is what I came up with so far. Upper idle air bleed is in the main body, float fulcrum is the early design and idle pass air is present.

Idle tube: not removed or measured yet
Idle channel: .046
Lower idle air bleed: .070
Upper idle air bleed (main body): .070
Accelerator pump discharge holes: .026
Main air bleed (main body): .060
Main air bleed (air horn): .040
Secondary POE well restriction: 0.36
Secondary POE restriction: .052
Secondary tube restriction: .035

Looks like everything matches what a 1970 Oldsmobile carb should have came with originally from the best I can tell. Also most measurement match recipe #1 except the MAB are smaller, which appears to be correct for an early Olds also.

I have 58, 70, 72, 73, and 76 jets as well as BE, AU, BP, CH, and CV secondary rods already but only the 49c priamary rods. My plan was to set it up as a 7040251 with 49c primary rods, AU secondary and 70 jets. Should I drill the MAB and use different rod/jet combo or leave the MAB and go with a more traditional olds setup? Also what size idle tubes should I have with the 49c/AU/70 combo?

Let me know if you need any more information or if you suggest any other mods, thanks Joel.

[4speed455]

Besides the idle bypass it also has 7/64 holes drilled in front of the primary throttle plate. Cliffs book says some “high-performance” quadrajet used 3/32 holes but these are bigger then that. I’ve also read that these are more common on olds then other brands.

[Kenth]

The 49c/AU/70 combo would be a nice start.
I would use .052"-.055" idle down channels and .038"-.039" idle tubes.

Also, your carb has a Carter made Chevy base plate and with .109" idle bypass air no need for drilling the throttle blades.
And the choke pull-off rod sits upside down and in the wrong direction for the proper function.

[Cliff Ruggles]

Good find Kenth, not exactly sure how you could install one of those backwards but it's on there that way?

You will have trouble tuning the carb without a working APT screw in the baseplate if you convert it to a power piston style carb.  49 rods are pretty "fat" on the upper section and teamed up with .070" jets you will need the ability to raise and lower those rods.

The Chevy baseplate will probably work, but it is different in several areas than the Olds variety including the length and location of the transfer slots, another good reason to have a working APT system.  I'd add here that "Frankencarbs" and commercially "remanufactured" carbs are the most difficult to tune, so prepare yourself.  This simply happens because of the differences in the parts used to build them, plus "builders" often make changes in several areas which are seldom for the better.

As far as tuning it will matter which "stock" 455 1968 Olds engine you are going to use it on?  Olds made several different 455's and specific camshafts for them, which determines the "tune" required to get it to work well.

I would also recommend getting one of my high flow N/S assemblies (non-windowed) and one of my accl pumps at a minimum even though it is a "fresh" rebuild.  If it has a brass float in it toss that in the trash and get a float for it as well........Cliff

[4speed455]

Thanks for the suggestions. First off the choke pull off rod was installed upside by the prior owner after opened it up to check the setup, it has been noted the correct orientation. As far as the base plate what tells you it’s a carter Chevy plate it has a similar casting number to the other carb pieces? It does appear to have APT installed complete with adjustment screw and tension spring. The opening is still capped so it doesn’t appear to have been messed with. As far as the cam it was originally a high compression 2bbl motor with the mild cam. As far as the idle tubes and down channel restrictions I thought I read that olds used smaller opening to work with the fatter 49 primary rods and the smaller 70 jets? That’s why they used smaller MAB from my understanding. I am still a newbie and here to learn though. Thanks again.

[4speed455]

[Cliff Ruggles]

The Olds Q-jets thru 1974 will have small MAB's and smaller jets in them.  They used APT to fine tune part throttle A/F on the primary side, mostly for emissions then capped it off to prevent tampering. 

Before you can do any carb tuning you'll have to make all the decisions on the engine build.  I've built thousands of Olds carburetors, matter of fact just finished up a W-30 carb a few days ago.  I've found that most of the Olds factory "high performance" early carburetors used very small MAB's and in 1970 with the manual trans carbs they are really tiny as they eliminated the power piston and primary metering rods then ran them straight off the jets.  I suppose the factory didn't think there would be enough vacuum with the really long duration camshaft.  They were wrong for the most part and over the years I've built a good many custom early Olds Q-jets for the 1970 455 manual transmission W-30 engines and they worked flawlessly.

Those carburetors are usually missing and difficult if not near impossible to find anyhow, plus pretty expensive when you do find one........Cliff

[4speed455]

Will this baseplate with the larger holes and idle bypass work well for a mild cam stock 455 or will I have problems getting it to idle? I have seen similar baseplates that are milled the same on the underside and drilled inside primary throttle bores. They were both on a 7029202 Carter carbs. So it looks like it is a Carter made Chevy base plate.

[4speed455]

So I got to the part of the book where they talk about adding idle bypass and drilling holes in the main body if they are not present. My baseplate has all the holes drilled in it for idle bypass but my main body does not have holes. So it looks like I do not have idle bypass on this carb, just the holes in the primary throttle bores next to the transition slots.

[Cliff Ruggles]

Idle bypass air is easy to add you just have to drill holes in the voids and make sure the airhorn has slots to allow incoming air to get to those voids.  It's not fussy to drill the casting and "notch" the airhorn if needed.

Might want to try the carb without bypass air and see if it actually needs it.  For a "stock" cammed engine it may be fine without it......Cliff

[4speed455]

Cliff, thank you for the advice. I am planning on trying it without the idle bypass first. My air horn does have notches so I am only missing the holes in the main body if I do need it.

Another thing I noticed is the gasket between the main body and baseplate doesn’t match the baseplate(BOP gasket and Chevy baseplate) so I will need to change that, and what gasket should I use between the baseplate and manifold, I see several different styles?

Do you think I will be okay with the .046 idle channel, and what size idle tubes would a stock 7028251 or 7040251 carb use or what do you recommend? What power piston spring will I need?

My plan is to order a rebuild kit from you with the correct gasket, power piston spring, needle and seat and acc pump. Use the .049c primary rods with 70 jets and AU secondary rods, no idle bypass and drill the idle tubes/idle channel if needed.

Does this sound like a good starting point? Anything else I should do or change?

[Cliff Ruggles]

I will put the correct gaskets, PP spring and anything else you need in the rebuild kit.  Most likely it will use the early style Chevy main body to baseplate gasket.  They made the change to the later style in 1973.......Cliff

[4speed455]

Another piece to the puzzle, it looks like this was a divorced choke carb originally(1968 Olds by my best guess) that has been converted to an integral electric choke. Based off other threads on this site that is possible but difficult. It seems you would need a 70-74 Olds donor carb for the parts then you would need to drill and tap the mounting holes for the choke pull off bracket. Is there anything that need to be done for it to work correctly, I would like to verify everything?

[Kenth]

The only Qjets with front inlet and no idle bypass in body would be the 1974 or earlier Buick 350 units.
What is the cast number at the back of bowl and beneith the fuel inlet, also the fine stamped number?
What are the size of secondary POE outlets in air horn at front edge of the air valves and where are are they located, at the edge, high above or under the valves? Pictures if you have.

[4speed455]

Cast number at the back of the bowl is 7032551 and under the fuel inlet looks like 11. The stamped number appears to be 35222. Secondary POE outlet are .052 and they are above the air valves. I was told most early Olds did not have the idle bypass in the main body drilled, Is that not what you have found? I have found at least one 7028251 with the same cast number on the main body and it appeared to have the same style baseplate but I could see the cast number in the pictures.