Success! I went through the hole on the underside of the power piston bore with a T handle allen wrench and tapped it out with a hammer. The plug came out in two taps. The bore still looks great and there was even still a spring inside. I need to find a power piston now but I don’t want to buy rods or jets until after I read Cliffs book.

 

I have a chart that shows all the rods and hangars, the hangar is another way to tune the back 2. I’ll dig it out and post it.

 

That would be great if you could post it. I haven’t come across any info on the hangers except they were not all the same on carbs of the same number. What I read was that when they were built the changed the hangers to get the flow correct

 

Here

 

Another pic, I have a handful of the sec rods, some are not avail anymore. Leme know if you need any, I also have some hangars

 

Cool! I'm glad it worked out for you.

 

If you’re on the hunt for qjet parts, I can recommend Cliffs High Performance rebuild kits and quadrajets.com.

If you want a great rebuild, I suggest (and recommend) SMI in Huntington Beach.

Good stuff all around.
Chris

 

I got 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 bypass 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

Externally it 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 would be really interested to see how this compares to a 251 carb. Does anybody know what those measurements would be from the factory?

 

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.

 

Olds used idle bypass air *and* holes in the primaries from the factory. You may or may not need to modify the idle air circuits.

 

On my thread at Cliffshighperfomance.com they mentioned that my baseplate may be a Carter Chevy style baseplate. They suggested that it may not have APT, however it does appear to be in place and the casting numbers on the baseplate seems to be consistent with Quadrajet numbers. Has anybody seen this same style baseplate on an Olds?

 

Duplicate

 

Looks more or less normal to me. The voids between the primaries might be a little different, but I haven't actually looked at a throttle plate in a couple of years. Actually, ignore that, I'm thinking 71-later which has a ported vacuum in that area, so the casting will be a little different anyway.

It has ""APT"". You have to do the modification like in Cliff's book. Drill out that plug in the front, pull out the threaded stud - you can see the rounded end of it under the tang there in the middle - and put in one of Cliff's machined screws.
Be careful. I thought the plug in mine was a bit more obvious in its retention - could see the stake marks.

 

Nice progress so far! This doesn't totally help you, but here's what I have on a stock 7028251 ('68 400/455 carb), using your format:

Idle tube: not removed or measured yet
Idle channel: .052
Lower idle air bleed: .065
Upper idle air bleed (main body): .065
Accelerator pump discharge holes: not measured
Main air bleed (main body): .060
Main air bleed (air horn): .040
Secondary POE well restriction: 0.037
Secondary POE restriction: can't find my notes on this
Secondary tube restriction: can't find my notes on this

It's pretty crazy if you have both idle bypass air AND a 0.109" hole in the primary throttle blades. I use an 0.077 hole in my throttle blades without idle bypass air installed. The stock '68 H/O carb (7028255) had 0.120" holes (no bypass air in casting), and I couldn't get it it to idle below 1,000 rpm.

 

Thanks, that does help. Did your 8251 have idle bypass? The holes are not in the throttle blades but in the baseplate, but I believe it does the same thing. I am still learning about idle bypass. Do you think this baseplate would work better on a motor with a bigger cam?

 

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.

 

Sorry for the delayed response. As far as I know, Olds never used idle bypass air through the airhorn and main body in '68-69. They may have used it by '73 from another carb I saw. To go through the main body, you need notches in the air horn (primaries), the holes in the main body, then the holes in the baseplate. Sometimes, carbs that went through mass rebuilders just mixed and matched parts so you end up with a

The advantage of going through the mainbody is you bypass the primary venturis and reduce the risk of nozzle drip. Drilling the holes in the primary blades seems to work pretty well, I have no nozzle drip on mine.

You can try living with your 0.109" holes, or swap on throttle plates from another carb core. The only risk you have is that it will idle too high or possibly nozzle drip a bit. You can always change the plates out later.

 

The holes inside the primary throttle bore are for the idle bypass on this style baseplate. From what I can find this appears to be a Carter baseplate mostly used on 68/69 Chevy applications. However I did find a 7028251 Rochester carb on eBay with the same style baseplate but I’m not sure from the pictures if the idle bypass is drilled. Also that 8251 carb has the same casting number on the main body as my carb. So my options are to not use idle bypass(which I probably don’t need for my mild cam stock 455), drill the main body for the idle bypass(air horn already has notches) or drill the primary throttle blades. My plan is to try it without idle bypass first and go from there.

 

I just looked back and realized you have a stock '68 455 bottom end and cam. I doubt it needs idle bypass air at all, like you just said.

 

So after further research I think I have positively ID the original application of this carb. It looks to be a 7028251 that has been converted to integral electric choke and has possibly a Carter baseplate for Chevy. I plan to add the power piston with the correct spring, 49b primary rods, 70 or 71 jets and AU rods. It should work well on my motor, if not I will likely go with an o2 sensor to dial it in.I had to recheck a few things but here are my final numbers.

Idle tube: not removed or measured yet
Idle channel: .052
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: .030
Secondary tube restriction: .035
Secondary POE outlets .028
Idle bypass none

Cast number
main body 7035221
air horn 7938161
baseplate 7037600(possibly Carter Chevy baseplate)