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Is there a way to determine how your carburetor left the factory? I'm talking about jet sizes, primary rod, secondary rod, power piston spring. Things of that nature. I have a 7029251 carb that I have had for a good 40 years. It has seen service on many of my previous cars. I have no idea what may have been done to it before it came into my possession. It sounds far fetched to me that all carbs with the same number would be built identically, but thought I would ask. Thanks guys.
Start with the Cliff Ruggles and Doug Roe printed books for this kind of information. Finding the factory rod and jet sizes for your Rochester Qjet is pretty easy, the information is out there, as is the data on what secondary rods and what rod hanger it left the factory with.
Carbs with them side number had many different combinations rods/jets/secondary rods/hangers so that the basic design could serve all of GM’s divisions, many different engines, many different cams, wildly varying displacements from maybe 250 cubic inches all the way up to the Cadillac 502. It’s a remarkable carb, I prefer the 170 series (75 and later) carbs for their center inlet.
What’s _not_out there is much of anything on power piston springs. I just cobbled up a little test rig this weekend to try and organize my power piston spring collection. It helped. I was able to get the part throttle cruise AFR/Lambda values very close to 14.2/ 1.00 with a longer lighter spring than I had used before.
A little web surfing and you may find the factory combination for your too, which would save the cost of the books.
Thanks Chris. The books are already on the way. I am a documentation person so books are always good. What you stated is exactly what I was thinking. The same number had to have been used on multiple engines with multiple cams, compression ratios and timing curves as well as other differences. Hopefully I'll be able to get a baseline setup for a certain application and go from there. Even if I had an exact setup, I'd still have to tune. I'm at almost 3000 feet so I'd already be rich with a factory sea level setup. Just want to get a starting point. Once I get initially setup, then I'm off to the exhaust shop to have an O2 bung installed. Like you, I want my AFR gauge.
My carb has all the original parts, with the exception of an electric choke coil.
I assume you're happy with the performance? Idle and part throttle response? My carb has the holes in the primary throttle plates. They are definitely factory holes not done after the fact. They are too perfect and clean. Clean meaning no marks that would indicate that they were not manufactured that way. My understanding is that the holes were on the carbs that went on the highest output engines with bigger cams. W-30 maybe. Toronado maybe. I don't know this as gospel, just what I've heard. It makes sense to me. It is a common practice on Holley's to drill the throttle plates on big cam motors with low idle vacuum. What is your idle vacuum? I'm at 10 inches. That's why I want to make really sure I choose the power piston and APT springs properly for my vacuum.
With the 217/221 cam the idle vacuum was bouncy 14-ish and the power brakes didn’t work very well. The engine has a GM HEI distributor from a 77-79 Olds 403 engine. I embarked on an extensive tuning journey with weights, center bar, and springs. Went full circle and ended back with the original factory weights and center bar, but one medium spring and one light spring from a Moroso kit to dial in the advance rate. I also made a vacuum advance limiter to get 10 degrees vacuum advance. The advance is connected to full manifold vacuum as that worked best and dramatically helped with the idle vacuum and smoothness.
The end result is 18 degrees initial advance, 36 degrees total advance, and 10 degrees vacuum advance. With these settings the idle is smooth with steady 16” vacuum, the brakes work fine, and the RPM doesn’t drop when the AC is turned on.
Last edited by Fun71; June 25th, 2023 at 08:39 PM.
The Rochester carb book will tell you exactly what parts were in the carb from the factory. I always trust factory documents over third party material.
The Rochester carb book will tell you exactly what parts were in the carb from the factory. I always trust factory documents over third party material.
Where do I find this book? What exactly is the title? All of my searches return "aftermarket" books. Is this available new or is it going to be used only?
Where do I find this book? What exactly is the title? All of my searches return "aftermarket" books. Is this available new or is it going to be used only?
I found mine at a swap meet.
There is a version of this book available on line. Click on the GM tab, where you will find Olds and Pontiac specs. Here are the pages for your '9251. The way to read this is that if the box under your selected carb is blank, the correct part number is the first one to the left. Secondary metering rods are 7033655, which are AU coded rods. Primary jets are 7031970. The last two digits are the jet size, so these are 0.070" jets. The primary metering rods are 7034849, which are 49B coded rods.
I found mine at a swap meet. There is a version of this book available on line. Click on the GM tab, where you will find Olds and Pontiac specs. Here are the pages for your '9251. The way to read this is that if the box under your selected carb is blank, the correct part number is the first one to the left. Secondary metering rods are 7033655, which are AU coded rods. Primary jets are 7031970. The last two digits are the jet size, so these are 0.070" jets. The primary metering rods are 7034849, which are 49B coded rods.
Now THAT is a book!! I'd love to find one, but I'm sure they are scarce and pricey. Thanks for the info Joe. I'll use the online version for now.
Good grief there's a lot of good info there but my god that needs some proper editing. It's pretty unreadable in that format.
Agree. I read/perused this page many moons ago and noticed the one item which gives it away pertaining to formatting. It was dragged over from days of old - a Listserv.
Good grief there's a lot of good info there but my god that needs some proper editing. It's pretty unreadable in that format.
In my day sonny, all we had were comma-delimited text lists and ASCII graphics, and we were damn glad to have them!
Seriously, that list was originally generated in the old Chubecto Oldsmobile listserver that was the only online forum we had in the late 1980s. The fact that nearly every Oldsmobile website today just does a copy/paste of that info should be worrisome, because it wasn't all correct.
Well I crawled around on the internet and found a book like yours Joe in decent shape for what I consider a good price considering all of the information in it. It arrived today and I'm very pleased with it.
Back to the topic of the carb and the car, I installed the carb today and those factory holes in the throttle plates have no business on my 350. Waaay too lean. If I close the choke butterfly down she cleans up nicely and the idle vacuum comes up to about 12in. I put a post in the parts wanted thread, but thought I'd mention it here also. So... Primary throttle plates... Anyone... Anyone. Oh, without holes.
My carb has all the original parts, with the exception of an electric choke coil.
So where does the "AS" and "AU" designator come from? Since having bought pretty much the same book as Joe P. I still don't see that information. I see it referenced on Cliff's site and other Quadrajet sites, but have no clue where it comes from.
The come from the original AC=Delco manual - which I have 'somewhere'. Someone else will most likely beat me to it since I'm turning into a couch potato when I'm done responding to this thread. I've (also) attached a document I put together many moons ago. A couple links may be broken, but does contain some useful information.
Here's a link from our site I perused ages ago when making a list. Unfortunately, it does NOT directly answer your question but does demonstrate the several codes used in the original AC-Delco manual. Joe's going to puke up the authoritative answer to your question in no time - I'm too tired to look any longer.
Here's a link from our site I perused ages ago when making a list. Unfortunately, it does NOT directly answer your question but does demonstrate the several codes used in the original AC-Delco manual. Joe's going to puke up the authoritative answer to your question in no time - I'm too tired to look any longer.
🤣 Thanks Norm. I appreciate it. Well, if it's in the AC Delco book, I should have it then. I haven't gone through it completely. It's only about 1500 pages🤣
Indeed they are, but I can’t recall what they actually mean/signify. I think I recall Doug Roe’s book is where I saw them first time. That book has vanished mysteriously.
Something tells me they had to do w/ the taper of the rod?
I think it's a profile. How long they are, how thick they are, and what the taper angle is. This is all done to give a different air flow as they are retracted.
A little spreadsheet on qjets. I hope it helps you.
Quadrajet hobbyist, computer nerd, and sometime MS excel user.
These are basically notes to myself from testing/trying various Quadrajet combinations over the years. This was the starting point at trying to limit the combinations I had to try on the road since I was short of time at the time. You'll see I take the Doug Roe book at its word and built out from there.
What I can tell you maybe 15 years later, are a couple of lessons:
1) Qjets use vacuum as a measure of driver demand. If there's leak the carb interprets it as a signal - right or wrong and will meter fuel accordingly right or wrong.
2) Primary throttle plates eventually wear & the well plugs eventually leak. You don't want either of these conditions as either or both will screw up your fuel metering
3) For Olds 455's, at least stock ones, with the later, safer 170 series 800 CFM design, you'll get pretty close to correct primary metering with a 74 jet/49 rod, but (big BUT) you have to have the right power piston spring or the right rods will be too high or too low in the jets to get near to the correct (i.e. stochiometric) metering. See my power piston posts for more on getting to the right spring.
4) Secondary metering is more forgiving, start with factory (see the graphs & tables for fuel deliver pictures) and evolve where you want to go.
5) A wideband oxygen sensor is a very valuable tuning tool when dialing in a qjet. You get to see where it's rich and lean in realtime as you drive.
I could go on, but I'm hoping this spreadsheet workbook helps you all. I'm not dying to get in the carb support business, but will happily answer a few questions to the best of my ability.
I wrote the spreadsheet on my computer which gets regularly virus scanned, so I think it's virus free, but run your virus scanner over it before firing it up just in case something nefarious happens between your binary device & mine.
I think it's a profile. How long they are, how thick they are, and what the taper angle is. This is all done to give a different air flow as they are retracted.
Note that metering rods control fuel flow, not airflow.
Everyday Performance LLC over at V8Buick has a new book out as well.
I havent seen it yet.
Has anyone here?
Curious if it would be another good source.
June 25th, 2023, 03:42 PM
