Q-Jet Factory Specs
#1
Q-Jet Factory Specs
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.
#2
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.
Hope that helps
Chris
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.
Hope that helps
Chris
#4
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.
#9
#10
Yes, very happy with the performance.
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.
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.
#12
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?
#13
#14
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.
#15
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.
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.
#16
#17
© 1996 - 2000 by the members of the Oldsmobile Mail List Server Community
#18
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.
#19
#20
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.
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.
#21
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.
#22
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.
#23
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.
https://classicoldsmobile.com/tech/79.shtml
https://classicoldsmobile.com/tech/79.shtml
#24
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.
https://classicoldsmobile.com/tech/79.shtml
https://classicoldsmobile.com/tech/79.shtml
I'll do some more reading.
#26
#27
#29
#30
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.
Cheers
Chris
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.
Cheers
Chris
#31
Here's a list that cross-references primary metering rod part numbers to stamped number on the rod.
Here's the list for secondary metering rods
Here's the list for secondary metering rods
#32
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