Leaner jetting for a Q jet carb
Registered User
Joined: May 2009
Posts: 2,255
From: Northern California
This is a worthy undertaking and not all that hard to do. Qjets are amazing analog technology, but complex enough that they got a bad rep among those who preferred the simplicity of Holley and others. At the time, and still to some extent today, the parts availability makes them almost infinitely adjustable for how and where you use your car. For my 455's I settled on the 170 (75 and later) series using the safe front inlet and 800 cfm. They start on a dime and work great after a great deal of tuning time on my part.
2 beginning / easy options:
You can go with smaller jets, but that will affect wide open throttle mix which you may want to remain rich. The other way to go is fatter primary rods while keeping your factory jets so that it's just the primary circuit that leans out. In a qjet, most of what you use around town is the primary circuit so that's a good place to start unless you do a lot of highway driving or WOT racing passes.
Suggest you read on up what the factory settings were back in the day, account for the crappy alcohol gas we get today, account for the altitude and get parts that vary around the factory settings as a beginning.
Also see - http://quadrajetparts.com https://cliffshighperformance.com/si...orum/index.php
http://www.sparkyscarbs.com
Somewhere here on C/O I have a write up on deploying oxygen sensors to really dial in a qjet. If you're interested look under my user name. The 02 sensors are one of the best mods I ever did to learn how to tune a car.
Cheers
Chris
2 beginning / easy options:
You can go with smaller jets, but that will affect wide open throttle mix which you may want to remain rich. The other way to go is fatter primary rods while keeping your factory jets so that it's just the primary circuit that leans out. In a qjet, most of what you use around town is the primary circuit so that's a good place to start unless you do a lot of highway driving or WOT racing passes.
Suggest you read on up what the factory settings were back in the day, account for the crappy alcohol gas we get today, account for the altitude and get parts that vary around the factory settings as a beginning.
Also see - http://quadrajetparts.com https://cliffshighperformance.com/si...orum/index.php
http://www.sparkyscarbs.com
Somewhere here on C/O I have a write up on deploying oxygen sensors to really dial in a qjet. If you're interested look under my user name. The 02 sensors are one of the best mods I ever did to learn how to tune a car.
Cheers
Chris
Registered User
Joined: May 2009
Posts: 2,255
From: Northern California
When you dig into to qjets, you’ll figure out that the jets have a heavy influence on WOT mixture.
Where you really get the drivability gains is in the primary rods, primary piston spring stiffness and the vertical start/stop position of the primary power piston. For both the 70-series and 170-series qjets, Rochester made many many many springs, power pistons and rods. The combinations are d*mn near infinite, so keep a log of what combination you started with, what you changed (1 item at a time…) and what driving behavior you liked or didn’t like.
As you get into the qjet, the first test to complete is spraying a bit of Berrymans B12 Chemtool at the primary throttle rod holes on each side at the throttle plate. If you get a rise in RPM, you probably have worn throttle rods which represent a vacuum leak. Solve that first before moving on to jetting if you have that problem. Otherwise you’ll chase your tail forever. Simply put, the primary throttle rods going through the plate rotates every time you touch the gas pedal. So they wear out and get leaky.
The qjet uses vacuum signal to interpret driver demand. High vacuum = low demand; Low vacuum = high demand in its analog metering language. If you have a vacuum leak, the qjet mistinterprets demand and metering will be off. Especially at low RPMs around town where metering is particularly sensitive. The ugly one is the primary throttle rods, but look around for bad gaskets or other vacuum leaks too. They can all contribute to bad metering in a similar way. You’ve won the game when you spray carb cleaner all around the carb and don’t find any rise in engine RPMs.
Get it right and qjet’s really shine as mixture devices. If they leak gas, leak vacuum, have a too-loose secondary wrap spring (aka qjet bog) they suck and people put on Holley’s for simplicity.
If this is helpful to you, dig around under my user name for what little carb stuff I have to offer. There are many who know a lot more than me, but I dug in pretty deep long ago and most of it stuck. Also hit me with a private message if you’d like my wildly complicated excel workbook on qjet metering. I haven’t looked at it in years, but it does have some details on factory combinations for various engines & years of Olds big & small blocks with details on jets & rods and (dig this) “square metering area” aka mixture at various points of rod/jet interaction…
Cheers
Chris
Where you really get the drivability gains is in the primary rods, primary piston spring stiffness and the vertical start/stop position of the primary power piston. For both the 70-series and 170-series qjets, Rochester made many many many springs, power pistons and rods. The combinations are d*mn near infinite, so keep a log of what combination you started with, what you changed (1 item at a time…) and what driving behavior you liked or didn’t like.
As you get into the qjet, the first test to complete is spraying a bit of Berrymans B12 Chemtool at the primary throttle rod holes on each side at the throttle plate. If you get a rise in RPM, you probably have worn throttle rods which represent a vacuum leak. Solve that first before moving on to jetting if you have that problem. Otherwise you’ll chase your tail forever. Simply put, the primary throttle rods going through the plate rotates every time you touch the gas pedal. So they wear out and get leaky.
The qjet uses vacuum signal to interpret driver demand. High vacuum = low demand; Low vacuum = high demand in its analog metering language. If you have a vacuum leak, the qjet mistinterprets demand and metering will be off. Especially at low RPMs around town where metering is particularly sensitive. The ugly one is the primary throttle rods, but look around for bad gaskets or other vacuum leaks too. They can all contribute to bad metering in a similar way. You’ve won the game when you spray carb cleaner all around the carb and don’t find any rise in engine RPMs.
Get it right and qjet’s really shine as mixture devices. If they leak gas, leak vacuum, have a too-loose secondary wrap spring (aka qjet bog) they suck and people put on Holley’s for simplicity.
If this is helpful to you, dig around under my user name for what little carb stuff I have to offer. There are many who know a lot more than me, but I dug in pretty deep long ago and most of it stuck. Also hit me with a private message if you’d like my wildly complicated excel workbook on qjet metering. I haven’t looked at it in years, but it does have some details on factory combinations for various engines & years of Olds big & small blocks with details on jets & rods and (dig this) “square metering area” aka mixture at various points of rod/jet interaction…
Cheers
Chris
Registered User
Joined: May 2009
Posts: 2,255
From: Northern California
If you find your 70 series throttle plate does leak vacuum, the fix is throttle bushings which you can do at home for not much $.
But in the 170 series, Rochester made 800 CFM carbs (vs 750 cfm for 70-series) and due to the increasing emissions regulations in the later 1970’s made them extremely precise.
The thing that really sold me was the front inlet of the 170-series is much safer than the side inlet of the 70-series. The later carbs avoid a “freeze plug” that is supposed to seal the 90° turn from the side fuel inlet. In the 80’s I had a rebuild of one of these leak and start a gas fire on the block. That sucked. Burned up the harness and what not. I was very happy to eliminate the fire threat using the later design.
For the purists, no it’s not 1966 factory. But to me, the later carbs were just better - more CFM, more metering precision, less fire risk.
For anyone following along I still smile about Chris Witt and his April fools joke about “1966 Lansing air” in the tires for a really proper restoration. I like to think he & I are on the same page about factory vs. functional. I also recall his joke about old parts being UBU - Used But Useful. That’s an important phrase.
Cheers
Chris
But in the 170 series, Rochester made 800 CFM carbs (vs 750 cfm for 70-series) and due to the increasing emissions regulations in the later 1970’s made them extremely precise.
The thing that really sold me was the front inlet of the 170-series is much safer than the side inlet of the 70-series. The later carbs avoid a “freeze plug” that is supposed to seal the 90° turn from the side fuel inlet. In the 80’s I had a rebuild of one of these leak and start a gas fire on the block. That sucked. Burned up the harness and what not. I was very happy to eliminate the fire threat using the later design.
For the purists, no it’s not 1966 factory. But to me, the later carbs were just better - more CFM, more metering precision, less fire risk.
For anyone following along I still smile about Chris Witt and his April fools joke about “1966 Lansing air” in the tires for a really proper restoration. I like to think he & I are on the same page about factory vs. functional. I also recall his joke about old parts being UBU - Used But Useful. That’s an important phrase.
Cheers
Chris
Thread Starter
Registered User
Joined: Feb 2025
Posts: 74
When you dig into to qjets, you’ll figure out that the jets have a heavy influence on WOT mixture.
Where you really get the drivability gains is in the primary rods, primary piston spring stiffness and the vertical start/stop position of the primary power piston. For both the 70-series and 170-series qjets, Rochester made many many many springs, power pistons and rods. The combinations are d*mn near infinite, so keep a log of what combination you started with, what you changed (1 item at a time…) and what driving behavior you liked or didn’t like.
As you get into the qjet, the first test to complete is spraying a bit of Berrymans B12 Chemtool at the primary throttle rod holes on each side at the throttle plate. If you get a rise in RPM, you probably have worn throttle rods which represent a vacuum leak. Solve that first before moving on to jetting if you have that problem. Otherwise you’ll chase your tail forever. Simply put, the primary throttle rods going through the plate rotates every time you touch the gas pedal. So they wear out and get leaky.
The qjet uses vacuum signal to interpret driver demand. High vacuum = low demand; Low vacuum = high demand in its analog metering language. If you have a vacuum leak, the qjet mistinterprets demand and metering will be off. Especially at low RPMs around town where metering is particularly sensitive. The ugly one is the primary throttle rods, but look around for bad gaskets or other vacuum leaks too. They can all contribute to bad metering in a similar way. You’ve won the game when you spray carb cleaner all around the carb and don’t find any rise in engine RPMs.
Get it right and qjet’s really shine as mixture devices. If they leak gas, leak vacuum, have a too-loose secondary wrap spring (aka qjet bog) they suck and people put on Holley’s for simplicity.
If this is helpful to you, dig around under my user name for what little carb stuff I have to offer. There are many who know a lot more than me, but I dug in pretty deep long ago and most of it stuck. Also hit me with a private message if you’d like my wildly complicated excel workbook on qjet metering. I haven’t looked at it in years, but it does have some details on factory combinations for various engines & years of Olds big & small blocks with details on jets & rods and (dig this) “square metering area” aka mixture at various points of rod/jet interaction…
Cheers
Chris
Where you really get the drivability gains is in the primary rods, primary piston spring stiffness and the vertical start/stop position of the primary power piston. For both the 70-series and 170-series qjets, Rochester made many many many springs, power pistons and rods. The combinations are d*mn near infinite, so keep a log of what combination you started with, what you changed (1 item at a time…) and what driving behavior you liked or didn’t like.
As you get into the qjet, the first test to complete is spraying a bit of Berrymans B12 Chemtool at the primary throttle rod holes on each side at the throttle plate. If you get a rise in RPM, you probably have worn throttle rods which represent a vacuum leak. Solve that first before moving on to jetting if you have that problem. Otherwise you’ll chase your tail forever. Simply put, the primary throttle rods going through the plate rotates every time you touch the gas pedal. So they wear out and get leaky.
The qjet uses vacuum signal to interpret driver demand. High vacuum = low demand; Low vacuum = high demand in its analog metering language. If you have a vacuum leak, the qjet mistinterprets demand and metering will be off. Especially at low RPMs around town where metering is particularly sensitive. The ugly one is the primary throttle rods, but look around for bad gaskets or other vacuum leaks too. They can all contribute to bad metering in a similar way. You’ve won the game when you spray carb cleaner all around the carb and don’t find any rise in engine RPMs.
Get it right and qjet’s really shine as mixture devices. If they leak gas, leak vacuum, have a too-loose secondary wrap spring (aka qjet bog) they suck and people put on Holley’s for simplicity.
If this is helpful to you, dig around under my user name for what little carb stuff I have to offer. There are many who know a lot more than me, but I dug in pretty deep long ago and most of it stuck. Also hit me with a private message if you’d like my wildly complicated excel workbook on qjet metering. I haven’t looked at it in years, but it does have some details on factory combinations for various engines & years of Olds big & small blocks with details on jets & rods and (dig this) “square metering area” aka mixture at various points of rod/jet interaction…
Cheers
Chris
Registered User
Joined: May 2009
Posts: 2,255
From: Northern California
Minor qjet geek. But I truly stink at replacing those shaft bushings.
I can go right through primary rods, jets, all those vacuum passages, secondary wrap spring, goop seal the leaky well plugs, seal the vacuum heat thing on the passenger side to change the intake heat choke to electric, etc, etc, etc, but I’ve never gotten the primary shaft process right.
If you have a process that works, please share. I’ve ruined at least 4 or 5 throttle plates trying to learn. I hate the waste, but just don’t have the skill to do it right. Or maybe the tools - or maybe both.
Looking forward to your process and any photo so you’d care to share.
Many thanks
Chris
I can go right through primary rods, jets, all those vacuum passages, secondary wrap spring, goop seal the leaky well plugs, seal the vacuum heat thing on the passenger side to change the intake heat choke to electric, etc, etc, etc, but I’ve never gotten the primary shaft process right.
If you have a process that works, please share. I’ve ruined at least 4 or 5 throttle plates trying to learn. I hate the waste, but just don’t have the skill to do it right. Or maybe the tools - or maybe both.
Looking forward to your process and any photo so you’d care to share.
Many thanks
Chris
Thread
Thread Starter
Forum
Replies
Last Post
