Q-Jet Question
Thread Starter
Safety Car
Joined: Sep 2011
Posts: 4,116
Likes: 1,230
From: Madeira Beach, FL
2024 C8 of the Year Finalist - Unmodified
2024 C2 of the Year Finalist - Unmodified
2023 C2 of the Year Finalist - Unmodified
2020 C3 of the Year Finalist - Unmodified
Q-Jet Question
Hello,
I have a carb question for the forum.
I looked today at my carb to see what# is on it.
I recently purchased a 68 427/390 hp 4spd car.
I was told the carb was wrong when I purchased the car.
The car runs great.
I do need to pump the accelerator briskly till it fires up.
What is on the car now is a 209202 DH 0169.
Other sites report this as a 69 350/300 hp auto application.
Am I cheating myself out of more performance with this carb?
It gets decent mpg and runs great.
Other than application/year am I cheating myself on the performance side?
Thanks in advance for all responses.
I'd like to have the right 7028209 FG if I can find one and if its a big improvement.
Marshal
I have a carb question for the forum.
I looked today at my carb to see what# is on it.
I recently purchased a 68 427/390 hp 4spd car.
I was told the carb was wrong when I purchased the car.
The car runs great.
I do need to pump the accelerator briskly till it fires up.
What is on the car now is a 209202 DH 0169.
Other sites report this as a 69 350/300 hp auto application.
Am I cheating myself out of more performance with this carb?
It gets decent mpg and runs great.
Other than application/year am I cheating myself on the performance side?
Thanks in advance for all responses.
I'd like to have the right 7028209 FG if I can find one and if its a big improvement.
Marshal
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,117
From: Crossville TN
Basically, all Q-jets made prior to 1975 can be made to work on any engine requiring 750 cfm airflow or less. All that needs to be changed (for reasonably good function and performance) are the rods, jets and maybe some throttle/choke linkage connections.
If your carb performs well, is reasonably economical on the highway, and the spark plugs look good when you inspect them, that's all you need to know. The excessive 'pumping of the throttle' to get the car started could be an accelerator pump seal issue or loss of fuel in the carb's fuel well due to syphoning back to the tank after the engine is shut down. And, if performance is not a problem, then your accelerator pump is likely fine.
You may want to change the fuel filter and replace it with one that has a check valve flap so that fuel cannot back-flow out of the carb.
If your carb performs well, is reasonably economical on the highway, and the spark plugs look good when you inspect them, that's all you need to know. The excessive 'pumping of the throttle' to get the car started could be an accelerator pump seal issue or loss of fuel in the carb's fuel well due to syphoning back to the tank after the engine is shut down. And, if performance is not a problem, then your accelerator pump is likely fine.
You may want to change the fuel filter and replace it with one that has a check valve flap so that fuel cannot back-flow out of the carb.
Thread Starter
Safety Car
Joined: Sep 2011
Posts: 4,116
Likes: 1,230
From: Madeira Beach, FL
2024 C8 of the Year Finalist - Unmodified
2024 C2 of the Year Finalist - Unmodified
2023 C2 of the Year Finalist - Unmodified
2020 C3 of the Year Finalist - Unmodified
Thank you for the education on the Q-Jet.
It is good to know what's on there is usable.
I see the cast# for my year and engine can be quite expensive.
The check valve in the fuel filter is a great idea like in my sump pump.
I wasn't aware fuel would drain back towards the tank.
Acceleration is good.
I'm new to the car so I'll drive awhile as such and monitor performance.
I'll check with the auto parts store for fuel filter with anti back drain feature.
Marshal
It is good to know what's on there is usable.
I see the cast# for my year and engine can be quite expensive.
The check valve in the fuel filter is a great idea like in my sump pump.
I wasn't aware fuel would drain back towards the tank.
Acceleration is good.
I'm new to the car so I'll drive awhile as such and monitor performance.
I'll check with the auto parts store for fuel filter with anti back drain feature.
Marshal
Race Director
Joined: Apr 1999
Posts: 11,520
Likes: 13
From: CORVETTE 77 385 C.I. TEXAS
If you are into perf., Holley makes a Q-J replacement DP. Also avail. as a VS.
Q-J are normally 750 & have very small primary. There is an 800 Q-J that has larger primary that would be good on 383+ even if more cfm was not needed. Any Q-J prior to the 80s electronic can be set up to work well.
You can check the accel. pump for a strong shot & there is normally an accel. pump adj.
Q-J are normally 750 & have very small primary. There is an 800 Q-J that has larger primary that would be good on 383+ even if more cfm was not needed. Any Q-J prior to the 80s electronic can be set up to work well.
You can check the accel. pump for a strong shot & there is normally an accel. pump adj.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
Contrary to myth, a carb different from a QJ won't get you more HP. There's been thousands of people who switched to a Holley or Edelbrock or brand X only to see no change.
Keep what you've got and tune it as required.
Keep what you've got and tune it as required.
Melting Slicks
Joined: Aug 2005
Posts: 2,683
Likes: 34
From: Working on the car NY
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
'69 did not have the "best" Qjets.
If u want the skinny on Qjets u need to read the book by Cliff Ruggles. The better Qjets came out in late '70s with electric chokes too. An original '69 Qjet won't make the power a later '70s series will.
Good luck,
cardo0
Good luck,
cardo0
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,117
From: Crossville TN
Actually, there are some changes to parts in a late 70's Q-Jet that need to be made so that it will perform as well as those from earlier years. I think you need to re-read Cliff's book.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Race Director
Joined: Aug 2004
Posts: 10,628
Likes: 132
From: Duvall, WA
I think his point was that Cliff said if you are going rebuild a carb, the later ones are a better choice than the earlier ones. Either way, you are going to have to build it for the engine that is in the car.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
U need to speciy what u think your talking about. What changes would a late 70's carb need that a 69 Qjet didn't??
But i do know an 800cfm qjet with APT will be an improvement over a stock '69 750cfm Qjet without APT. And the stock body plugs sealed much better than early year carbs too. Like Rochester was'nt improving their carb's? Its a no brainer they made big improvements to remain competetive. Maybe u need a tutor to help u read Ruggle book.
cardo0
Drifting
Joined: Mar 2006
Posts: 1,526
Likes: 2
From: Oak Ridge NC
Thank you for the education on the Q-Jet.
It is good to know what's on there is usable.
I see the cast# for my year and engine can be quite expensive.
The check valve in the fuel filter is a great idea like in my sump pump.
I wasn't aware fuel would drain back towards the tank.
Acceleration is good.
I'm new to the car so I'll drive awhile as such and monitor performance.
I'll check with the auto parts store for fuel filter with anti back drain feature.
Marshal
It is good to know what's on there is usable.
I see the cast# for my year and engine can be quite expensive.
The check valve in the fuel filter is a great idea like in my sump pump.
I wasn't aware fuel would drain back towards the tank.
Acceleration is good.
I'm new to the car so I'll drive awhile as such and monitor performance.
I'll check with the auto parts store for fuel filter with anti back drain feature.
Marshal
Race Director
Joined: Apr 1999
Posts: 11,520
Likes: 13
From: CORVETTE 77 385 C.I. TEXAS
CARBS Tests TQ/HP
...
I recently purchased a 68 427/390 hp 4spd car. ...
I do need to pump the accelerator briskly till it fires up.
What is on the car now is a 209202 DH 0169.
Other sites report this as a 69 350/300 hp auto application.
Am I cheating myself out of more performance with this carb?
It gets decent mpg and runs great.
Other than application/year am I cheating myself on the performance side?
...
Marshal
I recently purchased a 68 427/390 hp 4spd car. ...
I do need to pump the accelerator briskly till it fires up.
What is on the car now is a 209202 DH 0169.
Other sites report this as a 69 350/300 hp auto application.
Am I cheating myself out of more performance with this carb?
It gets decent mpg and runs great.
Other than application/year am I cheating myself on the performance side?
...
Marshal
The 350 Q-J is probably lean & accel. pump weak. You can pull the sec. rods & check the 2 letter code.
Since you asked about performance I mentioned the Holley Double Pumper. Do you know what that is?
PS Regardless of a 350 Q-J that probably needs tuning, the stock guys basically say leave it stock & forget performance in which case a stock Q-J works good for economy.
Some might find this interesting.
CARBS Tests TQ/HP
http://www.corvettefaq.com/c3/carbcomparo.htm
Safety Car
Joined: Oct 2000
Posts: 3,593
Likes: 103
From: Southern Indiana
If you switch to a Holley and your car doesn't run better you better haul your old horse to the glue factory and cash it in now.
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
If you want to modify a qjet then a 76 or later is better.
The later carbs were improved for emissions are are extremely lean. At minimum the air bleeds have to be changed to be able to go back down in jet sizes so the available jets and rods can be used. The later carbs are already maxed out in jet and rod sizes in stock form.
Without getting too detailed, that is basically why a stock old qjet will outperform a stock later carb.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,117
From: Crossville TN
Correct. Later Q-Jets were made to be lean for emissions, not tuned/constructed for performance. The APT can be an asset to tuning, but having it is not a requirement for improving the carb's performance or function.
Thread Starter
Safety Car
Joined: Sep 2011
Posts: 4,116
Likes: 1,230
From: Madeira Beach, FL
2024 C8 of the Year Finalist - Unmodified
2024 C2 of the Year Finalist - Unmodified
2023 C2 of the Year Finalist - Unmodified
2020 C3 of the Year Finalist - Unmodified
Well Gentleman,
After a bit of jousting and a very informative thread, I've been comforted to know that what I have is working satisfactorily.
I do not want to smoke the tires off just enjoy it and if I step in it have it respond as designed.
I will eventually pull the secondary rods and report back what's in there now and see if any improvement can be made for wide open throttle.
Thank you again and much more informed.
Marshal
After a bit of jousting and a very informative thread, I've been comforted to know that what I have is working satisfactorily.
I do not want to smoke the tires off just enjoy it and if I step in it have it respond as designed.
I will eventually pull the secondary rods and report back what's in there now and see if any improvement can be made for wide open throttle.
Thank you again and much more informed.
Marshal
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
Chevy Q-Jets to 1974 are basically the same. As stated above, rods and jets would be different depending on the engine and as stated above, 1974 and prior carbs are preferable for performance. They have a broader tuning range.
One thing I always do, since the Q-Jet's achilles heal is it's small fuel bowl, is swap to the larger needle and seat (.135" vs. the stock .125") which will allow the fuel bowl to keep up with demand if yo're running more HP. If you need, I can forward the GM part number.
Here's a great source for q-jet parts to help with tuning, rods, jets, etc.
http://www.carburetion.com/quadrajet.asp
But learn about the Q-Jet. The more you learn, the more you'll like it!
One thing I always do, since the Q-Jet's achilles heal is it's small fuel bowl, is swap to the larger needle and seat (.135" vs. the stock .125") which will allow the fuel bowl to keep up with demand if yo're running more HP. If you need, I can forward the GM part number.
Here's a great source for q-jet parts to help with tuning, rods, jets, etc.
http://www.carburetion.com/quadrajet.asp
But learn about the Q-Jet. The more you learn, the more you'll like it!
Last edited by 73, Dark Blue 454; Sep 29, 2011 at 11:24 PM.
Advanced
Joined: Apr 2011
Posts: 84
Likes: 0
From: Mobile Alabama
Basically, all Q-jets made prior to 1975 can be made to work on any engine requiring 750 cfm airflow or less. All that needs to be changed (for reasonably good function and performance) are the rods, jets and maybe some throttle/choke linkage connections.
If your carb performs well, is reasonably economical on the highway, and the spark plugs look good when you inspect them, that's all you need to know. The excessive 'pumping of the throttle' to get the car started could be an accelerator pump seal issue or loss of fuel in the carb's fuel well due to syphoning back to the tank after the engine is shut down. And, if performance is not a problem, then your accelerator pump is likely fine.
You may want to change the fuel filter and replace it with one that has a check valve flap so that fuel cannot back-flow out of the carb.
If your carb performs well, is reasonably economical on the highway, and the spark plugs look good when you inspect them, that's all you need to know. The excessive 'pumping of the throttle' to get the car started could be an accelerator pump seal issue or loss of fuel in the carb's fuel well due to syphoning back to the tank after the engine is shut down. And, if performance is not a problem, then your accelerator pump is likely fine.
You may want to change the fuel filter and replace it with one that has a check valve flap so that fuel cannot back-flow out of the carb.
Thanks Rob
Thread Starter
Safety Car
Joined: Sep 2011
Posts: 4,116
Likes: 1,230
From: Madeira Beach, FL
2024 C8 of the Year Finalist - Unmodified
2024 C2 of the Year Finalist - Unmodified
2023 C2 of the Year Finalist - Unmodified
2020 C3 of the Year Finalist - Unmodified
Thanks everyone,
I'm going to tune what I have and appreciate the responses.
Filter # was a bonus too
Marshal
I'm going to tune what I have and appreciate the responses.
Filter # was a bonus too
Marshal