Quadrajet Trouble
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,000
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
Quadrajet Trouble
This is my 4th Quadrajet in a month and I'm learning more about them, but need a little help. This last one was built by Jet Performance and looks to be assembled well. It surged at cruise speed so I changed from .069 jet, .039 rod to .067 jet, .039 rod. Surging was reduced dramatically, ran better but still rich (I think) with throttle hesitation. Changed to .067 jet, .042 rod, not much change. I'm using Lars tuneup sheets which has been a lot of help.
I can turn the idle mixture screws in all the way and the engine still runs. I can cover the air horn with my hand shutting off the air, no change in engine speed so I assume it's running very rich.
Am I going in too small increments with the jet & rods, or is there something else wrong? It seems I'm not getting anywhere fast.
I can turn the idle mixture screws in all the way and the engine still runs. I can cover the air horn with my hand shutting off the air, no change in engine speed so I assume it's running very rich.
Am I going in too small increments with the jet & rods, or is there something else wrong? It seems I'm not getting anywhere fast.
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,000
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
Checked timing, set at 10 deg. Did vacuum leak check. Car was running good with old quad, changed intake to performer, re-installed old quad, would not idle and the idle adjustment screws were corroded and broke off, so bought a new rebuilt one.
Then that one wouldn't idle, so they sent me another one, ran better but surged really bad.
Then that one wouldn't idle, so they sent me another one, ran better but surged really bad.
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
What is your idle vacuum? If you have less than stock vacuum, the power piston spring may be holding the primary rods too high. That would expose the tips of the rods and cause a rich condition. Try pulling a vacuum hose off the manifold to create a temporary air leak. See if that increases the idle speed. If it does, your rich condition is confirmed. You may be forced to open the carb and see what the rebuilder actually did to the carb. Read Lars' papers on problems with commercially rebuilt carbs.
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
Check ignition... 90% of all carburetor problems are caused by ignition!
Seriously, what's your cruise rpm?
What is your ignition initial timing set at?
What is the maximum centrifugal advance measured at the crank?
At what rpm do you have full centrifugal advance?
Are you using vacuum advance?
If so, how much at full vacuum?
Seriously, what's your cruise rpm?
What is your ignition initial timing set at?
What is the maximum centrifugal advance measured at the crank?
At what rpm do you have full centrifugal advance?
Are you using vacuum advance?
If so, how much at full vacuum?
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,000
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
What is your idle vacuum? If you have less than stock vacuum, the power piston spring may be holding the primary rods too high. That would expose the tips of the rods and cause a rich condition. Try pulling a vacuum hose off the manifold to create a temporary air leak. See if that increases the idle speed. If it does, your rich condition is confirmed. You may be forced to open the carb and see what the rebuilder actually did to the carb. Read Lars' papers on problems with commercially rebuilt carbs.
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,000
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
Check ignition... 90% of all carburetor problems are caused by ignition!
Seriously, what's your cruise rpm?
What is your ignition initial timing set at?
What is the maximum centrifugal advance measured at the crank?
At what rpm do you have full centrifugal advance?
Are you using vacuum advance?
If so, how much at full vacuum?
Seriously, what's your cruise rpm?
What is your ignition initial timing set at?
What is the maximum centrifugal advance measured at the crank?
At what rpm do you have full centrifugal advance?
Are you using vacuum advance?
If so, how much at full vacuum?
Cruise RPM 2000, 2500
10 deg.
don't know
not sure
yes
don't know
Like I said, car was running great until I changed manifolds and got rid of my old quad. No vacuum leaks.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
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
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
Too much spark advance at cruise will cause surging. Perhaps you have too much centrifugal advance AND too much vacuum advance because it's the total at cruise that is important to your surging problem.
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,000
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,000
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
First carb didn't have a surge problem and after changing the jets/rods almost went completely away. BTW, I still have a chronic rich condition.
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,000
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
Burning Brakes
Joined: Jan 2012
Posts: 943
Likes: 73
I am pretty sure that if the power piston has a real stiff spring, it will cause the carb to run rich, I guess the piston never gets to seat. I think I would try putting a new one in as a start. I am not a Q-Jet expert, maybe somebody has a way of telling if the power piston is seating correctly?
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Well we dont know what u have for a cam 59 but assume its stock and the only mod is the intake. If u bought 4 carbs in one month u can afford to buy a wide band O2 monitor. The prices for a WB O2 are so low these days and now i see a dual channel WB for
Last edited by cardo0; Sep 1, 2015 at 12:40 AM.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Well we dont know what u have for a cam 59 but assume its stock and the only mod is the intake. If u bought 4 carbs in one month u can afford to buy a wide band O2 monitor. The prices for a WB O2 are so low these days and now i see a dual channel WB for
Burning Brakes
Joined: Aug 2008
Posts: 878
Likes: 55
From: Deep South Denmark
Any change you warped the air horn when you installed all those carbs?
You have to be very careful when tightening the long bolts. "Thumb and pinky only".
With 18inHg the power piston should seat, but try the softer springs anyway.
Are you using a stock fuel pump? Check your fuel pressure.
You have to be very careful when tightening the long bolts. "Thumb and pinky only".
With 18inHg the power piston should seat, but try the softer springs anyway.
Are you using a stock fuel pump? Check your fuel pressure.
Last edited by Danish Shark; Sep 1, 2015 at 03:58 AM.
Thread Starter
Le Mans Master
Joined: Apr 2013
Posts: 5,000
Likes: 1,056
From: Arlington TX
2024 C1 of the Year Finalist - Unmodified
2023 Restomod of the Year Finalist
2023 C1 of the Year Finalist - Unmodified
I don't know what a wide band 02 monitor is. The carbs are guaranteed so they send me a new one. Yes, the engine is a stock '69 350, except the recently added edelbrock intake.
Yes, I know about the warpage, not the case. Using a Carter electric fuel pump with a new fuel regulator set at 3.5 psi (gauge verified).
Yes, I know about the warpage, not the case. Using a Carter electric fuel pump with a new fuel regulator set at 3.5 psi (gauge verified).