Carb Change
August 29th, 2021, 03:39 PM
#1
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Thread Starter
Join Date: Oct 2020
Location: Freetown, Massachusetts
Posts: 140
Carb Change
As much as I wanted to keep this build original, I have been
So I also have an 85 Monte SS with an HP 5.0 and its just pre tbi but computer controlled E4MC Quadrajet.
I was referred to a NJ carb builder with a flow bench etc...
Talked with him about my issues with and CCC Qjet, and then about my Olds 400 Qjet issues.
So as it turns out while the cam in this engine has base vacuum at 12 or so, the Qjet will forever be late
on fuel delivery as a full vacuum controlled carb.
So the next option is a perfect Holley 650 dual feed to be tested.
I am just wondering where the main jets should be where for most its .068-70 on a chev, but not sure with the
400 or its compression ratio where ideal is.
If you have done this, please let me know.
So I also have an 85 Monte SS with an HP 5.0 and its just pre tbi but computer controlled E4MC Quadrajet.
I was referred to a NJ carb builder with a flow bench etc...
Talked with him about my issues with and CCC Qjet, and then about my Olds 400 Qjet issues.
So as it turns out while the cam in this engine has base vacuum at 12 or so, the Qjet will forever be late
on fuel delivery as a full vacuum controlled carb.
So the next option is a perfect Holley 650 dual feed to be tested.
I am just wondering where the main jets should be where for most its .068-70 on a chev, but not sure with the
400 or its compression ratio where ideal is.
If you have done this, please let me know.
August 29th, 2021, 09:21 PM
#3
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Join Date: Jan 2010
Location: central Indiana
Posts: 5,401
Late on fuel delivery???? What does that mean?
The computer controlled Q-Jet isn’t much different than the old school Q-Jet. The metering rods are controlled by a solenoid to control part throttle mixture. The computer has ZERO influence on WOT, you can calibrated full throttle just like any other Q-Jet. Some of the later Q-Jets on smaller engines are mechanically limited to less than the usually 750 cfm.
Keep in mind, if you install a Holley, you need to swap distributors also. The computer will not be happy without the inputs from the TPS, the open circuit to the mixture control solenoid, the timing will be stuck at its base setting. Swapping carbs will also disable the computer from controlling the lockup torque converter.
Every Q-Jet is technically a vacuum secondary carb. You change the rate the secondary air door opens with the tension spring on the passenger side of the carb. The size of the orifice in the choke pull off also influences the air door opening. Any carb builder should know about that.
Before spending a dime with this guy, you need to ask a bunch of questions. Bolting on a Holley on your Monte Carlo is going to create a bunch of headaches. I’m betting this guy doesn’t know the first thing about Q-Jets, and even less about the GM CCC system.
The computer controlled Q-Jet isn’t much different than the old school Q-Jet. The metering rods are controlled by a solenoid to control part throttle mixture. The computer has ZERO influence on WOT, you can calibrated full throttle just like any other Q-Jet. Some of the later Q-Jets on smaller engines are mechanically limited to less than the usually 750 cfm.
Keep in mind, if you install a Holley, you need to swap distributors also. The computer will not be happy without the inputs from the TPS, the open circuit to the mixture control solenoid, the timing will be stuck at its base setting. Swapping carbs will also disable the computer from controlling the lockup torque converter.
Every Q-Jet is technically a vacuum secondary carb. You change the rate the secondary air door opens with the tension spring on the passenger side of the carb. The size of the orifice in the choke pull off also influences the air door opening. Any carb builder should know about that.
Before spending a dime with this guy, you need to ask a bunch of questions. Bolting on a Holley on your Monte Carlo is going to create a bunch of headaches. I’m betting this guy doesn’t know the first thing about Q-Jets, and even less about the GM CCC system.
Last edited by matt69olds; August 29th, 2021 at 09:29 PM.
August 29th, 2021, 09:52 PM
#4
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Join Date: Feb 2013
Location: Phoenix, AZ
Posts: 14,691
I think you have been given inaccurate information.
The QuadraJet doesn’t care what the engine’s idle vacuum is; the secondaries are air flow operated, not vacuum operated.
The 1970 manual trans W-30 engine used a QJet, and that engine didn’t make enough vacuum for a primary power piston to operate. You should be able to easily get your QJet dialed in, as long as you get correct information and/or someone who actually knows about them.
The QuadraJet doesn’t care what the engine’s idle vacuum is; the secondaries are air flow operated, not vacuum operated.
The 1970 manual trans W-30 engine used a QJet, and that engine didn’t make enough vacuum for a primary power piston to operate. You should be able to easily get your QJet dialed in, as long as you get correct information and/or someone who actually knows about them.
August 30th, 2021, 02:51 AM
#5
Running On Empty
Join Date: Jul 2018
Location: Earth
Posts: 20,107
See Post #31 >>> Quadrajet Question--- what causes the secondaries to open?
August 30th, 2021, 06:01 AM
#6
Old(s) Fart
Join Date: Mar 2007
Location: Northern VA
Posts: 48,229
The Q-Jet is not a full vacuum controlled carb....inaccurate information.
See Post #31 >>> Quadrajet Question--- what causes the secondaries to open?
See Post #31 >>> Quadrajet Question--- what causes the secondaries to open?
August 30th, 2021, 09:01 AM
#8
Registered User
Thread Starter
Join Date: Oct 2020
Location: Freetown, Massachusetts
Posts: 140
Ok, so a few things to cover
This isn't a ccc, it was the first topic of discussion when I called the guys at this shop per the Monte SS forum referral.
https://www.customrebuiltcarbs.com/
The subject of my Olds came up afterwards where the discussion of vacuum came up.
This is not a secondary issue, its a primary issue and it idles very nicely.
The primary metering is regulated by vacuum, yes.
When I say "late" I mean lean off idle fall on your face till you put your foot in it and the carb catches up.
Per another shop, a lighter spring has been put under the power valve plunger. https://quadrajetparts.com/quadrajet...ode-p-210.html
When I got this last year I put a kit in this and noted the spring had been cut, replacing it with a standard one from my salvage qjets and it worked perfectly on the 350 that was in the car when I got it.
Then the cammed 400 went in and its been a problem since.
So back to the advice from Custom Rebuilt, they recommended a Holley and I have a near perfect one that is getting a kit currently.
It came off a 75 350 Corvette, and for the sake of trying it led me to the question about main jets.
Pretty well seasoned on Qjets, but not looking to take this one down 10 times to make tuning changes if I can avoid it.
Yes the 650 might be a little small, but substitution might help isolate this for me also.
Thinking a 750 might be a little big.
The cam is a Comp 42-308-4 if anybody wants to look at whats in there.
Thanks for the feed back and I will certainly read those other threads on this.
r
This isn't a ccc, it was the first topic of discussion when I called the guys at this shop per the Monte SS forum referral.
https://www.customrebuiltcarbs.com/
The subject of my Olds came up afterwards where the discussion of vacuum came up.
This is not a secondary issue, its a primary issue and it idles very nicely.
The primary metering is regulated by vacuum, yes.
When I say "late" I mean lean off idle fall on your face till you put your foot in it and the carb catches up.
Per another shop, a lighter spring has been put under the power valve plunger. https://quadrajetparts.com/quadrajet...ode-p-210.html
When I got this last year I put a kit in this and noted the spring had been cut, replacing it with a standard one from my salvage qjets and it worked perfectly on the 350 that was in the car when I got it.
Then the cammed 400 went in and its been a problem since.
So back to the advice from Custom Rebuilt, they recommended a Holley and I have a near perfect one that is getting a kit currently.
It came off a 75 350 Corvette, and for the sake of trying it led me to the question about main jets.
Pretty well seasoned on Qjets, but not looking to take this one down 10 times to make tuning changes if I can avoid it.
Yes the 650 might be a little small, but substitution might help isolate this for me also.
Thinking a 750 might be a little big.
The cam is a Comp 42-308-4 if anybody wants to look at whats in there.
Thanks for the feed back and I will certainly read those other threads on this.
r
August 30th, 2021, 09:05 AM
#9
Registered User
Thread Starter
Join Date: Oct 2020
Location: Freetown, Massachusetts
Posts: 140
I do understand how the secondary air valves are operated and adjusted.
We had a gram indicator tool to measure the back force on these from Mercruiser.
Is there a base turn count setting that can be initiated before final setting?
This one is set to just close and an 1/8 turn I believe, but again this is a primary/off idle issue.
We had a gram indicator tool to measure the back force on these from Mercruiser.
Is there a base turn count setting that can be initiated before final setting?
This one is set to just close and an 1/8 turn I believe, but again this is a primary/off idle issue.
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