1975 Quadrajet bog issue
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Intermediate
Joined: Sep 2019
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From: SW Michigan
1975 Quadrajet bog issue
I bought a 1975 Corvette 4-speed. It drives fine on the primaries but if you floor it it bogs and pops and never accelerates. I can see the secondary butterflies open, both top and bottom seem to open ok.
I shipped the carb off and had it rebuilt. It looks like new now but after putting the carb back on and following all the "how to adjust air fuel idle screws" and getting the best vacuum at idle and then adjusting the idle speed the carb does the exact same thing as before the rebuild.
The car starts, runs and drives great on the primaries, if you never floor it you'd never know there was an issue.
Any thoughts on what I can try?
I shipped the carb off and had it rebuilt. It looks like new now but after putting the carb back on and following all the "how to adjust air fuel idle screws" and getting the best vacuum at idle and then adjusting the idle speed the carb does the exact same thing as before the rebuild.
The car starts, runs and drives great on the primaries, if you never floor it you'd never know there was an issue.
Any thoughts on what I can try?
Is my vette stock?? HAHA
Joined: Oct 2015
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From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
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2 things, first...
Did Lars rebuild it. If not you may want to talk to him or buy the Cliff Ruggles book and see if there is an issue internally
Second and good news for you, most bog issues is the air flap in the secondaries opening too fast. THere is a allen head set screw underneath the spring tensioner for the door that you cant see. You loosen that, adjust the spring so the door stays closed slightly longer and the bog will generally go away if all the jets and rods are the appropriate size. The bad news is, you say yours never accelerates after you floor it. If you slowly put the pedal to the floor so that everything opens slowly, what happens. IF you slowly transition into the secondaries that door will open slowly. If the door is the issue you will not get the bog if you go slow. If you get a bog after that then you have the wrong rods or jets in the secondaries. You would have to determine rich or lean issue at that point
If you get the book and take it apart yourself you can see the numbers on the jets and rods at that point
Did Lars rebuild it. If not you may want to talk to him or buy the Cliff Ruggles book and see if there is an issue internally
Second and good news for you, most bog issues is the air flap in the secondaries opening too fast. THere is a allen head set screw underneath the spring tensioner for the door that you cant see. You loosen that, adjust the spring so the door stays closed slightly longer and the bog will generally go away if all the jets and rods are the appropriate size. The bad news is, you say yours never accelerates after you floor it. If you slowly put the pedal to the floor so that everything opens slowly, what happens. IF you slowly transition into the secondaries that door will open slowly. If the door is the issue you will not get the bog if you go slow. If you get a bog after that then you have the wrong rods or jets in the secondaries. You would have to determine rich or lean issue at that point
If you get the book and take it apart yourself you can see the numbers on the jets and rods at that point
Last edited by Rescue Rogers; Oct 16, 2021 at 07:09 AM.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,218
Likes: 9,355
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Forgot about that...Really good point. Do that first
Le Mans Master
Joined: Mar 2004
Posts: 7,502
Likes: 1,509
From: Little Rock AR
The secondary windup needs to be checked (after the timing is verified)
Somewhere between 7/8 and 3/4 of a turn past closed is probably in the ballpark. If it is too soft a setting, you will get a bog. Too tight and it will open slowly or not at all.
Lars also could fix it. I would just send it to him if you can't get it to work right.
Somewhere between 7/8 and 3/4 of a turn past closed is probably in the ballpark. If it is too soft a setting, you will get a bog. Too tight and it will open slowly or not at all.
Lars also could fix it. I would just send it to him if you can't get it to work right.
Thread Starter
Intermediate
Joined: Sep 2019
Posts: 41
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From: SW Michigan
2 things, first...
Did Lars rebuild it. If not you may want to talk to him or buy the Cliff Ruggles book and see if there is an issue internally
https://www.amazon.com/Rebuild-Modif...8&sr=8-1-fkmr1
Second and good news for you, most bog issues is the air flap in the secondaries opening too fast. There is a Allen head set screw underneath the spring tensioner for the door that you cant see. You loosen that, adjust the spring so the door stays closed slightly longer and the bog will generally go away if all the jets and rods are the appropriate size. The bad news is, you say yours never accelerates after you floor it. If you slowly put the pedal to the floor so that everything opens slowly, what happens. IF you slowly transition into the secondaries that door will open slowly. If the door is the issue you will not get the bog if you go slow. If you get a bog after that then you have the wrong rods or jets in the secondaries. You would have to determine rich or lean issue at that point
If you get the book and take it apart yourself you can see the numbers on the jets and rods at that point
Did Lars rebuild it. If not you may want to talk to him or buy the Cliff Ruggles book and see if there is an issue internally
https://www.amazon.com/Rebuild-Modif...8&sr=8-1-fkmr1
Second and good news for you, most bog issues is the air flap in the secondaries opening too fast. There is a Allen head set screw underneath the spring tensioner for the door that you cant see. You loosen that, adjust the spring so the door stays closed slightly longer and the bog will generally go away if all the jets and rods are the appropriate size. The bad news is, you say yours never accelerates after you floor it. If you slowly put the pedal to the floor so that everything opens slowly, what happens. IF you slowly transition into the secondaries that door will open slowly. If the door is the issue you will not get the bog if you go slow. If you get a bog after that then you have the wrong rods or jets in the secondaries. You would have to determine rich or lean issue at that point
If you get the book and take it apart yourself you can see the numbers on the jets and rods at that point
No, Lars did not rebuild it.
If I put my foot slowly to the floor it behaves the same way, at some point it just bogs and pops and hiccups. ....the bog never clears up.
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Intermediate
Joined: Sep 2019
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From: SW Michigan
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Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,218
Likes: 9,355
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Is your vacuum advance working? Is your mechanical advance working?
Thread Starter
Intermediate
Joined: Sep 2019
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From: SW Michigan
It was rebuilt by a eBay seller named Danny Aldridge (eBayID dlataa), he is located in Aiken, SC and the cost was $125 return shipping included.
He installed a new fuel filter. I paid a little extra to replace a bad choke diaphragm? I think and then i also had it converted to an electric choke from heat pipe.
He installed a new fuel filter. I paid a little extra to replace a bad choke diaphragm? I think and then i also had it converted to an electric choke from heat pipe.
Thread Starter
Intermediate
Joined: Sep 2019
Posts: 41
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From: SW Michigan
Lars, what do you charge to look over a carb?
My 75 Corvette is just a stock L-48 engine.
Thread Starter
Intermediate
Joined: Sep 2019
Posts: 41
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From: SW Michigan
I'm wondering if I'm just better off going with this....
https://www.jegs.com/i/Edelbrock/350...RoC7y4QAvD_BwE
$360 for a Edelbrock AVS2 1906 Carburetor [650 CFM Electric Choke]
https://www.jegs.com/i/Edelbrock/350...RoC7y4QAvD_BwE
$360 for a Edelbrock AVS2 1906 Carburetor [650 CFM Electric Choke]
Le Mans Master
Joined: Mar 2004
Posts: 7,502
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From: Little Rock AR
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Absolutely the WORST thing you can do is install an Edelbrock square bore carb on a GM spread bore intake.......then you can chase down why it runs poorly after it is installed. The mismatch that the required "adapter" creates is pretty terrible.......and it will 100% never run right.
Get the carb that came with the car repaired. Send it to Lars and see what he says.......
There is no carb that will function better than a Q-Jet on a mild 350. None.
I do a lot of them and every engine I bolt one to runs like a watch......it is crazy how well a well done Q-Jet runs.
Jebby
Get the carb that came with the car repaired. Send it to Lars and see what he says.......
There is no carb that will function better than a Q-Jet on a mild 350. None.
I do a lot of them and every engine I bolt one to runs like a watch......it is crazy how well a well done Q-Jet runs.
Jebby
Race Director
Joined: Oct 2000
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From: central FL, near the beach.
Jebby
the holley 4165 is better. Had a shoot out with Lars years ago- the 4165 won.
But i highly recommend Lars- he knows qjets!
Since the OP has a qjet, it will cost less to keep it.
the holley 4165 is better. Had a shoot out with Lars years ago- the 4165 won.
But i highly recommend Lars- he knows qjets!
Since the OP has a qjet, it will cost less to keep it.
Dr. Detroit
Joined: Mar 2012
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From: New Braunfels Texas
Jebby
Melting Slicks
Joined: Nov 2006
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From: Apple Valley, MN
The last properly rebuilt/setup Q-jet on the planet will have been done by Lars.
Remember the “Porsche - there is no substitute” ads? Think “Lars/Q-jet - there is no substitute “.
Remember the “Porsche - there is no substitute” ads? Think “Lars/Q-jet - there is no substitute “.
Thread Starter
Intermediate
Joined: Sep 2019
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From: SW Michigan