Quadrajet tuning success - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Mertz · 04-12-2010, 05:49 PM

I just wanted to give everyone some information about my attempts to get a quadrajet running on my 72 Chevy truck with 350 and 350 auto. First you should know that Cliff Ruggles has been helping me solve problems and has been very willing to email me with answers to all my questions. I have both his and Roe’s book but find his more helpful.

First thing I learned was to check the carb you are using for the original jets and rods. You can find this on several web sites if you have the carb number. The next and most overlooked is the size of the air bleeds and how many air bleeds the carb has. Large air bleed carbs require larger jets. I started this project with what I thought would be an easy bolt on and go. I found a Jet Performance Stage II carb in almost new condition. The guy had it on a Dodge pickup with a 318 and took it off to sell the truck. I don’t believe it ever ran well on the truck and that is why he took it off. The carb number showed it came off a 77 Corvette and should have 77 jets and 52 rods and had large air bleeds. It actually came from Jet with 73 jets and 43 rods (a pretty standard combination). I put it on the truck and stomped on it and it fell flat on its face. I tried a number of things but could not get it running right (not too much help from the Jet tech line). I finally switched to another carb I had but one I could not find the specs for. I ended up putting in 71 jets and 43 rods. By this time I got my Innovate LC-1 installed. After running it and checking the mixture on the computer I found it was running very rich. I changed to 64 jets with 43 rods and got it to lean out but now it was too lean so I went to 66 jets and have it running at about 13.5 at cruise, a little rich at idle (12.8) and 12.8 or so at WOT. It is leaning out a little when I get on it at cruise so I am going up to 68 jets. I have a carb with an APT that helps fine tune the mixture. If you are looking for a good tunable qjet get one from after 1975 but before they went electronic. These will have the APT for fine tuning the A/F.

The long and short of all this is that the truck has never run better. It had an Edlebrock 1406 on it when I got it and it ran fine. I just wasn’t satisfied with the power and the fuel economy. I can now spin the big tires from a stop and am getting 16+ mpg in a ¾ ton truck with highway gears and regular gas. Best mileage with the Edelbrock was 14 unless I was running premium gas and cruising on flat ground. I think I can still do a little better than 16 with the final jet change. My goal was 18. I can now tap the gas a little and it launches. Some have said a well tuned qjet is better than EFI and I can now say that I believe it. And there is no better sound than when the secondarys kick in (there goes the gas mileage).

Sinister · 04-12-2010, 05:55 PM

Congrats. I also have an LC-1, takes a lot of guess work out of tuning a carb.

jaros44sr · 04-12-2010, 08:28 PM

OK, so what is a LC-1 and where do I get one...

Sinister · 04-12-2010, 08:41 PM

Quote:

Originally Posted by jaros44sr

OK, so what is a LC-1 and where do I get one...

An Innovate LC-1 is a wideband O2 sensor controller. I got mine off fleabay with a Innovate DB gauge. Spendy but worth it.

6D8_C10 · 04-12-2010, 09:51 PM

Hey Mertz,

I am currently running an Edelbrock 1406, and I got a QJet from jegs.
I have put it on my truck a couple of times and can't seem to get it to run.

I can start it, and it will run, by adjusting the fast idle. But, when I put it in gear it dies. I have tried turning up the idle adjustments, turning a full 4 turns on each side, and still dies on me.

Anyone have suggestions?

Quackaddict · 04-13-2010, 09:36 AM

I am with you guys, my Q-jet on my truck needs a rebuild and was thinking of replacing with a Holly Truck Avenger or an edelbrock but tuning a carb scares the fire out of me because I have never messed with one.

dan42 · 04-13-2010, 10:05 AM

Quote:

Originally Posted by 6D8_C10

Hey Mertz,

I am currently running an Edelbrock 1406, and I got a QJet from jegs.
I have put it on my truck a couple of times and can't seem to get it to run.

I can start it, and it will run, by adjusting the fast idle. But, when I put it in gear it dies. I have tried turning up the idle adjustments, turning a full 4 turns on each side, and still dies on me.

Anyone have suggestions?

You might check for vacuum leaks, make sure everything that needs to be capped is capped on the carb.

ChevLoRay · 04-13-2010, 10:27 AM

You gotta be careful when you turn those idle air screws. A lot of them have been adjusted over their lifetime and can cause idle problems when they become grooved where the needle seats in the throttle body. At that point, you are not getting the setting you are trying to achieve.

If you suspect a vacuum leak, simply put your hand over the open carburetor. If there is a leak, it should smooth out. If no leak, it should die when you deprive the carb of any air.

I used to seat them (gently), then back 'em out 2.5 turns as a reference point. Then, I'd use the Sun machine to monitor the engine conditions as I sought to balance the idle air settings.

6D8_C10 · 04-13-2010, 10:48 AM

Quote:

Originally Posted by ChevLoRay

You gotta be careful when you turn those idle air screws. A lot of them have been adjusted over their lifetime and can cause idle problems when they become grooved where the needle seats in the throttle body. At that point, you are not getting the setting you are trying to achieve.

If you suspect a vacuum leak, simply put your hand over the open carburetor. If there is a leak, it should smooth out. If no leak, it should die when you deprive the carb of any air.

I used to seat them (gently), then back 'em out 2.5 turns as a reference point. Then, I'd use the Sun machine to monitor the engine conditions as I sought to balance the idle air settings.

Thanks I will give that a try.

Mertz · 04-13-2010, 10:59 AM

I would suggest taking it apart and throughly cleaning it. My Jet Performance carb, although partically new, was plugged is several places. Check the carb number on line and see what jet and rod sizes it is suppose to have and then see what is actually in it. Change if you have to. Before you do any of this get Cliff Ruggles book on the quadrajet. It will help a great deal. Even new carbs can have crap in them that will make the carb not work. Some rebuilders don't really know what they are doing. I found a lot of my later carbs have early rods which don't work in the later carb. Also make sure your ignition system is working properly before trying to adjust the carb. It makes a big difference. I have an MSD ready to run on mine and I can get any carb on it to run so I can make adjustments. Cliff has some ways to find out if you are running rich or lean if you don't have an LC-1 or LM-1. You can get real close using his methods. A good running Qjet will out perform that Edelbrock any day. The Edelbrock is still a good carb and I used it for 5 years but I am really please with the Qjet. And as said above check for vacuum leaks and your choke setting.

'72customdeluxe · 04-13-2010, 12:59 PM

Is there any good way to adjust the square idle screws while it's on the engine?

hotrod100 · 04-13-2010, 01:53 PM

Quote:

Originally Posted by '72customdeluxe

Is there any good way to adjust the square idle screws while it's on the engine?

http://www.autozone.com/autozone/cat...&brandName=OEM

I use one of these. They have different size ends and the flexible shaft make it pretty easy to get at the adjusting screws.

6D8_C10 · 04-13-2010, 02:30 PM

Quote:

Originally Posted by '72customdeluxe

Is there any good way to adjust the square idle screws while it's on the engine?

I need one myself.
Snap on sells a socket called the double D.
They are the only one I have found so far.

I was using a cotterpin and spread it out a little (backyard engineer)

Mertz · 04-13-2010, 03:36 PM

Take them out and put slotted ones in. I haven't seen any square idle mixture screws on a qjet.

'72customdeluxe · 04-13-2010, 05:03 PM

eh, they're rectangular. Can't get any needle nose pliers in there to turn them nor will a flathead

Mertz · 04-13-2010, 05:22 PM

I built a device to turn the APT using a bolt and a hack saw. I cut a slot in the bolt wide enough to go over the retangular part of the screw. It works perfectly. It might work for your idle mixture screws. Is this a newer carb that had plugs over the idle mixture screws so you could not mess with them?

'72customdeluxe · 04-13-2010, 06:26 PM

'78 model, I can get to them with small pliers, but with the engine off

kyle fitz · 04-13-2010, 10:08 PM

i like my q-jet. when i first got my truck it ran like crap. it wouldnt change at all when i adjusted the mixture screws. so i rebuilt it and i havent had a problem with it at all

Jim_PA · 04-13-2010, 11:22 PM

May the force be with you

**LostMy65** · 01-26-2011, 03:18 AM

I read in another forum where a guy took a piece of brake line and flattened it just enough so it fit over GM's double 'D' screws.

Here's Snapon's Double-D Socket:
Stock#: S6139
Socket, Double "D" Carburetor Adjusting (3.5mm across flats)
http://buy1.snapon.com/catalog/item....&group_ID=1441

And here's what Mac has:
http://www.mactools.com/productdetai...tor-tools.aspx

04-12-2010, 05:49 PM	#1
Mertz Registered User Join Date: Aug 2007 Location: Reardan, WA Posts: 132	Quadrajet tuning success I just wanted to give everyone some information about my attempts to get a quadrajet running on my 72 Chevy truck with 350 and 350 auto. First you should know that Cliff Ruggles has been helping me solve problems and has been very willing to email me with answers to all my questions. I have both his and Roe’s book but find his more helpful. First thing I learned was to check the carb you are using for the original jets and rods. You can find this on several web sites if you have the carb number. The next and most overlooked is the size of the air bleeds and how many air bleeds the carb has. Large air bleed carbs require larger jets. I started this project with what I thought would be an easy bolt on and go. I found a Jet Performance Stage II carb in almost new condition. The guy had it on a Dodge pickup with a 318 and took it off to sell the truck. I don’t believe it ever ran well on the truck and that is why he took it off. The carb number showed it came off a 77 Corvette and should have 77 jets and 52 rods and had large air bleeds. It actually came from Jet with 73 jets and 43 rods (a pretty standard combination). I put it on the truck and stomped on it and it fell flat on its face. I tried a number of things but could not get it running right (not too much help from the Jet tech line). I finally switched to another carb I had but one I could not find the specs for. I ended up putting in 71 jets and 43 rods. By this time I got my Innovate LC-1 installed. After running it and checking the mixture on the computer I found it was running very rich. I changed to 64 jets with 43 rods and got it to lean out but now it was too lean so I went to 66 jets and have it running at about 13.5 at cruise, a little rich at idle (12.8) and 12.8 or so at WOT. It is leaning out a little when I get on it at cruise so I am going up to 68 jets. I have a carb with an APT that helps fine tune the mixture. If you are looking for a good tunable qjet get one from after 1975 but before they went electronic. These will have the APT for fine tuning the A/F. The long and short of all this is that the truck has never run better. It had an Edlebrock 1406 on it when I got it and it ran fine. I just wasn’t satisfied with the power and the fuel economy. I can now spin the big tires from a stop and am getting 16+ mpg in a ¾ ton truck with highway gears and regular gas. Best mileage with the Edelbrock was 14 unless I was running premium gas and cruising on flat ground. I think I can still do a little better than 16 with the final jet change. My goal was 18. I can now tap the gas a little and it launches. Some have said a well tuned qjet is better than EFI and I can now say that I believe it. And there is no better sound than when the secondarys kick in (there goes the gas mileage).

04-12-2010, 05:55 PM	#2
Sinister Between Trucks... Join Date: Mar 2002 Location: Los Angeles, CA. Posts: 3,830	Re: Quadrajet tuning success Congrats. I also have an LC-1, takes a lot of guess work out of tuning a carb. __________________ Beat it to fit, Paint it to match...

04-12-2010, 08:28 PM	#3
jaros44sr Registered User Join Date: May 2006 Location: Philadelphia, Pa. 19454 Posts: 9,421	Re: Quadrajet tuning success OK, so what is a LC-1 and where do I get one... __________________ Semper Fi...Uncle Sam, you da man All parts offered to help are free, unless otherwise noted Dont try this stuff in my build thread, unless you have 55 years of mechanical OTJ training SAFETY FIRST AS usual, off topic They say your mind goes second, can't remember the first Jim

04-12-2010, 09:51 PM	#5
6D8_C10 Registered User Join Date: Dec 2009 Location: Saint Peters, MO Posts: 171	Re: Quadrajet tuning success Hey Mertz, I am currently running an Edelbrock 1406, and I got a QJet from jegs. I have put it on my truck a couple of times and can't seem to get it to run. I can start it, and it will run, by adjusting the fast idle. But, when I put it in gear it dies. I have tried turning up the idle adjustments, turning a full 4 turns on each side, and still dies on me. Anyone have suggestions? __________________ 68 c10 350/th350 winter daily driver 90 Nissan 180sx, sr20det gt2871r turbo, track car,summer daily driver "Just so we are reminded of those who are held back, Up front there ought to be a Man in Black". Johnny Cash.

04-13-2010, 09:36 AM	#6
Quackaddict "Hooked on Quack" Join Date: Apr 2010 Location: Midlothian,TX Posts: 154	Re: Quadrajet tuning success I am with you guys, my Q-jet on my truck needs a rebuild and was thinking of replacing with a Holly Truck Avenger or an edelbrock but tuning a carb scares the fire out of me because I have never messed with one.

04-13-2010, 10:27 AM	#8
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Re: Quadrajet tuning success You gotta be careful when you turn those idle air screws. A lot of them have been adjusted over their lifetime and can cause idle problems when they become grooved where the needle seats in the throttle body. At that point, you are not getting the setting you are trying to achieve. If you suspect a vacuum leak, simply put your hand over the open carburetor. If there is a leak, it should smooth out. If no leak, it should die when you deprive the carb of any air. I used to seat them (gently), then back 'em out 2.5 turns as a reference point. Then, I'd use the Sun machine to monitor the engine conditions as I sought to balance the idle air settings. __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.

04-13-2010, 10:59 AM	#10
Mertz Registered User Join Date: Aug 2007 Location: Reardan, WA Posts: 132	Re: Quadrajet tuning success I would suggest taking it apart and throughly cleaning it. My Jet Performance carb, although partically new, was plugged is several places. Check the carb number on line and see what jet and rod sizes it is suppose to have and then see what is actually in it. Change if you have to. Before you do any of this get Cliff Ruggles book on the quadrajet. It will help a great deal. Even new carbs can have crap in them that will make the carb not work. Some rebuilders don't really know what they are doing. I found a lot of my later carbs have early rods which don't work in the later carb. Also make sure your ignition system is working properly before trying to adjust the carb. It makes a big difference. I have an MSD ready to run on mine and I can get any carb on it to run so I can make adjustments. Cliff has some ways to find out if you are running rich or lean if you don't have an LC-1 or LM-1. You can get real close using his methods. A good running Qjet will out perform that Edelbrock any day. The Edelbrock is still a good carb and I used it for 5 years but I am really please with the Qjet. And as said above check for vacuum leaks and your choke setting. Last edited by Mertz; 04-13-2010 at 11:00 AM.

04-13-2010, 12:59 PM	#11
'72customdeluxe Registered User Join Date: Jan 2008 Location: Tejas Posts: 691	Re: Quadrajet tuning success Is there any good way to adjust the square idle screws while it's on the engine? __________________ '72 cheyenne super step, '05 long bed gmc

04-13-2010, 03:36 PM	#14
Mertz Registered User Join Date: Aug 2007 Location: Reardan, WA Posts: 132	Re: Quadrajet tuning success Take them out and put slotted ones in. I haven't seen any square idle mixture screws on a qjet.

04-13-2010, 05:03 PM	#15
'72customdeluxe Registered User Join Date: Jan 2008 Location: Tejas Posts: 691	Re: Quadrajet tuning success eh, they're rectangular. Can't get any needle nose pliers in there to turn them nor will a flathead __________________ '72 cheyenne super step, '05 long bed gmc

04-13-2010, 05:22 PM	#16
Mertz Registered User Join Date: Aug 2007 Location: Reardan, WA Posts: 132	Re: Quadrajet tuning success I built a device to turn the APT using a bolt and a hack saw. I cut a slot in the bolt wide enough to go over the retangular part of the screw. It works perfectly. It might work for your idle mixture screws. Is this a newer carb that had plugs over the idle mixture screws so you could not mess with them?

04-13-2010, 06:26 PM	#17
'72customdeluxe Registered User Join Date: Jan 2008 Location: Tejas Posts: 691	Re: Quadrajet tuning success '78 model, I can get to them with small pliers, but with the engine off __________________ '72 cheyenne super step, '05 long bed gmc

04-13-2010, 10:08 PM	#18
kyle fitz Registered User Join Date: Mar 2010 Location: pinetop az Posts: 55	Re: Quadrajet tuning success i like my q-jet. when i first got my truck it ran like crap. it wouldnt change at all when i adjusted the mixture screws. so i rebuilt it and i havent had a problem with it at all

04-13-2010, 11:22 PM	#19
Jim_PA Registered User Join Date: May 2005 Location: PA Posts: 2,696	Re: Quadrajet tuning success May the force be with you

01-26-2011, 03:18 AM	#20
LostMy65 But Found Her 25yrs Later! Join Date: Oct 2010 Location: Oregon City, Oregon Posts: 10,482	Re: Quadrajet tuning success I read in another forum where a guy took a piece of brake line and flattened it just enough so it fit over GM's double 'D' screws. Here's Snapon's Double-D Socket: Stock#: S6139 Socket, Double "D" Carburetor Adjusting (3.5mm across flats) http://buy1.snapon.com/catalog/item....&group_ID=1441 And here's what Mac has: http://www.mactools.com/productdetai...tor-tools.aspx __________________ I lost my 65 - Found it 25 years later: http://67-72chevytrucks.com/vboard/s...d.php?t=426650 66 C20 Service Truck: http://67-72chevytrucks.com/vboard/s...d.php?t=428035 Last edited by LostMy65; 01-26-2011 at 03:26 AM.



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