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1972 Pontiac Lemans GTO
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Discussion Starter · #1 ·
Well I went through my first rebuild on this carb with the help of Harold Demes on YouTube. Ran the numbers on it it is 800 cfm off of a Buick same period as the Pontiac 70-72. Fired it up choke came on nicely hit the throttle no idle. Would not idle to save your soul. I said well something I screwed up. Had to move it to work on the wife’s ride this weekend, then I noticed no vacuum going to the brake booster. So something was definitely a miss! Checked all vacuums all ok. So carburetor back on the bench tear it down all looks good. With the air horn off and all internals out I shined my light down and noticed a 16th of a gap between the throttle plate and the main body. The light clearly showed the gap. The cause? Too much JB Weld on the secondary wells. Filed them down fits like a glove. I will let you guys know how it is running after I find my missing choke rod. Dammit man!
 

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1972 Pontiac Lemans GTO
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Discussion Starter · #2 ·
I need some guidance now. Found the choke rod put it back on the car, it is idling better but any load via ac or in gear it wants to die. Got the mixture screws at 2 and a half now which has helped on idle but not when it has a load. Suggestions?
 

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I need some guidance now. Found the choke rod put it back on the car, it is idling better but any load via ac or in gear it wants to die. Got the mixture screws at 2 and a half now which has helped on idle but not when it has a load. Suggestions?
Idle is too low? Mixture screws are about right, I always do 2 1/2 turns out. With AC, you want an electric idle solenoid which these cars used. It will up the idle speed when it is energized via the key switch that powers up, and will drop the idle speed when the key/power is shut off. Your Chassis manual should show this and give you the setting for the idle speed with the solenoid energized and not energized.
 

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1972 Pontiac Lemans GTO
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Discussion Starter · #4 ·
Idle is too low? Mixture screws are about right, I always do 2 1/2 turns out. With AC, you want an electric idle solenoid which these cars used. It will up the idle speed when it is energized via the key switch that powers up, and will drop the idle speed when the key/power is shut off. Your Chassis manual should show this and give you the setting for the idle speed with the solenoid energized and not energized.
Thanks PJ
 

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Thanks PJ

Check this out. It should be of help and have the info you need. But, the Chassis Manual should show the electric solenoid. They can be purchased aftermarket, but not sure if you might need the brackets to attach it or not. Have seen them used as well.

 

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1972 Pontiac Lemans GTO
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Discussion Starter · #6 ·
Well Jim I got her to go on a test run at 4 turns out. When she comes to a stop still sounds like she is loading with fuel but running. At 2 1/2 turns will just die. I still think I might be catching air somewhere. Anyway she is back apart on the bench😡
 

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Well Jim I got her to go on a test run at 4 turns out. When she comes to a stop still sounds like she is loading with fuel but running. At 2 1/2 turns will just die. I still think I might be catching air somewhere. Anyway she is back apart on the bench😡
OK, you may have overlooked something and it never hurts to double check. Did you install new bushings in the base throttle shafts? These can wear the plate out and become a little sloppy and air can be drawn it or the throttle plates may not close/sit correctly. They make a do-it-yourself kit with drill/bushings. I am sure YouTube has a how-to on this.

Make sure both the front & rear throttle plates seat completely closed.

New float? I always install a new float that is compatible with ethanol. The old neoprene floats can soak up gas and run heavy so you cannot get a correct setting on float level.

Another thing to consider is the spring under the power piston. If you don't have enough vacuum, ie big cam, the spring is designed for a stock engine pulling a lot of vacuum, so it pulls down on the power piston and seals the delivery of gas. If you have too low of vacuum, the heavy spring will keep the power valve up/open and you are dumping additional gas into the carb all the time - thus running in a rich condition.

They do offer different tension springs in a kit as I recall. But a cheap fix is to measure the length of the spring as it is now, cut off a couple coils, then stretch the spring to your original measured length. This reduces the spring's tension. I used to do this per a Q-jet rebuild article I went by. Now you won't know what the spring tension is, as this method is not "scientific" by any means - it is backyard mechanic stuff and heaven forbid we ever use such old school techniques in these precise modern times. LOL
 

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1972 Pontiac Lemans GTO
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Discussion Starter · #8 ·
Yes I did get the brass float, I will check on the other things you suggested. Thanks buddy
 

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1972 Pontiac Lemans GTO
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Discussion Starter · #9 ·
Yes I did get the brass float, I will check on the other things you suggested. Thanks buddy
 

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I’m sure this has been repeated multiple times in this forum, but I’d recommend giving Cliff Ruggles a call and also buy his Quadrajet book. His carb rebuild kits are by far superior to any of the generic one’s bought at the local stores or online and he’ll custom build your kit for your exact specs (engine size, cam, altitude, etc.). He has also been invaluable talking in person about specific mods to help your situation. The book also does a great job at explaining how to match/modify the carb to suit your specific engine. It’s a small investment for fine tuning such an intrical part for the car. These carbs run great when set up right.
 

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1972 Pontiac Lemans GTO
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Discussion Starter · #11 ·
Yeah Duke ,Integrity already messaged me about Cliff . I think he used his product and book along with the video by Harold Demes. Harold is old school mechanic who smokes. Those are the best with carburetors lol
 

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1972 Pontiac Lemans GTO
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Discussion Starter · #12 ·
Just got off the phone with Cliff , he informed me it is actually a 850 cfm Q Jet. Anyway I’m dropping back to punt starting with one of his kits and see how it goes. Thanks PJ,Integrity and the Duke for suggestions I will keep you posted 👍
 

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1972 Pontiac Lemans GTO
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Discussion Starter · #14 ·
144351

Received the Q Jet Bible and kit. Hopefully all goes well this time. Can really tell the difference between it and the store bought kit.
 

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Another thing to consider is the spring under the power piston. If you don't have enough vacuum, ie big cam, the spring is designed for a stock engine pulling a lot of vacuum, so it pulls down on the power piston and seals the delivery of gas. If you have too low of vacuum, the heavy spring will keep the power valve up/open and you are dumping additional gas into the carb all the time - thus running in a rich condition.
This was my first thought too
 
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Glad to see that your kit showed up and the book is a great addition. I hope for many years ahead that Cliff is supporting these carbs. The things that I can recommend is 1-make sure the base plate is flat! 2-make sure your getting full travel on the primaries and secondaries 3-depending on what it’s going on (engine size) opening up the idle circuits really helped with responsiveness.
 

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1972 Pontiac Lemans GTO
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Discussion Starter · #18 ·
Thanks Duke in the process now of cleaning all the idle circuits and such and checking the base plate.
 
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