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Old 02-10-2018, 11:24 AM   #1  
lars
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Default More Q-Jet Rebuilding Problems and Observations

From time to time, I try to post up some of the recent observations and issues I see when rebuilding Q-Jets. As noted in my earlier posts, more and more of these carbs have been subjected to some really bad previous rebuild attempts, so it is no longer possible to simply disassemble a carb, clean it up, slap a “kit” in it, and put it back together to make it run right. Every carb must be carefully examined to find all the issues and errors induced by the past 20 guys who have messed with it, and all these issues must be correctly repaired. Often, parts must be replaced, since people will actually destroy parts in the carb when working on them.

I posted a few interesting notes and findings about Phil’s (“Philgran’s”) carb in his post here – this is worth checking out if you haven’t already been following the thread:
https://www.corvetteforum.com/forums...979-l48-2.html

This next carb comes from Derrin, whose wife just picked up a ’76 Vette with a badly-running QuickFuel carb. Derrin found a nice, used, original 1977 Vette California carb, which looked like a solid, rebuildable carb that would be perfect for his application. The carb was really dirty (which is good – it usually means it hasn’t been messed with), but all parts were there.

Upon disassembly, it became obvious why the carb was removed from service and put up for sale: The previous builder had caused so many problems with the carb that there is no way the carb could have run. If it was running, it was running so badly that the car would have been nearly inoperable. I’m sure someone was saying, “It’s that $%#^!! QuadraJunk Carb – those things are all crap!!!”

Here’s what I found:
  • They installed the incorrect fuel filter: 1966 – 1974 Q-Jets take a “short” filter. Starting in 1975, they went to a “long” filter. The builder of this carb put a "short" filter into the long inlet housing, which allowed all fuel to bypass the filter, filling the bowl with dirt and debris.
  • They failed to install the seal on the carb inlet fitting – the carb would have had an incurable fuel "drip" at the inlet in spite of the fitting being tightened so tight I could barely get it out (they really tried to stop the leak..!). I used my ” drive impact gun to remove the fitting.
  • Choke intermediate shaft seals were not installed, allowing dirt ingestion into carb.
  • The idle mixture screws were grossly misbalanced - all idle fuel was being metered on one side of carb only – idle fuel was shut off on the other side.
  • The Choke Pulloff was intentionally disconnected and disabled – the carb would have had horrible flooding at cold start, with no control of the secondary airvalve.
  • The Power Piston Spring had been intentionally stretched out and destroyed. Here is the damaged spring on left. A stock, correct, unaltered spring is on the right. The stretched spring would cause the power piston to never seat, putting the carb into a permanent rich condition:

  • The Power Piston hanger arms were intentionally bent upwards, pulling the rods permanently out of the jets – the main metering system in the carb was basically inoperable, and the carb was running in a massive full-rich condition with no rods in the jets. The "rebuilder" probably was unable to get the rods to slip into the jets at assembly, so the problem was solved by bending the hanger arms. This actually made the stretched-out spring somewhat irrelevant, so it solved that problem, too...

  • The Accel Pump diaphragm was collapsed – accel pump inoperable.
  • The Float had been bent to lower it so far that it was bottomed out in the bottom of the float bowl – I’ve never seen a float level so low. Probably lowered in an attempt to compensate for the rods not being engaged in the jets.
  • Undersized jets installed. They dropped some really small jets into this thing to try to fix the other problems.
  • Very low APT (power piston height) setting, further running the carb lean (if the rods had been in the jets...)
  • They also bent the Secondary Hanger arms with 1/8" difference in height from left to right – the one side of the carb would have run prematurely really rich, with the other side of the carb running super-lean at wide open throttle.
  • Not caused by the previous builders, but the carb had severe corrosion on all parts - all steel parts covered in rust, and all aluminum/zinc parts covered in oxides.

I’m going to have to replace the power piston spring, but all other items can be fixed and corrected. The carb just came out of the de-rusting tank, and is actually looking pretty good. Later today, the carb should be up and running on the test engine, and I think Derrin’s wife is going to have a really nice running carb.

So look closely and critically at those carbs and carb parts when you tear into your Q-Jet during rebuild: The object of the game, and the only way to make these carbs run right, is to find all the horrors induced by 40 years of bad work, and bring these things back to life. And make sure you know the correct, original specs for the carb you're working on (that applies to you Holley guys, too...) - most carbs have been altered, and you cannot assume that the jets and rods in any carb are correct for your application.

As always, feel free to contact me if you need assistance or comments as you work on your own carbs.

Lars

Last edited by lars; 02-10-2018 at 02:38 PM.
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Old 02-10-2018, 11:58 AM   #2  
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excellent
lot of info if the reader
thinks about it/
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Old 02-10-2018, 12:04 PM   #3  
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Knowledge is power. Lars knows these things inside out in his sleep.

I sent him a quadrajet years ago I took off a motorhome. It had never been off the low millage Winnie.

He was like a kid in a candy store. Totally unmolested and pure.
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Old 02-10-2018, 02:34 PM   #4  
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If I were a kid in a candy store, that candy store would not go unmolested...
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Old 02-10-2018, 03:11 PM   #5  
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And this is why I like to have someone knowledgeable rebuild these carbs.

There's a rebuilder local to me who has done decent work for me in the past, but they have no way to test a carburetor on a running engine. Given the great work Lars did rebuilding a 1975 Quadrajet for me (including machining all of the emissions ports to work properly on my 79 Corvette) and the unknown history of this 77 Quadrajet, I'd rather have it tested on a running engine and verified with an AFR meter so I don't have to worry about much of anything when getting it back.

Thanks again, Lars! If it runs anywhere near as good as the 75 Quadrajet on my (daily driver) 79 that you built, my wife and I will be happy!
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Old 02-10-2018, 05:00 PM   #6  
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Quote:
Originally Posted by Big2Bird View Post
Knowledge is power. Lars knows these things inside out in his sleep.

I sent him a quadrajet years ago I took off a motorhome. It had never been off the low millage Winnie.

He was like a kid in a candy store. Totally unmolested and pure.
I think he was like that with mine the first go around. Still had the mixture screws plugged.

I meant to post this up from the second time....

I have the carb all set up and tested, and it's running great.

I did not waste your money with a complete re- rebuild, and I did not do any cleanup of the carb - it was not looking too bad, and any cosmetic cleanup would not be adding any value you cannot add on your own by spraying it down with a little carb cleaner if you chose to do so. So just charged you the standard fee for a setup and test along with return postage. Here is what I did:

Raised float level to .300
Increased Idle Fuel Restrictor Orifice to .039
Raised APT
Increased secondary rods from CH (.057 short) to CV (.053 long)
Removed idle mixture screw seal plugs
Set mixture screws to 6 turns
These settings created idle speed of 930 rpm. Dropped idle speed to 850.

I tested the carb on my test engine which has the same XE268 cam that you're using, along with headers, so it should be a very good setup comparison. Your idle mixture now is dead-on the numbers at 14.7:1 and throttle response is instant. The slightly increased IFR sizing should enrichen your transition nicely to give you very good throttle tip-in with your new engine configuration. I also installed a pair of slightly richer secondary rods with the long tips, giving you quicker fuel enrichment on the secondary side. Give these a shot and see what you think. If they're too rich too quick, drop your "CH" rods (included with your carb in a baggie) back in and I'll return your $35 for the rods upon your return of the rods to me. But I think these will run really well for you.

Indeed it does.

Last edited by Capt. Shark; 02-10-2018 at 05:07 PM.
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Old 02-10-2018, 05:17 PM   #7  
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Quote:
Originally Posted by dmruschell View Post
And this is why I like to have someone knowledgeable rebuild these carbs.

There's a rebuilder local to me who has done decent work for me in the past, but they have no way to test a carburetor on a running engine. Given the great work Lars did rebuilding a 1975 Quadrajet for me (including machining all of the emissions ports to work properly on my 79 Corvette) and the unknown history of this 77 Quadrajet, I'd rather have it tested on a running engine and verified with an AFR meter so I don't have to worry about much of anything when getting it back.

Thanks again, Lars! If it runs anywhere near as good as the 75 Quadrajet on my (daily driver) 79 that you built, my wife and I will be happy!
Derrin -
As you noted, there is no way anyone can build a carb and assume it's going to run right without testing it. I've done about 800 of these Q-Jet rebuilds now over almost 40 years, and I still have carbs fail the run test for one reason or another. Including your carb just now...

The carb fired up immediately and ran great, but the cold fast idle rpm was really high (1750 rpm). I was about to back off the fast idle screw when I noticed that the fast idle screw was fully backed out and not touching the fast idle cam. The 1750 rpm was set by the previous builder using the hot idle speed screw (the carb was probably running so bad that he had to crank the idle speed way up to get the carb to run). After backing off the hot idle, the fast idle was observed to be 700 rpm. I cranked that up to about 1320, at which point the carb/engine idled really nicely. When the engine warmed up, it came off fast idle, but the hot idle could not be lowered below 900 rpm...

Shutting the engine down and checking the linkages, I found that "Ace Carb Tuner" had altered/bent the secondary safety tang in a misguided attempt to "tune" the secondaries for quicker opening. The secondary linkage was so tight that the primary throttles could not fully close. A quick correction to the linkage, and the carb ran perfectly.

Here is Derrin's carb actually running on the test engine:



Air/fuel mixture right on the number at 14.6:1 and 1.00 lambda:


The carb is smooth, with instant throttle response. And all the rust is gone...!

Lars


Quote:
Originally Posted by Capt. Shark View Post
I think he was like that with mine the first go around. Still had the mixture screws plugged.
Glad to hear the setup ran well for you - I get lucky every once in a while... I actually made note of the stock idle mixture screw setting on your carb for reference - I've never had a performance carb run well at that setting using today's pump gas, but it's always fun to know what the original settings were (as noted, I altered your idle/transition orifice size and the mixture screw settings to get it to run well for you).

Lars

Last edited by lars; 02-10-2018 at 05:22 PM.
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Old 02-10-2018, 07:07 PM   #8  
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I'm definitely looking forward to running this carburetor!

It's amazing to me that, even with the purchase price of the Quadrajet and the cost to have it rebuilt, I'm still going to come out ahead when compared to buying a new Holley or Edelbrock carburetor.
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Old 02-10-2018, 08:14 PM   #9  
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...and this is going to run better than either of those two.

Lars
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Old 02-10-2018, 08:35 PM   #10  
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Quote:
Originally Posted by lars View Post
...and this is going to run better than either of those two.

Lars
Having run Edelbrocks, a Holley, and a Quadrajet on my 79, I agree.
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Old 02-10-2018, 10:02 PM   #11  
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love these real world posts, very educational, thanks for posting. I think this could be a great sticky with updates on the various rebuilds as they are processed
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Old 02-10-2018, 10:24 PM   #12  
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Pierre -
The problem with keeping it going as a single thread post is that it gets very long. After a while, people don't pull up a thread that's been running for months or years because it's long and boring. So I prefer to post up new, fresh threads as odd situations and new carb problems pop up to share on a real-time basis. This gets more people involved, and anyone "tuning in" can see what the "Problem du Jour" is from the opening post. So just watch for my name or the "Q-Jet" topic, and I'll keep posting up photos and comments for everyone to participate in some unique carb rebuild projects.

Lars

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Old 02-13-2018, 08:49 AM   #13  
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lars, thanks for all of your in sight I always enjoy learning from you
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Old 02-13-2018, 10:28 AM   #14  
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CAN WE CLONE LARS??????? I want someone like him down in our neck of the woods. Of course I love Colorado!!!
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Old 02-13-2018, 10:55 AM   #15  
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Thanks for the kind words and comments - you guys make it fun posting up this tech stuff! More carbs are on their way to my workshop right now for rebuilds, so I'll post up some more stuff as I come across the interesting ones.

Lars

Last edited by lars; 02-13-2018 at 10:56 AM.
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Old 02-13-2018, 05:58 PM   #16  
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Lars,you rebuilt my 78' L82 carb,wound-up perfect! If you ever need a Arctic cat snowmobile carb/eng done let me know. I am the guru in Mich. Do you have Michelob Amberbock long neck beer in Col? Game changer,for sure......Hdal
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Old 02-13-2018, 07:33 PM   #17  
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The next time I'm in the market for a snowmobile engine you'll be my first contact..! No, I haven't seen Michelob Amberbock long necks out here, but I'll check the store next time to see if I can buy any on your recommendation...

Lars
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Old 02-14-2018, 05:43 AM   #18  
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Lars IS the man! I was in awe when he was at the Northern Virginia Tuning for Beer event light years ago. Very knowledgeable, humble and just an all around great dude. And he taught us all something that day.

I can't wait to send you my carb so you can work your magic!

Last edited by theandies; 02-16-2018 at 04:35 PM.
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Old 02-16-2018, 01:51 PM   #19  
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Lars did one of my carb a few years back , I have done alot myself but sometimes you need an expert. Keep up the great work and articles, we love them , Ron
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Old 04-30-2018, 01:58 AM   #20  
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Howdy all Im brand new here , mainly here to learn from the Guru Lars about Qjets , I will try to cut to the kill here , and I apologize , I wanted to start a new thread like Lars suggested , but I failed to figure out how on this forum yet .
I am trying to refurbish carb # 7045213 to use on a convertable 70 GTO w/ a 1974 400 w/ 4x heads edelbrock performer int. HEI dist, this is a temporary engine setup for now. I have a few questions , and I will try to attach a few pix to explain , the thing that threw me is the needle and seat ? Ive never seen anything like it , notice the pix , the rubber seat end is not tapered ? , however it looks like it might seat, the top is rounded ? and had NO clip , it looks like it might function , but Im wondering if I should put a Normal needle/seat in it .
heres some of the specs on the carb for now , it looks like 71 primary jets , w/ 393 ? primary meter rods { man those numbers are tiny ! } for the life of me could not find markings on sec. rods , but hanger # 33797 suspends them, I know Lars advises against the truck carbs , however I miced the primarys and they miced out at.026 ? , so those are the decent primary rods , correct ? I cant remember from the article , for right now I just wanna get this thing running , it will mainly be run at high altitude between 4500 - 7500 plus feet, again this car has been a jigsaw puzzle for 30 years , and all the pieces i thought would be a breeze to work with have kinda bit me ...oh well, I do have a kind of rare ? 7040507 70 vette manual carb , that I may rebuild or try to trade a vette guy for a 70 goat carb . I have toyed w/ these qjets before , but left off 30 plus years ago, however I did find a nice stash of oem qjet carb parts stuff
anyway I believe that covers that , anybody ever seen aneedle /seat like this ? thanks to All !
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