Quadrajet question

[AuroraGirl]

Try disconnecting the vacuum advance (assuming your using it?) and see if that changes anything. If it does, you probably need a vacuum advance canister that has a higher vacuum rating. The idle could be jumping around because the engine vacuum is really close to the rating on the advance canister. The timing might be jumping around a little as the advance reacts to engine vacuum.

Make absolutely sure there are no vacuum leaks anywhere. As Aurora Girl mentioned, leaky throttle shafts are a possibility. Spray a little carb cleaner around the carb base (make sure the air cleaner is on so the engine doesn’t ingest the spray they the carb) and listen carefully for any changes in idle. If the throttle plate doesn’t have bushings, changes are after 40 plus years of use the throttle bores are worn.

Don’t get too hung up on the actual numbers. Give the engine what it wants, not what you think it needs.

Take the truck for a long drive, make sure to run it kinda hard to ensure all the carbon is burned off the plugs. Connect a vacuum gauge, and slowly turn each idle mixture screw in or out a little at a time until you have the highest vacuum reading. Reset the idle speed and fine tune the mixture again.

You might try spraying the idle air bless in the throttle bore.

if it wasnt just next to the opening of the carb id say use propane torch but i feel that would inevitably suck into the throat making false positive. unless he had the air cleaner duct i guess lol

 

[AuroraGirl]

I'll try that. No EGR, but it does have the charcoal cannister and the PCV valve does rattle so that may be my culprit

rattling is usually a good sign its when it doesnt that its alarming, but if its old you should get a new one. make sure the grommet and the opposite corner of the engine the breather is nice and tight/not leaking

Then also you may consider undoing your characoal setup from the carb, cap it so you dont have a leak, and see if issue exists still. crack your gas cap if youre worried about pressure for the test. if somehow thats not working right it maybe is causing inconsistent behavior.

 

[Matt69olds]

Once the idle is straightened out, then you can concentrate on the APT.

You lean out the part throttle cruise by turning the screw clockwise. That limits how far the metering rods come out of the jet. The rods are held down by engine vacuum pulling on the APT piston.

Do your part throttle tuning on a fair weather day. Drive the truck at normal highway speed, stopping occasionally to turn the APT screw in a little at a time. Keep leaning it out until you get a lean miss, it will kinda feel like a fish nibbling on the bait. Once you feel the “fish bite”, you have found the lean limit. Turn the screw counterclockwise until the lean miss disappears, then go just a fraction further. If the screw bottoms out and you haven’t felt the lean miss, then the jet is already too big. Drop a jet size or t2, run the APT screw up 5-6 turns, and try again.

Now is the time you can really use the AFR gauge. Get the truck up to highway speed, then slowly accelerate. Ideally, you will see the AFR go slightly lean, then richen up. That’s due to engine vacuum falling off, and then the metering rods raising out of the jets. If it takes a lot of throttle to get the metering rods to come up, you need a lighter spring under the piston. If it goes rich under the slightest load, you need a heavier spring.

The very things that make people say Q-Jets are junk is what makes them so versatile. You will never get the balance of economy or power with a Holley or Edelbrock. When I had a Q-Jet on my Olds, it was no big deal to get 17mpg, and still run mid 11s. My dads 455 swapped 81 GMC gets similar mileage, and cruises effortlessly at 75 mph while doing it. Try that with a Holley!!!!

https://static.speedwaymotors.com/pdf/455-35004.pdf

 

[Tim Cyrus]

You may have been on track with your original post thinking the float level is to high. The higher the float level the "Sooner" the nozzles start to feed. At an idle air rate with the float level high you may get some drips of fuel out the nozzles this would be seen as a rich spike and associated roughness. The nozzles should not drip or feed at an idle air rate. you can look down the carb at idle and try to see if the discharge nozzles are dripping and adjust accordingly or just dive in and lower the level 1/16th at a time. As mentioned earlier the QuadraJet is a great carburetor, it has many knobs to turn to talor the fuel delivery to what your engine needs.

 

[olnick]

I have owned my "78" since 1980 It has always had an issue with fuel! So 42 years later I finally get it to run like a champ! No more black puffs when starting when hot no more after run etc. etc. all it took was to get the book from Clif Ruggles read it and apply! My problem all along was NOT my Qjet, never thought it was however the now offered fuel pumps are NOT ANY GOOD. I could never find one that put out less than 7 PSI (5-6 attempts) I finally got a HOLLY 7.5 PSI MAX Wrong again try 9 1/2 PSI. being I was able to remove the fulcrum pin and removed the main PSI spring above the diaphragm. In order to reduce the PSI I needed to "CRUSH" the OAL of the spring down by 1/4 of an inch! re assembled pump, bench tested 5 5-1/2 PSI MAX running the test several times. Installed on truck have NEVER needed to look back! Thank you Cliff for stating " NO need for high PSI just volume 6 PSI is more than adequate.
Wish I found his book 40 years ago!

Olnick

 

[Paladin]

Ahhhh..... another end all, be all Quadrajunk doing what it does best, not operating correctly!!!! Get a HOLLEY!!!!!

 

[80BrownK10]

If Holley was the best carburetor wouldn't GM have just pulled a Holley off the shelf and put it on their truck in the first place. If it got the best MPG and power that they could get they would have just used an off the shelf carb and saved all the time designing another carb. Especially early on when they didn't have to work to meet emissions requirements.

Not trying to start an argument about carbs which I am not an expert on not even the slightest bit to be a keyboard expert on a forum. It's just what makes sense to me.

 

[Backfoot100]

If Holley was the best carburetor wouldn't GM have just pulled a Holley off the shelf and put it on their truck in the first place. If it got the best MPG and power that they could get they would have just used an off the shelf carb and saved all the time designing another carb. Especially early on when they didn't have to work to meet emissions requirements.

Not trying to start an argument about carbs which I am not an expert on not even the slightest bit to be a keyboard expert on a forum. It's just what makes sense to me.

Pretty much all of GM's highest performance engine applications did come from the factory with a Holley. 327 Corvette was the first one in '65. Next was the 427 in '66 followed by the 396 in '67.
If you had the one of the highest HP engine options in 327, 396, 402 and 427 form, you can pretty much guaranty that it had a Holley.
It was also the carb of choice for almost every manufacturers highest HP engines which is why the vast majority of performance buffs choose Holley. The legendary Mopar monster engines all used Holley. That's also why you'll hear repeatedly, if you want MPG, get a QJ. If you want performance, get a Holley.

 

[Matt69olds]

Pretty much all of GM's highest performance engine applications did come from the factory with a Holley. 327 Corvette was the first one in '65. Next was the 427 in '66 followed by the 396 in '67.
If you had the one of the highest HP engine options in 327, 396, 402 and 427 form, you can pretty much guaranty that it had a Holley.
It was also the carb of choice for almost every manufacturers highest HP engines which is why the vast majority of performance buffs choose Holley. The legendary Mopar monster engines all used Holley. That's also why you'll hear repeatedly, if you want MPG, get a QJ. If you want performance, get a Holley.

Build the Q-Jet right, you can have both.

The only way a properly calibrated Q-Jet will hold back performance is if the engine needs more airflow than a Q-Jet can provide. Considering every Q-Jet is capable of at least 750cfm, you need a pretty stout engine to need more than that.

There are many cars competing in the F.A.S.T (Factory Appearance Stock Tire) classes, many are deep into the 10s. The entire point of this class is to bend any rule possible, while maintaining a stock appearance. That means no aftermarket headers, no aftermarket aluminum intake manifolds, no aftermarket wheels and tires (factory size, and factory tire construction) and factory carburetor. For the majority of GM cars, that means the Q-Jet. You are allowed more compression, camshaft, big stroker engines, any gear ratio, etc as long as the car looks stock it fair game.

I’ll say it again, when these cars and trucks were new, they were expected to start easily in any weather, in any environment, by anyone from 80 year old ladies on their way to bingo, to priests on their way to deliver the Sunday sermon, and start and perform without drama and without fiddling with them, or have the owner stinking of exhaust. Properly maintained, there is ZERO reason they won’t today.

The Q-Jet has by far the largest production run. Nothing else is close. The same basic design from the mid 60s all the way into the late 80s.

Btw, ford used Q-Jets on a few of its performance cars.

 

[AuroraGirl]

Build the Q-Jet right, you can have both.

The only way a properly calibrated Q-Jet will hold back performance is if the engine needs more airflow than a Q-Jet can provide. Considering every Q-Jet is capable of at least 750cfm, you need a pretty stout engine to need more than that.

There are many cars competing in the F.A.S.T (Factory Appearance Stock Tire) classes, many are deep into the 10s. The entire point of this class is to bend any rule possible, while maintaining a stock appearance. That means no aftermarket headers, no aftermarket aluminum intake manifolds, no aftermarket wheels and tires (factory size, and factory tire construction) and factory carburetor. For the majority of GM cars, that means the Q-Jet. You are allowed more compression, camshaft, big stroker engines, any gear ratio, etc as long as the car looks stock it fair game.

I’ll say it again, when these cars and trucks were new, they were expected to start easily in any weather, in any environment, by anyone from 80 year old ladies on their way to bingo, to priests on their way to deliver the Sunday sermon, and start and perform without drama and without fiddling with them, or have the owner stinking of exhaust. Properly maintained, there is ZERO reason they won’t today.

The Q-Jet has by far the largest production run. Nothing else is close. The same basic design from the mid 60s all the way into the late 80s.

Btw, ford used Q-Jets on a few of its performance cars.

At least they arent autolites
yeah I went there
POS's lol.

 

[Conejo_K10]

A common issue with the q-jet is the power valve doesn't move/transition smoothly in the bore. This valve is located front and center and moves the primary jet rods. With the top and gasket off you can check if it has any stiction by pressing on it. In a Youtube video by Q-Jet power, it was suggested that the PV be polished with scotch-brite when rebuilding. Also make sure you have the right spring, there are 3 to choose from. IDK which version you have, but some have an adjustable PV stop which will influence mixture.
While you have it open perhaps upgrade to the later accelerator pump [blue] which is compatible with E85 and other fuel supplements.
Good luck..

 

[59840Surfer]

I've got it leaned out in the idle mixture to try and prevent dieseling at shutdown but it will still read like 13.4-13.8 afr with the screws 1.5-2 turns out.

That's backwards --- LEAN at idle PROMOTES "dieseling" because of the extra heat in the combustion chambers from the lean mixture.

During the Retro-NOX years in California --- dieseling was a really big BIG problem 'cause of the very lean mixtures and retarded timing that in reality, created a lot of NOX in the exhaust --- a gas the CARB hadn't considered at that time as they were mostly concerned with the visual brown haze that everybody complained of and so, stupidly .... it was the priority.

Later on came the Retro-NOX control add-on systems that created such a fury and lawsuits and .... believe me, it was just nasty.

Anyway --- shortening this war story --- when you lean out a mixture, you run the inner cylinder operating temperatures UP and that becomes a feedback problem, making dieseling at shutdown worse and worse.

I know this is almost a ZOMBIE post, but it grabbed me by the short hairs and I had to make a comment. Sorry.

PS ---> I LOVE my Q-Jets! They are the simplest and easiest to work on --- unlike Holleys that need jet changes every time the sun goes behind a cloud.

 

[idahovette]

Waiting patiently for @Paladin to respond^^^^^^^^.....lol

 

[59840Surfer]

Waiting patiently for @Paladin to respond^^^^^^^^.....lol

I got 15+1 rds of 10mm .... let him come!

 

[Bextreme04]

That's backwards --- LEAN at idle PROMOTES "dieseling" because of the extra heat in the combustion chambers from the lean mixture.

Anyway --- shortening this war story --- when you lean out a mixture, you run the inner cylinder operating temperatures UP and that becomes a feedback problem, making dieseling at shutdown worse and worse.

I know this is almost a ZOMBIE post, but it grabbed me by the short hairs and I had to make a comment. Sorry.

PS ---> I LOVE my Q-Jets! They are the simplest and easiest to work on --- unlike Holleys that need jet changes every time the sun goes behind a cloud.

This is not necessarily true. Well... you are technically right, but some context of the reason is probably worthwhile. Combustion temps are highest when the fuel/air mixture is stoichiometrically balanced. For 10% ethanol blend gas, that is about 14.27:1 and for pure gas, it's about 14.7:1. If you go rich or lean, the combustion and exhaust temps will drop.

The statement that lean mixture will promote hotter CHAMBER temps is correct though. What happens is that a richer mixture is basically evaporating fuel in the combustion chamber and that evaporative action is pulling heat out of the cylinder walls and piston in order to support the phase change.

In rocket engines they do the same thing by running fuel through the combustion chamber walls and then pointing some of the fuel nozzles towards the inside wall of the chamber to create a film of fuel along the chamber wall to keep the metal from melting at the 5,000+ degree adiabatic flame temps.

In aircraft engines you can adjust your mixture at cruise to lean of peak by adjusting the mixture knob until the exhaust gas temperature has peaked and then falls by about 50 degrees. This is the same thing that factory manuals have you do for idle and cruise adjustments. For idle you find peak RPM and then lean the mixture until you get an RPM drop. For cruise you lean the mixture slowly until you start getting some surging while cruising, then richen slightly. When done properly, I've NEVER seen an engine have any issues with dieseling. The problem with trying to run it richer to avoid dieseling is that you are producing more unburnt fuel and carbon in the chambers and that can lead to hot spots in the chamber that can lead to preignition and dieseling after shutdown eventually. Plus your mileage will be less than it could be.