Thursday, December 20, 2018

Quadrajet Tuning: Adjusting the Idle Mixture Screws

GOOD Vs. BAD JUJU

For some of us who own C3 Corvettes equipped with Rochester Quadrajet carburetors, getting them properly adjusted seems to be a difficult task. And at first glance, it appears only a select few know the secret or the magic juju spell required to get a Quadrajet to run right.

I've had professional mechanics attempt to get my 1976 Corvette Stingray to run properly, with mixed results (no pun intended), and I've never been 100% satisfied. From rough idle to hard-starting to extreme gas smell, my car experienced all of those and more.

One "professional," for example, convinced me Quadrajets were trash, so I spent money on a brand new Edelbrock. This was not the solution and that carb was, in my opinion, worse than the factory carburetor.

Eventually, I sent my old and tired Quadrajet to All American Carburetors in Orange Park, Florida, and they remanufactured it. A few weeks later, I had my old carburetor in like-new condition.


The mechanics who did the engine swap reinstalled it and adjusted it a bit so the car would run okay. And it did. My Corvette ran "okay." Call me crazy but I wanted it to run great after all the money I'd spent.

I owned another '76 Stingray back in the early '80s, and it was my daily driver for a while, and the mechanics who worked on it were always able to tune it properly. It started up right away, whether the motor was cold or hot, there was no flat-spot, no too-lean or too-rich issues, and it idled steadily at 750 rpm.

Where are those mechanics today?!?!?

Probably retired or dead, and the few who remain in business are nowhere near me, so I was left to my own devices and had to attempt to slay the dragon myself.

I read several articles, some which were VERY technical and went right over my head, and others that outlined the basics but without any "how-to" tips. I also watched lots of YouTube videos on Quadrajet tuning.


THE CARB DIET

Carburetors—by today's standards—are obsolete relics. Be that as it may, they are, in my opinion, fascinating marvels of engineering. I think they get a bad rap because very few people know who to tune them properly.

And so I realized that if I wanted my Corvette to run well, I would have to adjust the carb myself.

Admittedly, I'm a do-it-yourselfer, not a mechanic. But it appears that some of the mechanics out there know as much about dentistry as they do about carburetors, so I figured I might as well learn as much as possible so I could tune my Quadrajet to operate like it did the day it rolled out of the St. Louis, MO assembly plant.

If you follow my blog, you know that I had the original engine completely rebuilt, so the compression is good, the timing was set correctly during initial tune-up, and that the engine has new spark plug wires as well as spark plugs.


WEIRD SCIENCE


The first thing you'll hear is that you will need a vacuum gauge in order to get your carburetor tuned properly. And that without one, you'll never be able to fine-tune your Quadrajet with precision.

It's true!

So I went ahead and purchased an inexpensive engine vacuum tester gauge online. I didn't have one and, as you know, one cannot have too many tools.

I used the brake booster vacuum line for the manifold vacuum source, and I adjusted the idle mixture screws with okay results since I did not have a decent baseline to work from.

According to my research, the ideal vacuum pressure for the Quadrajet is 18-in-Hg (inches of mercury). I adjusted the idle mixture screws and the best reading I got was about 17½-in-Hg.

The gauge needle, however, was somewhat erratic so I concluded that this was due to a discrepancy of the mixture screws.

Yes, I should have adjusted the idle mixture screws before attempting to fine-tune the carburetor with the vacuum pressure gauge. Assuming they were set properly was dumb.

Idle mixture screws on the Quadrajet must be adjusted equally in order to have a balanced air/fuel mixture.

Using my misguided approach, I ended up running from one side of the car to the other like a madman, losing track of how much I had turned the previous mixture screw and in which direction.

I was on the right path but putting the cart in front of the horse.


YOUTUBE® TO THE RESCUE

I watched a whole bunch of how-to tuning videos and the takeaway was to set the mixture screws anywhere from two to four-and-a-half turns out (from fully-seated). I decided to give this approach a try to see if it would solve my idle issues.

It is important to note that "fully seated" means snug but lightly.


Fortunately, my Corvette has accessible idle mixture screws. There's one on the driver's side and one on the passenger's side. Notice their exact location and the head style. Mine can be adjusted with a flat-head screwdriver.


It's important to note that the engine needs to be at normal-operating-temperature in order to fine-tune the idle mixture screws.


HOW TO ADJUST IDLE MIXTURE SCREWS

Once the car is at normal operating temperature, shut it off and turn the idle mixture screws until they're fully seated by turning them clockwise.

DO NOT overtighten them!

Also, make sure you count the exact number of turns for each side, and you may want to write those numbers down just in case you need to reset your carburetor to the previous setting.

Once they are fully seated, start turning them counterclockwise (to the left) counting each full turn. I don't want to state the obvious but it is important to know what a quarter-turn, a half-turn, three-quarter-turn, and a full-turn is.

Remember, both idle mixture screws must be adjusted equally!

And don't expect your carburetor's idle mixture screws to be exactly as shown in the diagram. Your starting point may be at 2-o-clock or elsewhere, but the same principle applies. You must establish a starting point and start counting from there.

Like I said before, the suggested number of turns ranges from two full turns counterclockwise to four-and-a-half.

I started with the lowest digit and I turned each screw two full turns out. I then started the car.

Honestly, I thought it was going to be hard to start and/or idle poorly.

WRONG!

It started right up and idled so smooth it surprised me.


But I had to make sure this was the right setting for mine, so I tried 2½ turns, then 2¼. Too rich, in my opinion, so I returned to 2 full turns.

I then adjusted the idle speed screw so the car would idle at 750 rpm.

I reinstalled the air-cleaner assembly and did another test run to ensure the car was running well.

It ran great!


Late that evening I took the car out for a long drive, and it ran better than ever.

When I pulled back in the garage I shut the engine off and restarted it a couple of times. No issues other than I felt it was still running a bit rich. So I reset the mixture screws and went with 1½ turns from fully seated for each side.

I drove the car again the next day for about an hour. The engine is running smooth and is very responsive, and there's hardly any gasoline odor either while driving or after I pull in the garage. But I had to experiment some more, so I adjusted the idle mixture screws one more time to 1¼ turns.

After another long test drive it seems that's the magic number.

My plan is to drive the car like this for a few days and then I will use the vacuum gauge to fine-tune the carb since I cannot leave well-enough alone. Frankly, I want to adjust it to the best of my ability.

So to recap... if you do a little bit of research and follow standard procedures, your car will idle and operate properly. You do not need to spend money (sometimes lots of it) with a mechanic. And you certainly don't need to buy a new carburetor. Quadrajets are great!

Now, the adjustments that worked for me may not work for your carburetor. Every situation is different, so my advice is to educate yourself as much as possible on the subject. Having said that, the numbers I shared may be a good starting point.

Adding an affordable vacuum gauge to your toolbox is a good idea, but you can tune your Quadrajet carburetor pretty well—provided the carb and engine are in good operational condition—without one.


You can get your C3 Corvette—or any other carbureted vehicle, for that matter—to run properly. And knowing what to do and how to do it will ensure your car will run great for as long as you own it.

I hope you'll stay tuned for a follow-up to this article. I just ordered a pair of new idle mixture screws and a used Dwell/Tach/Volt Analyzer so I can set the idle speed without having to look inside the car every time I make an adjustment.

Also, I understand that there are many carburetor tuning experts out there. I'm not one of them and my approach may have some shaking their heads. Having said that, I've gotten my car to run great so, obviously, I've done a few things right.

Thanks for following.


More Quadrajet Articles from my Blog

• Getting the Rochester Quadrajet Rebuilt | 2/23/2016
• Remanufactured Rochester Quadrajet Carburetor | 4/1/2016
• Carburetor to Air Cleaner Gasket | 2/2/2018
• Quadrajet Tuning: Replacing the Idle Mixture Screws 12/26/2018


Product Links... (#sponsored)

8-Mile-Lake Fuel Pump & Carburetor Vacuum Tester
Actron CP7677 Automotive Digital Multimeter and Engine Analyzer
ACDelco Air Filter | Part No. A348C
FRAM Fuel Cartridge Filter | Part No. CG3389
Dorman 55144 Fuel Filter Spring (Pack of 2)
Dorman 55143 Carburetor Inlet Gasket Assortment

1 comment:

  1. I run a 454 Chevy in a 50 Mercury. I am in agreement the the Quadrajet carb is a great bit of engineering, as they went on so many different engines with different displacements. My engine has a mild cam and 15 in. of vacuum is about the best that I can do, but it runs very well. I always adjust with a vacuum gauge, but I did adjust for high RPM years ago. Whatever works.

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