Wednesday, December 26, 2018

Quadrajet Tuning: Replacing the Idle Mixture Screws

I wasn't sure if the company that rebuilt my Corvette's Quadrajet carburetor replaced the idle mixture screws, but from the looks of them, my guess is that they did not.

They were clean, but they showed signs of pitting and wear, and I wasn't sure if that would have a negative effect on how my car idled.

So, I removed them to inspect them closely as well as take measurements to make sure I would order the correct screws. I also messaged the eBay seller with the details as to what I needed, the carburetor ID number, a photo of one of the screws, plus measurements I took.

A reply from Quadrajet Power confirmed I was ordering the correct idle mixture screws for my Rochester Quadrajet.

The $10 cost for the pair, shipping included, made this decision super easy.

Like I mentioned before, I have no idea if the condition of the original idle mixture screws would have any sort of effect on how smooth or rough the idle behaves, but for ten bucks I had nothing to lose and (maybe) lots to gain.

The photos fail to capture all the minute scratches and pitting, plus one of them was bent a bit, something that was particularly noticeable when turning it to make adjustments.


I measured the screws with my trusty Vernier caliper, which is one tool that I always have trouble interpreting, but a couple of YouTube videos refreshed my memory enough to determine that the total length was 1.625" (or 1 5/8") and the body (shank, threads, and tip) was 1 1/8".

The guys at Quadrajet Power quickly replied to confirm that the ones I was looking at were indeed the right replacement idle mixture screws for my carburetor.

By the way, like all automotive parts and components, Quadrajet carburetors have identification numbers that tell the whole story about the carb sitting on top of your engine. I found a site that will allow you to figure out a lot about your carburetor simply by interpreting the Rochester Quadrajet casting numbers.

The first thing to do is locate the carburetor number. Mine is stamped on the left side of the carb toward the back of the secondaries.



With that data available, I was able to determine the following:


This information helps you figure out if the carburetor on your motor is the one it left the factory with. And based on the dates and other data, mine is correct for my car and, most likely, the original one.

I received the new idle mixture screws in a matter of days and swapped them today.

You have the option to order new replacement springs, but mine looked fine so I decided to reuse them.


If you look closely at the photo above, you can tell that the original idle mixture screw is bent a tiny bit right toward the end of the threads. How that happened should remain a mystery, but I do know that it would take quite a bit of force in order for it to bend.


The magnified picture above shows the overall condition of one of the idle mixture screws threads and tip. And even though I am no expert to categorically say "They are bad," I don't think one needs to be to make the decision to replace both of them.

It's worth mentioning again that they have to be seated (very gently) and then turned out whatever number of turns works best for your Corvette. In my case, the magic number is 1¼ turns out, and both idle mixture screws must be adjusted equally.

And like I mentioned in Quadrajet Tuning: Adjusting the Idle Mixture Screws, it is a good idea to write down the number of turns on your carburetor before you remove them. Just count the turns as you re-seat them so you'll know how far to adjust the new ones.

After I'd buttoned everything up, I took my Vette out for a long drive to see how it performed. Call it a placebo effect but it appeared to idle a lot smoother than before and it stayed that way throughout the entire 30-mile trip. So I'm going to guess that the overall condition of the idle mixture screws can have noticeable effects, but I'll let you be the judge of that if you decide to replace yours.

It's a very easy job that can be done in less than fifteen minutes and, again, for a very reasonable amount of money.

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: Adjusting the Idle Mixture Screws | 12/20/2018

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