Putting these articles together is no simple task.
Between photos, videos, all the necessary editing, then putting the whole process into words—after actually taking the carburetor apart and then reassembling it—takes long hours.
Needless to say, I was happy when the process was over. I stayed up late and put my thoughts into words. However, with so many words plus photos, I had to break them down into three installments.
The next day, a Saturday, I test-drove my Corvette and recorded the hot-engine-start video I published in Part 3, but as I mentioned at the end of that article, I was not 100% happy with the car and it bugged the heck out of me.
So early next morning, I was back in the garage taking the carb off of the engine and I disassembled my Quadrajet... again!
Well, I didn't take the whole carb apart. Just the air horn.
Why? you may be asking?
My car was not performing to my standards. It ran okay, but okay is not great, and great is what I was after.
Please, don't misunderstand; my Vette ran OK, but it just didn't feel it had as much punch as before, and that bothered me. Plus, the two times I floored it, it sort of bogged down, and that was simply unacceptable.
As I lay in bed that night, thinking about a possible solution, my mind kept going back to the two changes I had made.
Number one was the primary metering jets, and two was the float level angle.
So I decided I was going to go back to the original jets and also reset the float angle back to 5/16" (from 15/32").
Since I do not have precision measuring instruments (not that I would know how to properly use them, anyway), and I was unable to determine the jet size from the original jets themselves (I could bearly read a number 6 stamped on them), I went with the vendor-recommended (.077") size.
The next day I crudely measured the original jets with a small drill bit (yes, I used the shank), and a 5/64" bit fit perfectly in the jet hole. However, that same drill bit would not fit the 0.77" jet hole.
Why? Because 5/64" is the equivalent of 0.781 so, in other words, my original jets were a bit larger even though the new ones were .077 inches which seems to indicate they would be larger than .076-inch jets, which—I learned later—is the size of the jets my carb came with.
Were my carb jets stamped wrong? Were the new ones?
When it comes to carburetor jetting, I admit I have no clue what I'm talking about. But numbers don't lie and my crude measuring proved there was a minuscule yet significant difference.
The Air Horn
Fortunately, this time I did not have to take the whole carb apart. The vacuum diaphragm and the accelerator pump lever were the only external components that had to be removed, followed by the secondary metering rods and the hanger, plus the nine screws that secure the air horn to the body.
Once the air horn was off of the carburetor body, I removed the gasket, the primary metering rods, the float bowl plastic insert, the float, and the primary jets.
I reinstalled the original jets, readjusted the float angle, and reassembled the carburetor. I was done in about 30 minutes, from start to finish.
Did It Make A Difference?
Yes, it did!
It literally feels like a different car, and the bog is no longer an issue when I floor it.
Unfortunately, I cannot tell you specifically if it was the jets or the float angle, but I am guessing it was a combination of the two.
If anyone knows the right carburetor primary jet size for a 1976 Corvette with a 4-speed transmission, please let me know.
How About The Hot-Engine Restarting Issue?
It is gone. I just have to lightly pump the gas pedal once and it starts right up. And the rough-running condition is no longer an issue, so it appears that resealing the Quadrajet well plugs took care of that problem.
And even though my Vette is running better than ever, one item remains on my to-do list, and that is to fine-tune the idle-mixture screws with a vacuum gauge. I think this is crucial for the carburetor and engine to operate optimally.
Any Other Issues?
Yes. And I should've mentioned the importance of replacing the fuel inlet nylon gasket.
These gaskets are a one-time-use item. Once you tighten the fuel inlet nut, they get crushed and must be replaced if you remove the fuel inlet to replace the fuel filter, or for whatever other reason.
I failed to do that during the carburetor reassembly process and eventually, discovered... (smelled, is a better word), a gas leak.
The ACDelco Rebuild Kit shown at the end of this article includes one. If you do not need a complete rebuild kit, you can buy these nylon gaskets from Dorman. That way you'll have a spare when one is needed.
You can only get them in an assorted 10-pack, but you need the white nylon gaskets, so you'll get five. There may be other vendors that sell just the white nylon gaskets but make sure the inside diameter (ID) is .785 in. (2 cm).
Window Seat?
No, I am not talking about airplane seating arrangements. Instead, I am talking about carburetor needle seats, which in the case of Quadrajets come in two varieties: Window Seat or No-Window Seat. Mine was the former.
However, the ACDelco rebuild kit only comes with the non-widowed seat.
Now, there are dozens, nay, hundreds of online discussions as to the pros and cons of either version, and I am not qualified to render an opinion on the subject. I've heard that both versions work the same so if you need to replace your needle seat you're probably okay with the non-windowed seat even if your carb came with the other.
However, needle seats rarely get damaged, so you may be just fine if you give yours a good cleaning. But again, your car would probably run just fine with either seat.
The needle is a different story and it should be replaced during a rebuild. And yes, the ACDelco rebuild kit includes one along with the hanger wire.
Parting Comments
A properly tuned Rochester Quadrajet carburetor should allow the engine to run as well and smoothly as a fuel-injected one. Unfortunately, many of these cars run poorly due to a myriad of causes. From air leaks to improper jets to bent or cracked components.
I recently purchased two 750 CFM Quadrajet carburetors at swap meets, which I have rebuilt. One came off a 1979 Corvette with a 4-speed transmission, and the other one from a 1976 Chevrolet with an automatic.
Upon disassembly, I discovered that in one of the carbs the fuel filter was installed backward (and you really need to force it to go in that way), I also found a couple of loose or damaged butterfly screws, and a torn gasket so they could make it fit around the power piston.
The other one had two different secondary metering rods. One was stamped CH (which is the correct one), and the other was a CA rod. The needle was installed incorrectly as was the choke mechanism. And lastly, the float was damaged somehow to the point of failure. Why it was reused I do not know.
And some people have the audacity to call these carburetors "QuadraJunks."
Also, and mainly for trivia purposes, if you are curious as to whether your Quadrajet is a 750 cfm or an 800 cfm carb, the easiest way to determine this is by a visual inspection.
The 800 cfm Quadrajet will have a little bump inside each of the primary Venturis (red arrow below). The 750 cfm carbs do not.
Last but Not Least
Please remember that I am not a mechanic and even though my articles may sound like instructions or—God forbid—advice, they are not intended as such. I just happen to believe that they sound better when presented in such a fashion.
Having said that, there are plenty of authoritative articles and videos online that could be used by the average DIYer to completely rebuild their Quadrajet to factory specs.
Over the years I've seen repairs done by so-called "professionals" that can only be described as shoddy, to be kind. And because of that, I decided to learn as much as possible in order to do my own repairs, or in this case, rebuild my Quadrajet carburetor.
You can do the same!
Thank you for following my '76 Vette Blog!
Product Links... (#sponsored)
• J-B Weld MarineWeld Marine Epoxy - 2 oz.
• J-B Weld Dark Grey TankWeld Gas Tank Repair
• Gumout Carb and Choke Cleaner | Spray 16 oz.
• Rochester Carburetors by Doug Roe
• How to Rebuild & Modify Rochester Quadrajet Carburetors by Cliff Ruggles
• Dorman 55143 Carburetor Inlet Gasket Assortment - 10 Piece
• ACDelco Carburetor Repair Kit: Ball, Clips, Gaskets, Screws, and Seals
• 1975-1985 Rochester Quadrajet Hot-Air Choke Remanufactured Carburetor
