78 Corvette Quadraaaajet
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Instructor
Joined: Jun 2022
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78 Corvette Quadraaaajet
Hey guys, it's the original quadrajet, rebuilt with new floats, we solved the backfire through the exhaust problem, let's try this.
Car fired up fine cold, ran great for about 35 minutes while I did some errands, got onto an open street and gave it some gas. No issue, it accelerated fine. Then it sputtered, spit and died. I pulled over and checked the plug wires, hei wires etc and I saw a little gas on the intake.
I got back in, opened the throttle all the way and it finally came alive. It was spitting and sputtering but it made it home. I took the air cleaner off, and when it stalled out fuel spit out of the bowl.
The idle air screws were out 4 turns, so I adjusted those now it idles at least. It's odd that it was completely fine until I got on it and the issue persisted until it cooled a little. Weather today is about 78 and sunny.
Car fired up fine cold, ran great for about 35 minutes while I did some errands, got onto an open street and gave it some gas. No issue, it accelerated fine. Then it sputtered, spit and died. I pulled over and checked the plug wires, hei wires etc and I saw a little gas on the intake.
I got back in, opened the throttle all the way and it finally came alive. It was spitting and sputtering but it made it home. I took the air cleaner off, and when it stalled out fuel spit out of the bowl.
The idle air screws were out 4 turns, so I adjusted those now it idles at least. It's odd that it was completely fine until I got on it and the issue persisted until it cooled a little. Weather today is about 78 and sunny.
Race Director
Joined: Mar 2017
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From: Virginia
That sounds like vapor lock. My 79 does it all the time. My 80, not so much for whatever reason.
You can eliminate the fuel tank vent issue by cracking your gas cap. Do you have a return line on your fuel pump?
BTW, if searching for this thread ever helps anyone, the correct term is afterfire (or after fire) if it is in the exhaust. Backfire is ignition in the intake manifold and carb.
You can eliminate the fuel tank vent issue by cracking your gas cap. Do you have a return line on your fuel pump?
BTW, if searching for this thread ever helps anyone, the correct term is afterfire (or after fire) if it is in the exhaust. Backfire is ignition in the intake manifold and carb.
Thread Starter
Instructor
Joined: Jun 2022
Posts: 176
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That sounds like vapor lock. My 79 does it all the time. My 80, not so much for whatever reason.
You can eliminate the fuel tank vent issue by cracking your gas cap. Do you have a return line on your fuel pump?
BTW, if searching for this thread ever helps anyone, the correct term is afterfire (or after fire) if it is in the exhaust. Backfire is ignition in the intake manifold and carb.
You can eliminate the fuel tank vent issue by cracking your gas cap. Do you have a return line on your fuel pump?
BTW, if searching for this thread ever helps anyone, the correct term is afterfire (or after fire) if it is in the exhaust. Backfire is ignition in the intake manifold and carb.
Race Director
Joined: Mar 2017
Posts: 11,947
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From: Virginia
It could also be a carb adjustment, but vapor lock seems most likely. What is your timing set to? A hot running engine makes it worse.
Thread Starter
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You don't need to drive around like that. If the tank vent isn't working (and you'd need to burn a LOT of gas too see an issue), you'd hear a hiss as air enters the tank when you crack the cap.
It could also be a carb adjustment, but vapor lock seems most likely. What is your timing set to? A hot running engine makes it worse.
It could also be a carb adjustment, but vapor lock seems most likely. What is your timing set to? A hot running engine makes it worse.
I didn't hear a hiss when I got home, I was just frantically screwing with the carb, wasn't til an hour or so later I had a moment of oh **** its vapor lock. And it had a slight one when I checked it. The engine does seem to be running a bit hot, I still may dial back the timing.
My 72 Spitfire did it to me once when I first got it on the road, that was a plugged vent canister.
Race Director
Joined: Mar 2017
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From: Virginia
it's around 10° with the vacuum advance disconnected.
I didn't hear a hiss when I got home, I was just frantically screwing with the carb, wasn't til an hour or so later I had a moment of oh **** its vapor lock. And it had a slight one when I checked it. The engine does seem to be running a bit hot, I still may dial back the timing.
My 72 Spitfire did it to me once when I first got it on the road, that was a plugged vent canister.
I didn't hear a hiss when I got home, I was just frantically screwing with the carb, wasn't til an hour or so later I had a moment of oh **** its vapor lock. And it had a slight one when I checked it. The engine does seem to be running a bit hot, I still may dial back the timing.
My 72 Spitfire did it to me once when I first got it on the road, that was a plugged vent canister.
Look for a post by @lars where he shares his email, and email him for his timing, vacuum advance, HEI, and Q-Jet papers. Then follow along.
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Went out to go to work this morning, fired up on the first hit and then proceeded to dump fuel out the vent all over the place.
Removed it, had one of my guys pick me up and disassembled on the bench at my shop, the float isn't sticking, but the needle isn't moving much in the seat, float level was set where it was before and it is not binding. It is a Nitrofil float.
I'm going to see about getting another needle and seat and trying it again tonight, I'll also swap the fuel filter, but inside the carb is just as clean as when it was rebuilt.
Removed it, had one of my guys pick me up and disassembled on the bench at my shop, the float isn't sticking, but the needle isn't moving much in the seat, float level was set where it was before and it is not binding. It is a Nitrofil float.
I'm going to see about getting another needle and seat and trying it again tonight, I'll also swap the fuel filter, but inside the carb is just as clean as when it was rebuilt.
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From: Indiana
it's around 10° with the vacuum advance disconnected.
I didn't hear a hiss when I got home, I was just frantically screwing with the carb, wasn't til an hour or so later I had a moment of oh **** its vapor lock. And it had a slight one when I checked it. The engine does seem to be running a bit hot, I still may dial back the timing.
My 72 Spitfire did it to me once when I first got it on the road, that was a plugged vent canister.
I didn't hear a hiss when I got home, I was just frantically screwing with the carb, wasn't til an hour or so later I had a moment of oh **** its vapor lock. And it had a slight one when I checked it. The engine does seem to be running a bit hot, I still may dial back the timing.
My 72 Spitfire did it to me once when I first got it on the road, that was a plugged vent canister.
https://www.corvetteforum.com/forums...l82-q-jet.html
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Instructor
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Ok, I disassembled it, there's no schmoo in the bowl at all, but the operation of the float seems a little odd.
Inside the carb there are two needles that go down into jets, and one of the tips seems to be broken off, aside from that I could not find anything wrong so I ordered a reman one from summit and am having it shipped air mail next day delivery. Since it's a Canadian long weekend and every car guy I know is going up racing it is next to impossible to get it looked at.
The reason I went reman was a corvette shop in town told me he bought his quadrajet on summit and has had ZERO issues. My city seems to have completely gone away from carb rebuild and this one isn't like the Holley 4160's that I can do with my eyes closed.
Inside the carb there are two needles that go down into jets, and one of the tips seems to be broken off, aside from that I could not find anything wrong so I ordered a reman one from summit and am having it shipped air mail next day delivery. Since it's a Canadian long weekend and every car guy I know is going up racing it is next to impossible to get it looked at.
The reason I went reman was a corvette shop in town told me he bought his quadrajet on summit and has had ZERO issues. My city seems to have completely gone away from carb rebuild and this one isn't like the Holley 4160's that I can do with my eyes closed.
Instructor
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From: Texas
Racer
Joined: Jan 2020
Posts: 388
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From: Tacoma, Wa
I highly recommend installing a 1/4 inch extra thick carb to intake gasket. I was chasing down vapor lock issues during the first summer after I bought the car. Everything I tried kinda helped, but didnt solve the problem. This gasket solved the problem and then some. Last summer I drove in 110* heat and no vapor lock issues whatsoever.
Advanced
Joined: Apr 2020
Posts: 80
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From: Ont, Canada
Ok, I disassembled it, there's no schmoo in the bowl at all, but the operation of the float seems a little odd.
Inside the carb there are two needles that go down into jets, and one of the tips seems to be broken off, aside from that I could not find anything wrong so I ordered a reman one from summit and am having it shipped air mail next day delivery. Since it's a Canadian long weekend and every car guy I know is going up racing it is next to impossible to get it looked at.
The reason I went reman was a corvette shop in town told me he bought his quadrajet on summit and has had ZERO issues. My city seems to have completely gone away from carb rebuild and this one isn't like the Holley 4160's that I can do with my eyes closed.
Inside the carb there are two needles that go down into jets, and one of the tips seems to be broken off, aside from that I could not find anything wrong so I ordered a reman one from summit and am having it shipped air mail next day delivery. Since it's a Canadian long weekend and every car guy I know is going up racing it is next to impossible to get it looked at.
The reason I went reman was a corvette shop in town told me he bought his quadrajet on summit and has had ZERO issues. My city seems to have completely gone away from carb rebuild and this one isn't like the Holley 4160's that I can do with my eyes closed.
Pro
Joined: Nov 2019
Posts: 614
Likes: 56
From: Dover, Delaware
Well this is my 2cents worth....................If your QJ wasn't rebuilt/reengineered by Lars it just ain't right!!!!
Had two done by Lars and never had any issues except that mine takes several extra pedal pumps bc I only start it and drive it every few weeks and Lars hit the nail on the with regards to the ethanol fuel the east coast offers...ugh!!!
JJ78
Had two done by Lars and never had any issues except that mine takes several extra pedal pumps bc I only start it and drive it every few weeks and Lars hit the nail on the with regards to the ethanol fuel the east coast offers...ugh!!!
JJ78
Pro
Joined: Nov 2019
Posts: 648
Likes: 353
From: Portland, Oregon
Remanufactured carbs are almost never reassembled from just a single original. Highly recommended to post some pictures here from all angles and the inside; before even trying it out. Lots of subtle, non-obvious anomalies possible that might frustrate your future carb tuning. Pictures of the problems with your old carb will get you better advice on whether it’s still worth fixing. A previous post of mine included pictures of a Summit carb they sent me with multiple major problems; sent it right back.
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Went to a local shop that does work on c1 to c7 Corvettes. In the lot he had a 78 Indy 500, so I rang the bell and said "hey man, what do I do with this thing?" He said we bought one from summit, $400 and it ran out of the box.
Well, I'm in a pickle, my wife is already complaining about a Triumph GT6 on a trailer in the driveway, currently blocked by my 78 vette that she's helped me push twice.. so I did it, I bought the reman.
God damn, a few modifications, add the electric choke, linkage, and a few other odds and ends.. I threw it on after work, set the air/fuel screws to 2 turns and after a few cranks it fired right up, idles perfect, and has zero leaks.
I'm getting my original one rebuilt, an old mechanic I know needs a couple parts but hes done hundreds of them, so I have the peace of mind knowing there's a backup.
Well, I'm in a pickle, my wife is already complaining about a Triumph GT6 on a trailer in the driveway, currently blocked by my 78 vette that she's helped me push twice.. so I did it, I bought the reman.
God damn, a few modifications, add the electric choke, linkage, and a few other odds and ends.. I threw it on after work, set the air/fuel screws to 2 turns and after a few cranks it fired right up, idles perfect, and has zero leaks.
I'm getting my original one rebuilt, an old mechanic I know needs a couple parts but hes done hundreds of them, so I have the peace of mind knowing there's a backup.