Gas Boiling in Quadrajet
Thread Starter
Drifting
Joined: Nov 2013
Posts: 1,806
Likes: 258
From: Dublin Ohio
Gas Boiling in Quadrajet
It's a 72 base engine. Stock appearing under the hood.
Car was running around 190 degrees. Turned it off and let it set for 10 minutes. Turned the key back on and temperature was up to 210.
Pulled the air cleaner lid off and saw this. I think it's been doing this for years. Just never pulled the lid at the right time.
I'd like to put an end to this. where do I start? I do have the GM heat shield under the carb but no spacer.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Float is probably set too high. But, you should also check to make sure the vent over the float bowl is clear. If plugged, internal pressure build-up can cause what you are seeing.
My 'guess' is that your poly'foam' float is absorbing fluid and can not shut off fuel flow as it should. If it has been years since last carb rebuild, just rebuild it with a kit from NAPA and also purchase a brass float for your P/N Q-jet.
My 'guess' is that your poly'foam' float is absorbing fluid and can not shut off fuel flow as it should. If it has been years since last carb rebuild, just rebuild it with a kit from NAPA and also purchase a brass float for your P/N Q-jet.
Burning Brakes
Joined: Mar 2014
Posts: 904
Likes: 46
From: Crestline CA
Not saying that float level isn't your problem but thats pretty common with modern fuels, especially with "winter blend" fuel. In the winter the fuel has more volatility to make up for the drop in ambient temp, higher volatility=lower boiling point. I used to have this problem before I started to mix 50/50 91 octane and VP100(available at the pump local to me).
People have reported good luck at eliminating this problem by blocking or restricting the exhaust crossover in the intake manifold.
People have reported good luck at eliminating this problem by blocking or restricting the exhaust crossover in the intake manifold.
Le Mans Master
Joined: Aug 2006
Posts: 7,427
Likes: 1,560
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
Hi Barry,
Here are some measurements that you might try or you can wait until Lars posts the exact number for you.
Make sure that you get this 'clip' on correctly.
Here are some measurements that you might try or you can wait until Lars posts the exact number for you.
Make sure that you get this 'clip' on correctly.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If the float is no longer buoyant and has absorbed fuel, you can't set the float height and have it mean anything. You have to replace the float. It could simply be set too high, so that the fluid level is too high. But, since it hasn't always done this, it's more likely that the float is "toast".
Thread Starter
Drifting
Joined: Nov 2013
Posts: 1,806
Likes: 258
From: Dublin Ohio
If I take it apart I'll definitely change out the float. Looks like NAPA is the place to get a good one.
As far as getting the carb apart to get to the float, from what I'm reading it's 9 screws to get the air horn off. For just changing a float, what else will I need to do? Assuming the carb is out and I'm sitting at my kitchen table.
As far as getting the carb apart to get to the float, from what I'm reading it's 9 screws to get the air horn off. For just changing a float, what else will I need to do? Assuming the carb is out and I'm sitting at my kitchen table.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Melting Slicks
Joined: Feb 2006
Posts: 3,105
Likes: 96
From: Sunrise FL
with above. I fought this with Holley carbs....SAME PROBLEM!!!1)add base insulator under carb (I have one if it does not come in rebuild kit)
2)Make sure heat riser is kept open
3)Change to new or metal float
Last edited by mikem350; Feb 19, 2017 at 04:20 PM.
Thread Starter
Drifting
Joined: Nov 2013
Posts: 1,806
Likes: 258
From: Dublin Ohio
Originally Posted by 69Vett
Install the original 1/4" (Thermal ) insulating gasket under the carburetor.
Fixed.
Fixed.
Thread Starter
Drifting
Joined: Nov 2013
Posts: 1,806
Likes: 258
From: Dublin Ohio
By the way, it has tried to die once or twice in 3 years during hard braking when I decided I shouldn't try to make a yellow light. could that be a sign of a high float as well?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Actually, the most difficult part of the 'partial' Q-Jet disassembly is managing the choke link that connects the choke plate to the choke mechanism (inside the right side of the carb body. You need to remove the screw at the choke plate without rotating that link (as it will dislodge from the transfer link to the choke shaft). The only way you can remove the top plate on the carb is to disconnect that choke link and to drive the roll-pin inward to free the accelerator pump arm.
Then you can remove the top plate screws and open it up. Getting it put back on without moving 'stuff' out of position is a little taxing, too.
Then you can remove the top plate screws and open it up. Getting it put back on without moving 'stuff' out of position is a little taxing, too.
Safety Car
Joined: Oct 2007
Posts: 3,729
Likes: 267
From: Austin Texas
Corvette of the Year Winner 2017
2016 C3 of Year Finalist
the thermal Insulator was not the problem for hot start issues.
by just adding the thermal insulator your carb. will drop temperature about 100 degrees,
test it. run car to normal temperature, hold your hand on the carb. (as long as you can!)
install insulator, same test, insulated carb. will literally be cold.
Hot start issues likely is a choke adjustment issue. good luck !
by just adding the thermal insulator your carb. will drop temperature about 100 degrees,
test it. run car to normal temperature, hold your hand on the carb. (as long as you can!)
install insulator, same test, insulated carb. will literally be cold.
Hot start issues likely is a choke adjustment issue. good luck !
Race Director
Joined: Feb 2001
Posts: 11,310
Likes: 2,096
From: Grapevine Tx
2018 C3 of Year Finalist
2016 C3 of the Year Finalist
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,221
Likes: 9,357
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Edelbrocks are notorious for this. I rebuilt the carb, adjusted the floats and used a 1/2" wooden insulator and changed to an Air Gap manifold and it solved my boiling problems.
I would suggest a rebuild and proper adjustment, and add that factory insulator, then go from there. The crappy 10% ethanol is ruining alot of engines across the board
I would suggest a rebuild and proper adjustment, and add that factory insulator, then go from there. The crappy 10% ethanol is ruining alot of engines across the board
Advanced
Joined: Jan 2017
Posts: 64
Likes: 1
Anyone have a suggestion on how I can get some more room to put a spacer under my carb? Here is my setup, I got some of my kid's play dough and set it on top of my air cleaner to find out how much play I had above it. The play dough about the air cleaner **** was clear! About 1/32" between it and the hood. I guess the only fix would be a shorter air cleaner. Any suggestions? Thanks.