Q-Jet fuel boiling issues
Thread Starter
Drifting
Joined: Aug 1999
Posts: 1,451
Likes: 84
From: Round Rock TX
Q-Jet fuel boiling issues
I took. My car out for a drive and parked it, and it didn't want to fire up. Took the air cleaner lid off and saw fuel bubbling out of the primary Venturi area. This is on an original 70 LS5 cast iron intake and carb, running Shell super unleaded fuel.
What are others doing to combat this?
What are others doing to combat this?
Is my vette stock?? HAHA
Joined: Oct 2015
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From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
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a wooden spacer will probably solve it. But I did see a quadrajet specific olastic heat spacer available from one of the vendors. It was a stock replacement, if I remember it was about 1/4 ich thick. I used a 1/2 inch wood spacer that worked
Pro
Joined: Oct 2009
Posts: 664
Likes: 89
From: Marysville Ohio
Common solutions include a spacer or heat shield as stated above, as well as a return line from carb back to tank to fuel doesn't sit still in the intake line. Modern fuels are engineered for fuel-injected vehicles, and IMO does not require as high a boiling point.
I would start with the suggestions above and do more (i.e., then return line) only if required.
I would start with the suggestions above and do more (i.e., then return line) only if required.
Safety Car
Joined: Sep 2007
Posts: 4,458
Likes: 331
From: Indy Indiana
1/4 inch ( no bigger as you will have hood clearance issues) phenolic spacer. this will solve your problem. before I installed one, you could hear my LS5 sizzle after shutting off the hot engine. Now it is barely warm to the touch. Best money I spent...
summit probably has them..
summit probably has them..
Safety Car
Joined: Sep 2005
Posts: 4,226
Likes: 643
From: Orange County Ca
My problem is that everything under the hood gets really hot after a decent run. So even with a spacer, the gas still boils off when it's parked. I do have the stock, exhaust cross over intake manifold which probably negates any help from the carb base gaskets/insulator. I have tried blocking off the exhaust passages, but it actually runs better with the passages open. It defies logic, but it does. Especially for the first 10 miles.
I also believe that the modern gas/alcohol fuel makes this worse.
With the fuel boiling over, you are flooding the engine. Hold the pedal to the floor and turn it over. This will purge the rich gas mixture and should start the engine. It may take a few tries.
I also believe that the modern gas/alcohol fuel makes this worse.
With the fuel boiling over, you are flooding the engine. Hold the pedal to the floor and turn it over. This will purge the rich gas mixture and should start the engine. It may take a few tries.
Last edited by Sayfoo; Jun 5, 2017 at 01:26 PM.
Safety Car
Joined: Oct 2007
Posts: 3,729
Likes: 267
From: Austin Texas
Corvette of the Year Winner 2017
2016 C3 of Year Finalist
return the original 1/4" isolation gasket under the carb. and your problems will go away.
I did some testing, adding the Isolation gasket will drop the carb temperature nearly 100 degrees.
I did some testing, adding the Isolation gasket will drop the carb temperature nearly 100 degrees.
Last edited by 69Vett; Jun 5, 2017 at 04:43 PM.
Thread Starter
Drifting
Joined: Aug 1999
Posts: 1,451
Likes: 84
From: Round Rock TX
I ordered this one, https://m.summitracing.com/parts/mor-65018.
69vette, which isolation one are you referring to. I just have the thin gasket currently installed. The car rebuild kit had one that was thicker, but seemed kind of waxy to me. Is that the one I should be running?
I need to keep a set of gloves in the car as the air cleaner lid was super hot. Once I removed the air cleaner and saw the issue, I did floor it and it started. Just would like to minimize drivability issues as much as I can, within reason (not putting fuel injection on). I'm running an original style fuel pump, with line from pump to carb, and have the return line connected as originally done.
69vette, which isolation one are you referring to. I just have the thin gasket currently installed. The car rebuild kit had one that was thicker, but seemed kind of waxy to me. Is that the one I should be running?
I need to keep a set of gloves in the car as the air cleaner lid was super hot. Once I removed the air cleaner and saw the issue, I did floor it and it started. Just would like to minimize drivability issues as much as I can, within reason (not putting fuel injection on). I'm running an original style fuel pump, with line from pump to carb, and have the return line connected as originally done.
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 unit you have ordered is .5" thick it can create clearance issues.
put the thicker oem 1/4" gasket under the carburetor. done.
Your in Texas I would suggest blocking off the heat crossover tubes,
in the intake also.
put the thicker oem 1/4" gasket under the carburetor. done.
Your in Texas I would suggest blocking off the heat crossover tubes,
in the intake also.
Last edited by 69Vett; Jun 5, 2017 at 05:15 PM.
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Is my vette stock?? HAHA
Joined: Oct 2015
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From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
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Its the ethanol, its basically just corn mash alcohol and it lowers the boiling point and dissolves old rubber fuel lines. I had a bunch of old yard tools and chainsaws that the fuel lines just melted. Wish it would go away with Obamas adminstration
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
I took. My car out for a drive and parked it, and it didn't want to fire up. Took the air cleaner lid off and saw fuel bubbling out of the primary Venturi area. This is on an original 70 LS5 cast iron intake and carb, running Shell super unleaded fuel.
What are others doing to combat this?
What are others doing to combat this?
Lars
Drifting
Joined: Dec 2004
Posts: 1,714
Likes: 42
From: Austin Texas
I just took some readings with an IR gun this weekend.
I have a small block with aluminum manifold with crossover blocked and a .25 phenolic spacer.
The manifold temp was 195* and the carb temp was 145* so about a 50* difference.
Take those readings on your car and see what you come up with as a difference. The temp guns are fairly affordable now.
Regards,
John
I have a small block with aluminum manifold with crossover blocked and a .25 phenolic spacer.
The manifold temp was 195* and the carb temp was 145* so about a 50* difference.
Take those readings on your car and see what you come up with as a difference. The temp guns are fairly affordable now.
Regards,
John
Thread Starter
Drifting
Joined: Aug 1999
Posts: 1,451
Likes: 84
From: Round Rock TX
Lars running the stock intake for a 1970 that doesn't have the gases getting to the bottom of the carb. Will put the thicker gasket on later this week. Should I just run the thicker or add the thinner one to the mix?
I really hate the ethanol fuels and wish we could get the corn out of our gas. I believe the ethanol is worse overall than straight gas. I don't recall having to dispose of gasoline like I do now. My mileage has gone down in my regular cars as well.
I really hate the ethanol fuels and wish we could get the corn out of our gas. I believe the ethanol is worse overall than straight gas. I don't recall having to dispose of gasoline like I do now. My mileage has gone down in my regular cars as well.
Pro
Joined: Nov 2008
Posts: 542
Likes: 3
From: Bloomfield Hills MI
Make sure you are running the correct, stock 1/4" thick insulating base gasket under the carb. Also, make sure you do not have one of the "smiley-face" exhaust crossover intake manifolds, and if you do, make sure you are running the correct 3-gasket "pack" under the carb.
Lars
Lars
Last edited by spinadog; Jun 22, 2017 at 06:04 PM.
Racer
Joined: Aug 2011
Posts: 429
Likes: 24
From: Charlotte NC
Sometimes the fuel gets warm before it gets to the carb and I've heard some move the line away as much as possible and then wrap the line in tin foil. Have not tried it but heard of good results. Also covering the float bowls with tin foil or other insulating material. I have the same problem but with no air-cond. I only drive at 70 degrees or less anyway.
Drifting
Joined: Nov 2016
Posts: 1,963
Likes: 579
From: Pittsburgh PA
im having issues with my qjet getting warm as well.
Would there be any benefit to adding one of the heat shields vs just using a 1/4" gasket?
https://www.ebay.com/itm/282529364798
Would there be any benefit to adding one of the heat shields vs just using a 1/4" gasket?
https://www.ebay.com/itm/282529364798
Thread Starter
Drifting
Joined: Aug 1999
Posts: 1,451
Likes: 84
From: Round Rock TX
I was using the thin gasket and then went to the 1/4" thick one, and that helped immensely. I put some DEI wrap around the fuel line all the way down to the pump, but haven't driven it since.
The 1/4" gasket I am using has plastic inserts around the carb mounting bolts. I am not running another gasket between the intake and carb base than that one.
The 1/4" gasket I am using has plastic inserts around the carb mounting bolts. I am not running another gasket between the intake and carb base than that one.
Safety Car
Joined: Oct 2007
Posts: 3,729
Likes: 267
From: Austin Texas
Corvette of the Year Winner 2017
2016 C3 of Year Finalist
it does not have to be GM part, any 1/4" thick Gasket will stop the heat transfer to the Carb.
do this and keep gas line away or insulated from hot engine, and you will have no problems.
do this and keep gas line away or insulated from hot engine, and you will have no problems.
Pro
Joined: Feb 2008
Posts: 556
Likes: 13
From: C3 Forum, Colorado
https://www.pure-gas.org/
Click on your state and scroll down to find if there's a city near you. It usually cost more but in my wife's computer controlled vehicle the on board MPG display shows me getting 3-4 miles per gallon better than running ethanol gas. So it actually works out to be the same price or cheaper than running ethanol. My carbureted cars don't care as much. Maybe 1-2 MPG better. But I feel better knowing they have non ethanol gas in the tank when they might happen to sit for prolong periods of time.