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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.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
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
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.
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..
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.
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.
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.
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.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
Originally Posted by cooper9811
Modern fuels are engineered for fuel-injected vehicles, and IMO does not require as high a boiling point.
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
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?
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 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.
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
OP & Lars, not derailing this thread, but I too would benefit greatly from knowing the right 'stack' of gaskets. Right now I have the following stack: manifold-thin gasket-heat shield-1/4" gasket-carb. Is this correct? '70 LS5, stock, correct carb.
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.
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.
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.
I have not been able to find a idle vent that fits. Anyone have some ideas?
It appears that you're missing the thin metal strip that holds the rubber valve.
Originally Posted by Duntov
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. My mileage has gone down in my regular cars as well.
Go to this website to see if there's a gas station in your area that sells ethanol free gas. 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.
06-04-2017, 10:25 PM
Q-Jet fuel boiling issues
