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My 1966 Impala 396 will sputter and sometimes cut off after cruising around 55mph for a while during the hot weather. It happens when I exit the highway, depress the clutch and go to neutral, and come to a stop at the stop sign. For now, I can usually get it to start back up again but feel like eventually I may get stranded. I adjusted the carb to idle a bit higher but this does not seem to help. I have read a few places this was due to high temps in summer ( usually around 90 to 95 degrees here). Any thoughts? Ideas on what this may be?
 

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Central Florida is the same but we heat up to that temp earlier in the year and stay there longer through out the year. Never has hit above 99 degrees here, ever, because we are surrounded by 90 degree water in the "summer months of March through November: but have I ever mentioned how humid it gets here with the Gulf boiling away to keep us "cool"?

As to your issues I would venture to guess that you are suffering vapor lock. That is where the gas evaporates (changes from a liquid to a vapor in side the fuel line) due to ambient heat combined with heat off of your exhaust. This is aggravated by the vapor pressure dropping under the suction of a mechanical fuel pump that also adds into the gas the heat of the engine's oil as it reaches the carburetor. The boiling point of gas is just 112 degrees, so it doesn't take a lot of radiant heat off your exhaust pipes to boil it, or from heat off of your intake manifold under the fuel bowls.

I recommend an electric fuel pump back by the tank if you don't have one already. This pressurizes the gas to help keep it from boiling (how a pressure cooker works), and it doesn't add any heat if it is a by pass style pump (requires a return line). If however it is a dead head pump such as the old Red, Blue or Black Holley fuel pumps then it will really heat the gas up at idle and low demand situations.

I used to have a gas pressure gauge inside the car just so I could keep tabs on gas supply and I could watch the pressure drop at idle at a railroad crossing or in thick traffic as the engine temp increased at idle. Finally got rid of my twin Holley pumps and installed one Aeromotive A1000 pump with a return line and the problem went away.

Big Dave
 

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I had the exact same symptoms on my 65, and after changing fuel pump, plugs, wires, and converting to electronic ignition... I found the actual problem: a vacuum leak. There was a bit of rust on the front of the intake at the carb base, and that would allow unmetered air in only when hot. I found it when I pulled the carb to rebuild it as the next step.
 

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I have read a few places this was due to high temps in summer ( usually around 90 to 95 degrees here). Any thoughts? Ideas on what this may be?
where exactly is 'here'? and have you done any fuel system upgrades/replacements/rebuilds to your Impala, such as carb, intake, fuel line, pump, etc? do you know what thermostat you have installed (180,190?)? if it only happens in warm weather then I'm inclined to think vaporlock too, like Big Dave. how long have you been driving it and has it ever done it before (like last summer)?
 

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My 1966 Impala 396 will sputter and sometimes cut off after cruising around 55mph for a while during the hot weather. It happens when I exit the highway, depress the clutch and go to neutral, and come to a stop at the stop sign. For now, I can usually get it to start back up again but feel like eventually I may get stranded. I adjusted the carb to idle a bit higher but this does not seem to help. I have read a few places this was due to high temps in summer ( usually around 90 to 95 degrees here). Any thoughts? Ideas on what this may be?

Is vacuum advance connected to full manifold vacuum (it should be)? Not enough initial timing advance? Both?

Recommended reading:

https://www.chevelles.com/forums/374-ignition/189195-ignition-101-a.html

Simply possibilities. Free to check.

Pete
 
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