[freebird69]

Originally Posted by JLMounce

Sure sounds like the fuel is boiling into vapor and the pump can no longer push pressure.

Modern e10 pump fuel begins to boil as low as around 110* at zero pressure and is mostly vapor by about 175* under zero pressure. Modern pump fuel as a result is designed to be delivered under pressure, typically 43-58psi where it's boiling point is much higher.

In a car like your 69 Firebird, with a factory style fuel system, you are sending fuel from the pump to the carb between 5-7psi, so you're not getting much of a pressure bump there. Add to that the fact that the fuel once bypassed from the pump to the carb sits there in the fuel line and in the carb bowls for a period of time before being consumed. This is why most vapor lock incidents tend to occur at low speed cruise, or while sitting at a light. This is where fuel consumption is the lowest and the fuel has more time to soak heat from the engine bay.

There's a couple things you can do.

1. Source ethanol free gasoline. Your car was made to function on pure gasoline. The problem here is that only a few stations here and there usually have pure gasoline and in many cases it's low octane designed for lawn equipment. If you need premium fuel, it's hard to find pure gas. This means you have to buy and pump your own and that leaves you without the ability to travel beyond a certain point.

2. Attempt to remediate the heat from your engine bay into the fuel system. Get a thicker heat shielding gasket for the fuel pump. Cover the fuel line in a heat shield material. Add a phenolic spacer under the carburetor. If the car has a vapor return line, make sure it's not clogged in any way and is returning vapor to the tank.

3. Fix the problem through modernization. A carb can still be used with modern fuels, it just needs a bit more help. An electric pump, inside the fuel tank with a bypass regulator close to the carburetor allows fuel to continuously circulate. This reduces the amount of time that fuel is sitting in the hot engine bay. Because the fuel is always there for you, it allows you to also run the floats a little lower, so you're not relying on a pool of fuel that is heat soaking in the carburetor.

4. EFI

If you're a stock appearing purist, you can build a system from option 3 that looks really close to what a factory setup would look like.

For what it's worth, I dealt with the exact same issue with my 1969 Firebird. The cramped engine bay doesn't breath well and during the summer I usually got 15-20 minutes of drive time before it started acting up at stop lights. I had every heat shield and phenolic barrier I could put between the engine bay and the fuel system. That bought time, it never cured the issue. I put a Throttle Body Injection system on the car with a proper fuel system and never dealt with it again, even in temperatures above 100 degrees and altitudes in excess of 8000 ft.

Jason, thanks for that detailed response. I think that I will start with the easy stuff first and work my way up... Fuel pump spacer and some well placed heat shielding plus a carb spacer.