This reduced pressure also lowers the boiling point of any liquid and especially gasoline which means a higher percentage of fuel will vaporize. However, the fuel is still subject to lower atmospheric pressure. The pump creates low pressure on its inlet side and higher atmospheric pressure pushing down on the fuel in the tank (because the tank is vented), is what pushes the fuel toward the pump. Before we go any further, it’s important to note that pumps do not “suck” the fuel. Now let’s introduce a mechanical fuel pump into this application where the pump must “pull” fuel from the tank all the way to the engine where it can now push the fuel into the carburetor. At this temperature, roughly 20 percent of the fuel has already converted to a vapor as measured at normal atmospheric pressure. Now let’s imagine we have a very warm day of 90 degrees. So we’re dealing with a fuel that is designed to vaporize. This allows a cold engine to start quickly. Gasoline is a complex mix of light and heavy hydrocarbons that are designed so the light or volatile portion of the fuel vaporizes quickly, even at normal or low temperatures. It’s the fuel that has vaporized that supports combustion. But first, it’s important to mention that fuel is designed to vaporize. Most enthusiasts tend to focus on the pressure side of the fuel delivery system when the real problems are more likely located on the low pressure or inlet side of the pump. Jeff Smith: Vapor lock is a term used to describe when fuel changes from a liquid to a vapor before it arrives at the carburetor. This doesn’t happen all the time, but it seems like it happens more when I drive in traffic. If I let it cool down for a couple of hours, it starts hard but eventually runs okay. Last summer, my small block 350 Camaro had problems in the heat of late summer where it would start running poorly and eventually quit running and it has already happened again this year. What exactly is vapor lock? My friends say I have a problem with vapor lock.
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