12/2/2023 0 Comments Carburetor vapor lock symptoms![]() ![]() What really happens is that enough fuel turns to vapor that a pump designed to push liquid is now trying to pump vapor. This creates a perfect scenario for vapor lock. Any combination of these situations will force the pump to work harder and create lower pressure on the inlet side in an effort to continue to pump the fuel. Or perhaps the fuel line has been kinked or bent or the existing tank vent has become plugged or restricted. For example, an older car with an original and a fossilized sock stuck on the fuel inlet in the tank could cause such a restriction. Then, if there is any kind of restriction in the fuel line between the fuel in the tank and the pump at the front of the vehicle, this restriction will create even lower pressure in the line heading to the pump inlet. So we have hot fuel (that has been percolating in the fuel tankexposed to heat coming off the pavement that might be 120 degrees F, and now we subject the fuel to lower pressure in the fuel line. ![]() 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. Don't let gas hit hot engine parts, though, as it is a fire hazard.What exactly is vapor lock? My friends say I have a problem with vapor lock. You can remove the gas line from the carb when having the problem, and see if gas flows. If the bike restarts then, it may be a venting problem. When the bike won't start hot, open the gas cap. Many people call these type of problems vapor lock incorrectly, but they do have similar symptoms. However, a bad vent on the fuel tank, an internally collapsed line, a stuck float valve, clogged screen, a bad petcock (A vacuum actuated petcock with a bad diaphragm too) can all stop the flow of gasoline to your carburetor. Newer vehicles, with the fuel pumps in the tank, cannot vapor lock either. Since your motorcycle is gravity fed, it cannot have vapor lock, at least in the strict definition. ![]() This prevents any liquid fuel from being pumped into the carburetor. The fuel pump, instead of receiving liquid fuel, receives only the vapor coming from the boiling fuel. The fuel, at the lower pressure than atmosphere, boils in the fuel line. TEchnically, vapor lock is when, during high temperature conditions (usually under the hood of a car) combined with the suction of the fuel pump, create a vacuum in the fuel line. ![]()
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