(quoted from post at 08:05:25 06/03/19)
The following paragraph is a quote from Wikipedia, Vapor Lock:
"Gravity feed fuel systems are not immune to vapor lock. Much of the foregoing applies equally to a gravity feed system; if vapor forms in the fuel line, its lower density reduces the pressure developed by the weight of the fuel. This pressure is what normally moves fuel from the tank to the carburetor, so fuel supply will be disrupted until the vapor is removed, either by the remaining fuel pressure forcing it into the float bowl and out the vent or by allowing the vapor to cool and re-condense".
Over all, I agree with the article however, I do believe it could use a review and clean up. For example, I would revise the above paragraph to read as follows:
Gravity feed fuel systems are not immune to vapor forming the line. If vapor forms in a vertical section of the fuel line, its lower density acts to reduce the pressure developed by the weight of the fuel. This pressure is what normally moves fuel from the tank to the carburetor, so the fuel supply can be reduced or disrupted until the vapor is removed. The vapor can be removed, either by the remaining fuel pressure forcing it into the float bowl and out the vent or by cooling causing it to re-condense.
The carburetor inlet on my 8-N is 12" below the bottom of the fuel tank and the fuel tank is 8" deep, approximately. The sediment bowl inlet extents into the tank approximately 2" so we need to subtract that. Therefore, with the tank full we have a column of liquid gasoline; 12" + 8" - 2" = 18". For each foot of gasoline we have 0.312 psi, therefore an 8-N would have 0.312 x 1.5 = 0.468 psi of pressure at the inlet of the carburetor, with a full tank of fuel.
From the above it can be seen that should the fuel line contain vapor it would act to reduce the pressure available to push fuel into the carburetor bowl. Since the pressure is reduced the importance of clean screens, as others have posted, proper operation of the inlet needle (float) valve and venting of the carburetor is apparent. Running with a full tank of fuel and the sediment bowl valve opened to the reserve position would improve the pressure available. In addition to having clean screens I would suggest the screens should not be finer than that used when produced by Ford. In line filters should not be needed if the original screens are in place.