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This and numerous other alleged vapor lock cures have been discussed here many times. I can tell you that from a scientific viewpoint, the wooden clothes pins make no sense. Wood is an insulator, so it's hard to imagine it would help radiate heat away from the fuel line. Even if it did, it would be working in the wrong direction; the problem is due to heat from the engine compartment getting INTO the fuel line, not fuel that was heated elsewhere needing to cool off as it traverses the engine compartment. While wetting the clothes pins down would provide some very short term evaporative cooling, wood absorbs so little water that that rationale just seems silly to me; wet rags are a proven short term fix, but I don't believe anyone advocates driving around with rags on their fuel line.
The only basis for the clothes pin trick working that I can conceive of is possibly as a vibration dampener on the fuel line: vibrations will encourage bubble formation in an otherwise metastable slightly superheated liquid. But, that too seems pretty farfetched to me.
The whole business is very controversial. Some folks even claim that it's impossible for a gravity fed-no fuel pump system to have vapor lock (by their definition). Search under "vapor lock" and you'll hear it all. However, there is no doubt that fuel vaporization can occur in the fuel line, whether or not it causes a problem is another question. Here's a picture of a vapor bubble I could watch build up in the top of my sediment bowl, and then burp on down the line, and repeat endlessly with motor idling smoothly on a hot day.
The problem is undoubtedly aggravated by addition of ethanol to gasoline, which raises the fuel's vapor pressure. Gasoline starts to boil around 100 degrees F, then boiling point goes up as the lighter fractions leave.
There's always a rash of vapor lock threads on the forum about this time of the year, as folks burn off their winter blend gasoline and the gas stations transition to the higher-boiling summer blend. Paradoxically, folks who live down south often report never having vapor lock issues even though it's much hotter down there. I believe that's because they only ever are sold a higher-boiling blend.
Steve