steve s
Well-known member
Here are some photos that may clarify the vapor lock issue. These were taken while on my knees looking up into the top of my sediment bowl. As I watched, a vapor bubble would grow until it covered the entire screen surface, burp on out the fuel line, and then the process repeat over and over. The first series has the screen in place. The shiny vapor bubble can be seen to grow until it covers the entire underside of the screen surface. The growing bubble lolls off to one side because the bowl is not perfectly level. The fourth (and blurriest--sorry) photo shows the maximum bubble size. Shortly after it gets to this state, the whole bubble blurps out of the bowl outlet and the process repeats endlessly. Each cycle takes a minute or so. The second set of two are without the screen. The two photos without the screen show the inlet with no bubble and then with the nascent bubble just beginning to grow and dance in the hole. Eventually, this would grow and fill the top of the bowl, and blurp on through. The screen didn't seem to make any difference.
This was a fascinating and amazing phenomenon, but very easy to not notice--you can be standing right there watching your motor run and not see it until its pointed out. I observed it many times. The photos were taken on a warm but not real hot Michigan summer day with the motor warmed up and idling but not just driven. I apologize for marginal quality of some photos, but the glass reflections were a challenge. Click twice on the "thumbnail" photos to get a better view.)
The photos show beyond doubt that vaporization does occur. That bubble is vaporized gasoline; there's way too much of it to be air. In this situation, there was vapor, but no actual “lock”. Gasoline seemed to flow sufficiently around the growing bubble, and, amazingly, the idling motor barely hiccupped when the bubble passed. However, it is easy to imagine how under actual running conditions, e.g., after a hard pull, hood closed, and hot day, this sort of thing could occur up and down the fuel line and in the carb, eventually overwhelming the poor motor’s ability to cope.
The vaporization seen in the photos should not be a surprise. Gasoline is a mixture of chemicals that has an initial boiling point around 100 degrees F that gradually rises to around 400 F as the lighter fractions are boiled off. It doesn’t take a very warm day or a motor that is overheating to get the engine compartment up to 100 F. So, why doesn’t everyone have the problem? It is somewhat of a puzzle because there is so much variation, even among members of the same club. Here are some of my thoughts.
Bubble formation is a notoriously non-equilibrium process that can be very sensitive to the nature of the contacting surface and to flow turbulence to get it started, so some individual variation is expected. So-called superheating can result in the unpredictable, sudden emergence of a large bubble that could produce a blockage. Not to pick a fight or anything—but in my experience, fuel filters are particular “occasions of sin” in this regard, to borrow a phrase from the Church: They are not intrinsically evil but they do sort of invite trouble as sites to either initiate vaporization or trap a bolus of vapor. I am also including a picture of various fuel filters I have experimented with, all of which have caused some degree of trouble for me. The clear plastic, inline filter was the most interesting: I could watch it fill up with vapor, after which the engine would stall.
The exact composition and boiling point of gasoline vary by geographical region and season. This may be why vapor lock is not more common down south, where they know it's going to be hot and the gasoline has been formulated accordingly. Further, the big problem with incorporating ethanol into gasoline (beyond dissolving rubber parts) was that it forms a highly “non-ideal” solution with an anomalously high vapor pressure (Raoult’s Law does not apply). When the government mandated incorporation of ethanol, the petroleum companies were challenged to compensate for this lowering of the boiling point by changing other ingredients.
Many other possible vapor lock contributors have been implicated or sworn by or at: presence or absence of engine pans, painted vs. shiny carburetors, fuel filters, bent vs. straighter fuel lines, insulated fuel lines. Although each of these has its fierce partisans, as far as I know, none are definitive. Certainly, none of these are relevant to what is seen in my photos; this vaporization starts as soon as the fuel enters the engine compartment. Many people have advocated a variety of fuel additives; my only experience in that regard is that I saw exactly the same vaporization in the sediment bowl after mixing two quarts of diesel fuel in my tank. Many assume that it’s an engine overheating problem. Again, not in this case; my engine compartment did not feel particularly hot and the temperature gauge shows my motor at a steady 165 F (thermostatted).
Actually, the Model A vapor lock controversy sort of reminds me of the women’s PMS issue: women who don’t personally have the problem are often convinced that those who do are either faking it or have other major competency issues.
Steve Schullery
Kalamazoo
This was a fascinating and amazing phenomenon, but very easy to not notice--you can be standing right there watching your motor run and not see it until its pointed out. I observed it many times. The photos were taken on a warm but not real hot Michigan summer day with the motor warmed up and idling but not just driven. I apologize for marginal quality of some photos, but the glass reflections were a challenge. Click twice on the "thumbnail" photos to get a better view.)
The photos show beyond doubt that vaporization does occur. That bubble is vaporized gasoline; there's way too much of it to be air. In this situation, there was vapor, but no actual “lock”. Gasoline seemed to flow sufficiently around the growing bubble, and, amazingly, the idling motor barely hiccupped when the bubble passed. However, it is easy to imagine how under actual running conditions, e.g., after a hard pull, hood closed, and hot day, this sort of thing could occur up and down the fuel line and in the carb, eventually overwhelming the poor motor’s ability to cope.
The vaporization seen in the photos should not be a surprise. Gasoline is a mixture of chemicals that has an initial boiling point around 100 degrees F that gradually rises to around 400 F as the lighter fractions are boiled off. It doesn’t take a very warm day or a motor that is overheating to get the engine compartment up to 100 F. So, why doesn’t everyone have the problem? It is somewhat of a puzzle because there is so much variation, even among members of the same club. Here are some of my thoughts.
Bubble formation is a notoriously non-equilibrium process that can be very sensitive to the nature of the contacting surface and to flow turbulence to get it started, so some individual variation is expected. So-called superheating can result in the unpredictable, sudden emergence of a large bubble that could produce a blockage. Not to pick a fight or anything—but in my experience, fuel filters are particular “occasions of sin” in this regard, to borrow a phrase from the Church: They are not intrinsically evil but they do sort of invite trouble as sites to either initiate vaporization or trap a bolus of vapor. I am also including a picture of various fuel filters I have experimented with, all of which have caused some degree of trouble for me. The clear plastic, inline filter was the most interesting: I could watch it fill up with vapor, after which the engine would stall.
The exact composition and boiling point of gasoline vary by geographical region and season. This may be why vapor lock is not more common down south, where they know it's going to be hot and the gasoline has been formulated accordingly. Further, the big problem with incorporating ethanol into gasoline (beyond dissolving rubber parts) was that it forms a highly “non-ideal” solution with an anomalously high vapor pressure (Raoult’s Law does not apply). When the government mandated incorporation of ethanol, the petroleum companies were challenged to compensate for this lowering of the boiling point by changing other ingredients.
Many other possible vapor lock contributors have been implicated or sworn by or at: presence or absence of engine pans, painted vs. shiny carburetors, fuel filters, bent vs. straighter fuel lines, insulated fuel lines. Although each of these has its fierce partisans, as far as I know, none are definitive. Certainly, none of these are relevant to what is seen in my photos; this vaporization starts as soon as the fuel enters the engine compartment. Many people have advocated a variety of fuel additives; my only experience in that regard is that I saw exactly the same vaporization in the sediment bowl after mixing two quarts of diesel fuel in my tank. Many assume that it’s an engine overheating problem. Again, not in this case; my engine compartment did not feel particularly hot and the temperature gauge shows my motor at a steady 165 F (thermostatted).
Actually, the Model A vapor lock controversy sort of reminds me of the women’s PMS issue: women who don’t personally have the problem are often convinced that those who do are either faking it or have other major competency issues.
Steve Schullery
Kalamazoo
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) the can behaves in the hot garage. No science, just observation of unknown relevance. 
