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Craig wites:
"The vapor lock occurs IN the mechanical fuel pump on the engine."

Yes, ok. And if the fuel pump on the engine gets hot enuf to vapour lock then the carb has become even hotter than the fuel pump. !!!

The carb is setting directly over exhaust heat passages. PErhaps a zero exhaust flow occurs but the HEAT still rises thru the cavity.

IF the pump gets hot enuf to vapor lock then the carb has gotten much hotter even sooner than the pump.

PackardV8 Quote:

Keith, I guess you are not familiar with a state phase diagram. It is the combination of temperature and PRESSURE that cause the fuel state to transition from liquid to vapor, not just heat alone. The carb is at atmospheric pressure, whereas the internal cavity (on the suction side) in a mechanical fuel pump is less than atmospheric. Therefore, the "vapor lock" will occur in the mechanical pump way before it could also occur in the carb. As soon as "vapor lock" occurs, NO fuel gets transferred to the carb via it's incoming line. So, it might look like the carb is in "vapor lock", but it is really the mechanical fuel pump.

Craig

From post #17 :
"In my engineering opinion, it is WRONG. The vapor lock occurs IN the mechanical fuel pump on the engine."

Ok. I will stand at least partially corrected. But note that u indicate "IN" the pump which i assume means INSIDE of the pump. Which does NOT mean ANYWHERE in the Fuel line before the pump.

Vapor lock occurs in the feed lines when the fuel (gasoline) turns from fluid to gas (vapor). A mechanical, diaphragm-type fuel pump works on because fluids are NOT compressible. However, since gases (vapors) ARE compressible, the diaphragm-type pump cannot move vaporized fuel.

A related issue, known as a fuel percolation, occurs in the carburetor itself where the fuel literally "boils" in the float bowl. The boiling fuel bubbles out the vents and runs down into the throat of the carb and intake. This "fuel foaming" results in a condition known as a "hot soak", which occurs after a hot shutdown, followed shortly by a restart attempt. The engine then behaves not like it is starved for fuel, but flooded.

I walked into a real can of worms with this when I joined Chrysler's Service & Parts Division back in April 1985 - spending the first six weeks "chained" to the customer relations desk. most of the complaints, then, were about vapor lock and fuel foaming. FWD cars were the worst, perhaps because of their small engine bays, but all of Chrysler's carbureted vehicles were affected to varying degrees. Yet, no amount of wooden clothespins helped. The solution (hung-up, waiting on EPA approval, then) would be to install an in-tank, electric fuel pump to raise fuel pressure just enough to prevent fuel form vaporizing in the lines (yet, nowhere near high enough for FI), but with a regulator to drop the pressure back down at the carb (to prevent pushing the float needle off its seat).

There are many shop manuals and service bulletins, from decades earlier, that show vapor lock and fuel percolation was nothing new to the industry. Changes in Reid-vapor pressure of fuels (to improve cold-weather starting) actually magnify these problems when seasons change from winter to summer, but complaints at the CR desk persisted well into summer - when winter grade fuels should have been exhausted from the distribution channels. Turns out, increasing use of alcohol and other supplements aggravated the problem, as well.

Though I haven't driven them much in recent years, I've never had such a problem with any of my V8 Packards - not even with the trip home with one from WV, when temps were well into the 90sa. Perhaps the fuel lines being routed on the outside of the frame rail, shielded from the exhaust system, is of benefit.

One must be carful not to confuse vapor lock with other problems, like a weak fuel pump diaphragm or leaks in the tank-to-pump lines, or a badly deteriorated interior of the fuel tank.

The ultimate solution to the vapour lock problem is to follow, i think it was Randy Berger, to install the exhaust crossover restrictors into the intake manifold.

The question at this point is what is the rite size of restrictor to use to strike a happy balance between cold weather and hot weather operation???

PackardV8 Quote:

OK, if you want to get more technical, the fuel in the line on the inlet (suction) side of the mechanical fuel pump will see a drop in pressure also, just not as much as INSIDE the pump in the inlet chamber. Depending on conditions, one could have a liquid-vapor phase transition in both the inlet line and mechanical pump at the same time. But due to pressure drop off upstream in the line (which would be very quick), vapor in the line right next to the pump would lag what is happening in the pump inlet chamber. However, the engine would stop for lack of (liquid) fuel primarily due to the pump, not the line.

It's a bad situation in any case. As I stated previously, an electric pusher pump will increase the pressure in the mechanical pump inlet chamber to prevent the liquid-vapor phase transition in most cases. Under severe conditions (my aforementioned drive up a long grade in 110F), even a moderate output/pressure electric pusher pump will not be adequate. In that case, one is way into the vapor part of the phase diagram and a little more pressure will not cause the transition back to liquid. In that case, one has to resort to the "Fred's Corona" solution and just wait until things cool down.

Craig

QUote:

"Depending on conditions, one could have a liquid-vapor phase transition in both the inlet line and mechanical pump at the same time. But due to pressure drop off upstream in the line (which would be very quick), vapor..."

Yes. Depending on conditions! If the fuel is so sensitive to temperature and presure DROP so as to vapourize on the suction side of the pump then very likely the fuel is also very sensitve to vaporization upon higher temperature and higher pressure on the pressure side of the pump.

Pressure, volumn and temperature. Is there something else that effects the change of state of matter???

Craigs experiment:
" Under severe conditions (my aforementioned drive up a long grade in 110F), even a moderate output/pressure electric pusher pump will not be adequate. In that case, one is way into the vapor part of the phase diagram and a little more pressure will not cause the transition back to liquid. "

THat in and of itself is a complete test to prove that hte problem is NOT on the suction side of the pump but is SOMEWHERE between the pump and carburetor (including the pump itself).

edit: Craig that's a great real world test. Thanks for sharing it.

Let me put it another way:
Someone correct me if i am wrong:

A fluid will boil even under pressure if ENUF heat is applied and the pressure does NOT increase sufficiently enuf to keep it from boiling.

No???

Therefore, vapor lock CAN and does occur on the pressure side of the pump up to and including the carb.

PackardV8 Quote:

Yes. And the converse is true that if NO additional heat is applied, but the pressure is reduced, then, if at the "right" point of the state phase diagram, the liquid will "boil", i.e., transition from liquid to vapor.

PackardV8 Quote:

The thing about the state phase diagram of gasoline and gasohol is that it take very little pressure change to effect the phase transition from liquid to vapor whereas it take a lot more temperature change to cause the same thing. In other words, the "slope" at the liquid-vapor transition line greatly favors pressure change. Also your statement above "very sensitive to ... higher pressure" is backwards. It's sensitive to lower local pressure. Higher pressure will put the fuel back into the liquid state.

Craig