I don't understand how the relationship between pressure and boil temperature is supposed to work.

 

The simple quick answer: Under the hood temperatures can get hot enough to cause fuel in a carburetor to boil, especially fuel blended with alcohol. In a carburetor, fuel is not under pressure. Highest under hood temps are when an engine is shut down, called heat soak. In a fuel injected system, all under the hood fuel is under enough pressure that fuel boil is not a problem.
Just like your cooling system with a pressure cap. Coolant can run much hotter without boiling.

 

Fortyniner,
I have known for a long time that as pressure increases so does boiling point. Water, for example, will increase its boiling point 3* for every pound of pressure. e.g. 10 pounds of pressure and water now boils at 242* instead of 212*. This applies to all liquids so if the fuel pressure on a carb is 6 lbs and on fuel injection is say 40 lbs, the fuel in the carb will boil (vaporize) long before the fuel injection system. But know that once fuel is past the needle and seat in a carb it is at atmospheric pressure and no longer the 6 lbs in this example. (Pressurized carbs are a different animal)

Here is a quote from a physics page on the web.
The boiling point of a liquid is lowered if the pressure of the surrounding gases is decreased. For example, water will boil at a lower temperature at the top of a mountain, where the atmospheric pressure on the water is less, than it will at sea level, where the pressure is greater. In the laboratory, liquids can be made to boil at temperatures far below their normal boiling points by heating them in vacuum flasks under greatly reduced pressure. On the other hand, if the pressure is increased, the boiling point is raised.

Read more: boiling point: Definition from Answers.com, boiling point: Definition from Answers.com

 

Thanks fellas - so the more closed loop nature of the injector systems changes the physics. OK, got it.

DW

 

I have Hagerty insurance and you scared me into asking them the question. Their response "We appreciate you contacting us regarding your concern for towing coverage. I spoke with our Hagerty Plus Roadside department in reference to your question. If the weight of the truck is in excess of 10,000 lbs, it may not be able to be accommodated on a flat bed tow, but we can arrange a tow using different equipment. I hope this information is helpful."

 

On the pressure thing I remember my high school physics teacher putting a beaker of tap water under a bell jar and sucking out the atmosphere with a vacuum pump at at a certain amount of vacuum, the tap water boiled at room temperature.

 

So, new battery installed, she fired right up (cold start). I took her to a car cruise in the evening - about a 20 minute or so drive. As she warmed up, it became appaant that she was rumbling - not quite backfiring. Choke adjustment did little to correct it. I get to the show, shut her off, and see if I can't start her up again warm. No dice. Of course, this brought "mechanics" of all sorts out. There were two older gentelman who seemed to be on top of things and after explaning that I think I have finally ruled spark out and the fuel pump was working, they suggested we check the carb float, and so we dug into it right then and there. It looked good. Well, scratching out heads, I let her cool down and enjoy some music and food. Time to leave, and she just didn't want to start. The block was still warm to the touch, but not hot. I cranked, waited, and cranked, and waited so many times slightly adjusting the choke. Finally she coughed to life. Again, riding home, she seemd just a tad rich. In my reading and research, this seems to be pointing to the power control valve.

So, what the heck is a power control valve? How does it work? How can I tell it's bad? Is this the correct replacement (NAPA AUTO PARTS)? How do I replace it?

 

Take the carb off the truck, and remove the upper bowl cover and the lower throttle section. Set the carb on the workbench and fill the bowl with gas. Let it sit and watch the level of gas (do this when it's as cool as possible so evaporation isn't at play) if the PV is leaking, the gas level will drop fairly quickly and be seen coming out the bottom of the carb.

 

OK, looks like Ford used to call this the economizer assembly, notice there are two different one's depending on altitude.



Here it is in the diagram

 

The power valve (PV) is a device that enriches the engine when vacuum drops, e.g. when you are getting on the engine, heavy load; it should be held closed at all time unless engine load cause a vacuum drop. Typically it is the equivalent of increasing the main jet by approximately 10 jet sizes to compensate for the increased air entering the engine. Holley makes several PV's with different vacuum ranges and I have found them to work in other Ford carbs in the past so you can always try one from them. They also make a PV plug (a good hardware store may provide a bolt that may work) that can be used to determine if you have a leaking PV. You can also test them with a Mite-Vac or similar tool. Vacuum applied to the non-thread side should hold, if it don't the diaphragm is shot.

 

What's the stock rating for below 5000 ft? 6.5? 7.5?

 

7.5 was stock, lots of lowland people run 6.5 for better mileage. Charlie Price's Vintage Speed | World's largest supplier of 50's – 60's Style Hot Rod fuel systems has all kinds of them.

 

Explain what you mean by adjusting the choke, please. Normally the choke is used to start a cold engine and should not be touched once the engine is warmed. This thing does have a manual choke, does it not?

 

What is the real difference between 6.5 and 7.5? Typical local altitudes are 500-900 ft.

 

That is the manifold vacuum at which the PV lets in additional fuel. Anything below 5000' you can use stock.