To answer question #1: Yes. Gasoline, like any other liquid, has a "vapor point" where it turns from liquid to gas via evaporation. Obviously, as the temperature of the fuel rises, it turns into a gas more readily; this is exacerbated by high under-hood temperatures. Once the fuel reaches its vapor point, it will turn to gas and interrupt the proper delivery of fuel to the metering mechanisms in the carb. Keep in mind that the temperatures inside the combustion chamber may exceed 1800 deg F; exhaust gas temperatures reach 1200 deg F. This heat is transfered to your exhaust headers and subsequently to the air inside the engine compartment, heating up everything under the hood, including your fuel lines and carbs. This is why proper ventilation of the engine compartment is important. Fuel and air flow through the carb help reduce and stabilize the temperature of the fuel in the float bowls, but if the air temp is too hot, it will result in heating of the carb structure and subsequently the fuel in the float bowls. Also, if the fuel lines are routed too close to heat-producing parts, line exhaust headers, the fuel can vaporize in the line, interrupting the delivery of fuel to the float bowls.
BTW, modern gasoline is specifically formulated to raise the vapor point with the express purpose of reducing evaporative emissions from the fuel tank. This isn't a big problem with fuel injected cars, since the injectors are more efficient at atomizing the fuel (through the spraying action) than carburetors, which depend on venturis and pressure differential to atomize the fuel. Consequently, carb'ed engines tend to run less efficiently on the "new" gasolines than they did on the old blends, even after accounting for the reduction in octane rating.
As to questin #2, the purpose of the power valve, as discussed in other forums, is to provide a richer fuel/air mixture under conditions of increased power demand, such as when climbing a hill or pulling a trailer. As the throttle is depressed to maintain speed (read: engine rpm) under increased load, the throttle plates open up, reducing the manifold vacuum inside the intake manifold. The power valve senses this and opens up to admit more fuel to the metering circuit, resulting in a slightly higher fuel/air ratio. A higher fuel/air ratio will result in slightly lower combustion temps due to incomplete combustion of the mixture. (All old VW people know that to reduce combustion temps, which was vital to engine life in an air-cooled engine, you jetted the carbs richer.) To my knowledge, only the Holley and Holley-based carbs use power valves. GM Quadrajets, for example, accomplish the same function by using tapered metering rods which are pulled up through the main jets as the throttle linkage is moved, effectively varying the main jet size as a function of throttle position.
For more info on the Holley power valves, see:
http://www.holley.com/HiOctn/TechSer...o/TI-222A.html
Early Holleys had no protection in the air passage between the manifold and the valve. The valve itself has a rubber diaphragm against which the manifold vacuum acts. The valve is normally held closed by the manifold vacuum. As the manifold vacuum decreases the spring overcomes the diaphragm vacuum and causes the valve to open, admitting more fuel to the metering circuit. Since the passage to the back side of the diaphragm was open, a backfire through the intake manifold would cause the diaphragm to rupture, resulting in the valve being constantly open and an excessively rich fuel mixture throughout the normal operating range. This is what's referred to as "blowing the power valve." The only fix is to replace the valve, which requires a partial tear-down of the carb. The more recent Holleys have small ball check valves installed in the passage to prevent this. Holley also sells a kit to retrofit older carbs.
Ow, Ow, somebody's slapping me again.....
