Hello Richard,
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I would like to add some things regarding the power valve. Even after you rebuild your carb and are certain your plugs and ignition are up to snuff you still need to determine the proper power valve requirement. The power valve is the circuit that kicks in during or at full load. You mentioned your power valve is a 8.5 or 9. That <o></o>
valve may or may not be the one you need. The only way to know for sure is by going through the following steps mentioned in this article I copied from another site.<o></o>
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There is a lot of mystery surrounding power valve selection for Holley carburetors. Once you have an understanding of how power valves work and a basic vacuum guage, it is very simple to select the correct power valve for your application.<o
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Most four barrel carbs come with a power valve based on list number and application. These are select to work on the majority of vehicles that the carb will be used on, and will get you in the ball park, but may not be ideal for your particular setup.<o
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Many people believe that power valves cannot be trusted to work because if the engine backfires through the carb, it will blow out the power valve. Most Holley carbs now come equipped from the factory with built-in power valve blowout protection. If your carburetor does not have this feature, you can purchase an easy to install kit from Holley (item # 125-500) to prevent this problem.<o
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Some carb tuners will remove the power valve and replace it with a plug (Holley item # 26-36). This should be done with extreme caution, and only on vehicles that spend the majority of their time on the race track. When the power valve is plugged, the main jets must be increased from 6 to 10 sizes to correct for the amount of fuel lost by removal of the power valve. In addition a low rpm rich condition will be created that can lead to plug fouling and decreased part throttle fuel economy.<o
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To determine the proper power valve for your setup, you will need an inexpensive handheld vacuum guage.
*SAFETY NOTICE* When checking the vacuum on your vehicle, please make sure the wheels are chocked and the emergency brake is activated and properly working. Allow the engine to come up to normal operating temperature before taking the reading. On vehicles with automatic transmissions take the reading at idle speed with the engine in drive. On manual trans equipped vehicles the reading should be taken at idle while the transmission is in neutral. Use a power valve that is rated at 1 to 2 inches below the reading on your vacuum guage. For example, if your guage shows 8 inches of vacuum at idle you should select a power valve rated at 6.5 or 7.5.<o
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Holley standard flow power valves are available in ratings from 2.5 to 10.5 inches (in 1 inch increments) and also in 1.0 and 5.0 inch ratings. High flow power valves are available in ratings from 2.5 to 10.5 inches (in 1 inch increments) with the exception of those rated at 7.5 and 9.5 inches which are not currently available. <o
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Very often symptoms you describe are mistaken for other things when all along it was the power valve not opening when needed causing a lean misfire or opening way too soon causing an overly rich condition. Hope this helps.
pete
