Originally Posted by 406MOUSE
what power valve should I be using I have a 6.5 in a Holley 750 vacuum secondaries. i am concerned that it is opening too soon but I am not sure. How should I go about choosing what size. I know that if it is too small it will not let fuel into the circuit but I am wondering if mine is staying open because the car smells really rich and I have a radical camshaft and I think it is making about 10 inches of vacuum at idle. would i divide that in half and use a 4.5 power valve?
How is the idle air being accommodated, radical cams require a lot of throttle opening. If you get the blades too high they expose the transfer slots, then both the idle and transfer circuits start feeding fuel which will make a rich idle and a power dip just off idle. If you meed more idle air the secondaries can be cracked open if this isn't a four corner idle circuit. There is an adjustable screw stop on the secondaries accessible from under the carb. This can be screwed in toe open the blades a little, but the same rule applies here as on the primaries which is the top of the blade must be below the bottom of the transfer slot to keep that system from suppling fuel in addition to the idle circuit. The process of last resort for more air is to drill a small hole in the throttle blades for additional air. If you must start small like 1/32 and work up. Usually a 1/16 hole in each primary blade is sufficient. Another trick is to get an aquarium air supply valve from the pet store and hook it into the not timed vacuum source nipple of the carb. Then use the needle valve of the aquarium air adjuster to dial in enough air leak to satisfy the idle requirement. Keep in mind this leak will always be there which might push you to a jet size bigger on main metering.
The power circuit has two elements the vacuum at which the enrichment valve opens, that's the number you see stamped on the valve. There are also jets that control the amount of fuel from this circuit in the metering plate on the back side of the valve cavity. But I certainly wouldn't start by messing with them.
If cruise comes up too rich you can do as suggested and go to a valve that opens at a lower vacuum (more throttle opening). You can also swap back and forth with main metering jet sizes. That's to say if the power valve comes in early, you can dial the main meter back a bit to lean it out. But the purpose of the power valve is to allow a leanish cruise and richen the top end, which is why its also called the economizer valve.
Radical cams and rich jetting can also upset the air bleeds, typically a larger size is needed. If the bleeds are too small the mixture goes rich and if the mixture is lean the bleeds can be too big. But exhaust the fuel jet adjustments and power valve cut and metering before messing with these. An error here is expensive to fix unless your carb has replaceable bleeds.