[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?

[ericnova72]

That is one accepted way to find a size. Another is to have the PV opening point 1.5" below the vacuum reading at cruise, this will give a little sooner enrichment. Choice is yours and what your engine responds to the best.

Rich at idle has nothing to do with the Power Valve used, the Power Valve channels feed the main well in the metering block up high at the booster, fuel flow is not pulled that high in the block at idle because of the idle syphon break(idle air bleed) kills the signal up high in the block preventing the fuel from the PV being pulled over the top of the main well into the idle well. The only fuel being fed to the idle screws is coming from just the idle well and the power valve doesn't feed this.

You likely have too large an idle fuel feed restriction in the metering block for your combo, too small an idle air bleed, or have the initial timing set to low to produce a good burn. Or some combination of these and possibly a couple other issues.

Detail the cam, compression, intake manifold, exhaust, initial and total mechanical advance timing, and whether you are using vacuum advance to get more info on fixing the problem.

[oldbogie]

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.

Bogie

[302 Z28]

I try at all costs to avoid drilling holes in the throttle blades, but they can always be soldered closed if too big initially.

Vince