HARDCORE A/O

Here we go again. A power valve is simply a poppet-type valve that has a spring of a specific rate on one side and a diaphragm on the other end. The end of the valve with the spring protrudes through the metering block into the float bowl and is (if the float bowl maintains the right level of fuel) always submerged in fuel. The side of the valve with the diaphragm is connected, through an orifice in the bottom of the carb base-plate (or, on "boost referenced" carbs through an external hose which comes out the side of the base plate) to the manifold plenum, so that it can "read" manifold vacuum. When the vacuum which exists in the manifold plenum at idle (even in Roots and blow-through applications) drops below the rate of the spring, the spring pops the valve open and allows fuel to flow from the float bowl into the main wells in the metering block and thereby enrich the main circuits. (This is the reason why carbs that have a P/V on the primaries are jetted like 8 sizes smaller than the secondaries - the PV circuit simply augments the main jetting). This is done to allow a slightly leaner cruise A/F ratio (for enhanced fuel economy).
I have heard tuners theorize about the P/V seeing the difference between atmospheric pressure and vacuum at the P/V but the reality is, in my view, that the P/V only sees (or FEELS) the vacuum in the plenum. The vacuum reading you see on your gauge is the same thing the P/V sees.
In a blow-through setup, when the throttle is opened enough that the boost in the box (or bonnet) completely fills the manifold plenum, then the vacuum disappears and is replaced by a positive pressure in the the plenum. This usually will happen very quickly at even a slight throttle opening and therefore the power valve circuit is only beneficial if the boat can cruise at so slight a throttle opening that there is sufficient vacuum in the plenum to keep the P/V closed.
Therefore, the reason for using a low (numerically) rated P/V, like the 2.5s that Nickerson uses, is to DELAY the activation of the P/V circuit. The reality is that if you're into the throttle most of the time, it don't matter whether the P/V is a 10.5 or a 1.5 , It will be open anyways.
I use a 6.5 (in the primary M/B) on my M-3 blown 509 but I am considering plugging both P/Vs and jetting square to eliminate the possibility of uneven fuel distribution. (I like lots of throttle so it probably wouldn't make a difference anyways!)
In the final analysis, I would err on the side of a richer transition (earlier P/V activation/ higher P/V number) which would at worst produce a flat spot and very minor increase in fuel consumption). If the P/V comes in too late during throttle advance, you may experience stalling (like when you're trying to plane the boat with minimal throttle), which you might have to cover up by reworking the accelerator pump circuit (but that's a topic for another thread!). Probably the only solution is to buy a pair of 2.5s, 4.5s and a 6.5s and test them all on YOUR boat to see what works best for you. Before you even do that you should study your vacuum gauge during normal use and see what that tells you.
Somebody oughta write a book about these dang blow-through carb setups.

KNOT-RIGHT

This is Solid Advice!!!


Here is another thread regarding powervalves.
http://www.offshoreonly.com/forums/s...ght=powervalve

Let me know when your going out.. I will
try to be there.

Gerry

electrical1

thanks caveman this has been a great insight. look for me out there on the water and give me a heads up if you see me. I'm usually out on Saturdays. If it stops raining. can't miss me white 42' wellcraft with yellow stripe along top with yellow tribal design on sides.
Vinnie