Not being a carb. guy, I took my '74 F-100 with a recently installed 300cid down to the Carb. shop to have it tuned to perfection. After slapping down the bucks for a diagnostic check, they determined it was running a bit lean and adjusted it accordingly. They ended up adjusting it out as best they could and determined that what it really needed was a different power valve. They didn't have time to complete the repair today as they did not have the correct power valve for my application.

But it is just as well, because despite my limited knowledge of carburetors, I DO know that Ford specifically installed 2-stage power valves on the Pickup Trucks because of the differing levels of load occurring when a truck is full or towing and when they are empty. However, the mechanic said he was installing a SINGLE-stage power valve on my truck, saying that the 2-stage power valves are not needed on the smaller engines (like my 300).
He was going to put a single "8.5" power valve on my Holley 600 because my engine pulls 19.5kg of vacuum at the manifold.

QUESTION: Should I take the advice of a Pro Carburetor guy in regards to this single-stage power valve in a TRUCK application, or should I have someone else install a 2-stage power valve in the carb?


Fresh 300cid, Holley 600 with Adjust-a-jet, K&N filter, Offenhauser intake manifold, RV Comp-cam, Melling HD Oil pump, Mallory Unilite Distributor, Accell Super Coil & wires, Ceramic Coated EFI exhaust manifolds, Custom Y-pipe and 2.5" exhaust w/ Dynomax muff.

 

The two stage is more for emissions than anything. If you were to hook up a vacuum gauge and watch it while driving you would find that vacuum is under 10" anytime there is much load, and is zero lots of times other than WOT. I guess what I'm saying is that a two stage power valve will be open all the way most of the time anyway, so a single stage isn't a detriment. Changing the number of the PV isn't actually going to change your mixture anyway, it will just change the point at which power enrichment starts. With a 4 barrel carb you will also get some enrichment from running with the secondaries partially open, which they might be when pulling a trailer.

 

Power valves act as a switch controlled by manifold vacuum that turn on the flow of additional fuel into the main metering system. The point at which the power valve opens is determined by the spring tension built into the valve assembly and this value(in inches of mercury) is stamped on the valve assembly. A power valve that opens at 6.5-in (Hg) will have the number "65" stamped on the valve. Selecting the correct power vavle can only be accomplished with a quality, non-damped vacuum guage. Observe manifold vacuum readings during various STEADY cruising speeds. Choose a power valve that opens at about 2
in(Hg) below the lowest STEADY cruise reading. A number 65, 75, or 85 power valve often works well on stock or slightly modified engines. The higher-number power valves begin adding enrichment fuel sooner, often to the detriment of fuel economy. On the other hand, low-number power vavles may open too late, causing hesitation and lean surging at medium throttle. With two stage power valves, the first stage (partial enrichment) opens at relatively high vacuum levels (9- to 12.5-in[Hg]). This partial enrichment sometimes allows the use of smaller main jets that would otherwise be too lean for a single-stage vavle at very-light throttle. At lower vacuum levels (2.5 to 6.5-in[Hg]) the valve opens and provides full enrichmetn, exactly like a single-stage valve. Improvements with two-stage pwoer valves are most noticeable with heavy vehicles that have low power-to-weight ratios and or do a lot of towing. On these vehicles, such as trucks, heavy cars, RVs (especially when operated in stop-and-go traffic or on hilly terrain) the two stage power valve can improve throttle response and economy. A two-stage power valve may also DECREASE the fuel economy when used in light vehicles that have low power-to-weight ratios. In these situations, the partial enrichment at high vacuum just isnt' needed, since the engine is so lightly loaded. Most likely, you don't need a two-stage power valve unless you plan on doing a lot of towing or heavy hauling. They are easy to change, test them out and see what works the best.