It should be lean at part throttle. It should get leaner with more throttle. Somehow all the magazine experts somehow missed this and misled most of us for years.*
The power valve, or step up, is needed when the engine gets close to putting out maximum power for the rpm.
A somewhat practical method on a modified engine is to note the vacuum when increasing throttle doesn't bring a gain in acceleration. Then try a PV that opens at or before that vacuum.
Mike Urich, at the time a VP at Holley, discusses something like that method in
Holley Carburetors and Manifolds. Back then, it had to be done on a track with a stop watch while "crowding" a vacuum.
I've tried it using the vacuum gage and datalogger with acclerometers and its hard to do. A flat spot will show up though.
If you happened to be so lucky as to know a place with NOx gas measurement and a load dyno then it can be done by observing the combustion.
"
The purpose of the power valve selection is to enrichen the mixture prior to the combustion temperatures going too high. To set a powervalve correctly requires a NOx sensor. Different engine combinations need fuel enrichment at different power levels."
Bruce "Shrinker" Robertson.
post #4 "Another Power Valve Question" archived Innovate Motorsports forum
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In this engine, they found it needed relatively early enrichment. That would be something like a 10.5 Power valve on an engine that cruised at 17 or 18"Hg. An engine that didn't need to go rich until 80 % load might be more like a 8.5 PV, and so forth.
Wideband
