let me see if i can explain it a little clearer for you guys. if you take the jets out of your primary block you will see a well behind them. that well is where the fuel that feeds thru the boosters comes from .the power valve also feeds that well when open. if you pull the power valve out you will see two small holes .050 -.076 usually. that is where the fuel goes into that well when the power valve is open. they are the power valve resrictions. the size of those holes determines how much fuel is added when the pv opens. the only time that fuel is added is when the boosters are flowing. the way to tune the part throttle circuits of a carb (above say 2000 when its running on the jets) is to keep lowering your primary jet size till you get the part throttle afr you want. then you must drill those power valve restrictions larger to get the fuel back under wot. i hope this makes sense to you guys im just tryin to help. took me years to fully understand the tuning capabilities of a holley carb. theres a lot more than just jets. my metering blocks are all drilled and tapped and i installed my own brass set screws that i drilled to tailor my fuel curve to exactly how i want it. my car with a 248/254@ .050 cam 4500 stall and 4.10's cruises at a 13.5-8:1 afr and gets around 14-15 mpg. its no econo car but runs pretty clean compared to most.
