I've got quite a bit of experience with the Hoilley list #7448 2 barrel carb that's rated at 350 CFM. It was the designated carburetor for the circle track class we ran at Nashville Fairgrounds speedway for several years. I'm not sure if the same methods are used to rate the 4 barrel 390, but the numbers will be fairly similar, IMHO. A 300 cubic inch engine at 4000 RPM will have a theoretical CFM of 347. That's virtually impossible, since no street engine approaches a volumetric efficiency of 100%. 90% is much more in the ballpark unless some serious cam, valve, port and exhaust upgrades have been made. That brings the actual CFM at 4K down to around 312. The 7448 carb will let a 350 Chevy go to approximately 6100 RPM, which is WAY above its rating, but the manifold vacuum starts coming up as it exceeds its rated CFM. It eventually acts like a restrictor plate to limit top RPM. Can you give us an idea at what RPM you're experiencing the bog? I'm going to guess that it's probably in the neighborhood of 2K-2500, right? It appears to me that the midrange mixture is going lean- - - -too much air, not enough fuel, possibly because the main jets aren't flowing enough gas before the power valve kicks in at low manifold vacuum and allows the main jets to be bypassed during acceleration. A low float level in the primary venturis' float chamber can also produce a bog, since the venturi vacuum needed to draw fuel into the airstream is directly affected by float level- - - -the lower the float level, the more likely there will be a lean bog on acceleration.
Start your tuning by making SURE the floats are set right. A float level less than 1/8" below normal can cause major problems. Set it too high, and it causes flooding at idle. Once that's right, you might want to install a power valve with a stiffer spring that allows it to open at higher manifold vacuum. The valve is held closed by vacuum, and it opens to bypass the main jets and provide extra gas as the vacuum drops when the throttle is opened. Ford calls the same part an "economizer"- - - -same part, different terminology. It's there to give a mixture boost on acceleration, then it closes at high vacuum cruise to improve fuel economy. Have fun!
Jerry
