Unfortunately, the Holley 390 didn't pan out. I've never figured out why, but it dropped my gas mileage from around 20 on the interstate down to around 11 - 12. I fiddled and fought with it for about 2 years before finally giving up.
My next carburetor to try was a Holley 600cfm. (2011)
A little big for the engine, but my gas mileage jumped up to 19! Talk about a confidence booster that a better carburetor could be found.
However, due do its size, my throttle suffered and I had lost a lot of grunt. Great for cruising. Not so much for daily driving (although my mileage around town was 15).
So, (2013) next I decided to try something unique. I wanted to give a Quadrajet a try, due to their tiny fuel sipping primaries, and toilet flushing secondaries. Power when you need it, but not when you don't. It sounded like a great combination. However, they're all used, old, and have their known problems. Plenty of people know their way around them and how to overcome their issues, but I found out that Holley makes a Quadrajet styled replacement, so I decided to give it a try instead. I figured that even though it was 650cfm, most of that cfm would be on the secondaries, and if I didn't use it, it wouldn't matter.
The carburetor drove great! My throttle response was back, and it had all the power I could ever need.
However, after going through two of them (warrantying out the first one), nothing I did would prevent it from dumping gas through the engine. I was averaging 9 - 10 around town and 11 - 13 on the interstate. I tried leaning it out, but nothing ever made any difference, so I finally gave up on it.
My next choice was a Holley 465cfm. (2015) It seemed like a good, middle ground between the 390cfm and the 600cfm.
After all my previous issues, I wanted more adjustability, so I added a secondary metering block
It's an old design (came out in the late 50s), but it drove like a dream, and my mileage went way back up. If it wasn't so old, it would pretty much be the perfect carburetor.
I love extreme adjustability in my carburetors and had always had my eye on QuickFuel's Holley style carb. There is simply nothing that you can't adjust on them. However, the smallest they made was a 600cfm and I didn't feel like dealing with that large of a carb again, which is why I found the Holley 465cfm. As luck would have it, QuickFuel came out with and released a 450cfm version of their carburetor only months after my purchase of the 465. However, as time went on, I began to tire of little issues on the 465 due to its age and decided to make one final purchase and got myself the QuickFuel 450cfm. (2016)
The thing drives like fuel injection. The throttle response is insane. The sound of the engine is rich, deep, and beautiful. It's smooth as glass on the interstate. I will never own another carburetor again. This thing is AMAZING and it puts a grin on my face very day I go out an drive. I've been looking for reasons just to go somewhere so I can feel that amazing pedal.
It's amazing getting out on the interstate and flying by people up to 80mph and having them wonder how the heck a 35 year old truck flies like it does. Little do they know there's a six cylinder in there.