My new to me '77 K20 just isn't tuned right. Truck was built by somebody else, but it has a pretty fresh looking 350, with headers, edelbrock performer eps manifold, msd hei distributor, holley 670 truck avenger carb, and '70s vintage iron heads. I don't know about cam or compression. Throttle response just is not there and it smells of raw fuel....not much black smoke though. I know the truck won't be any fuel miser, but think I got like 7 mpgs on my first tank with it (of course it was running on choke most of the time!) and I wasn't pushing it real hard. So far I've dropped the timing back from the 14 degrees original owner had it at to 7 or 8 degrees. He had choke at 2nd from richest setting, dropped back to one less lean then halfway (still pretty cold in the morning here). I messed with the air screws yesterday, seemed to be best (but still pretty crappy) at 3/8 of a turn out. I found out that it has silver secondary spring, will change to heavier black spring to see if it cures any of the problem. Have not checked floats as the sight plugs have already been swapped out. Maybe he doesn't have full 12 volts to electric choke and its not working right.....will check this too. I have a edelbrock #1406 600 cfm w/electric choke on my '71 Nova with a 327 and I had same 1406 carb on my old '79 K10. I found this carb to be very easy to tune and very responsive yet decent on fuel at cruise. What are you guys all running for carbs? This is my first holley and I am less then impressed. Is it just me? Keep screwing with it and it'll work good or what? I'm just looking for a decent running honest 300 horse motor that may pull down 10 or 11 mpg.