My Holley 4150 came new with #64 primaries in it and I have always thought it was loading up sitting in traffic. I have an older Holley 4150 that has #612 jet size in it . Is this a #61 or a #62? It Idles great with these in it in place of the #64s. My truck was running good I was just trying to get my mileage back.
Idle is not effected by your main jets at all, unless they are completely plugged that is. Main jets control fuel flow at or near Wide Open Throttle (WOT) only, plus the power valve. Mid-range is controlled by the air bleeds built into the carb (nonadjustable) and float level (adjustable). #612 (I doubt this number, probably 615 for 61 and 1/2) would be a 61 with a 2/10 additional or .612 opening.
Yea I'm actually looking at a 612 and a 65 jet right next to each other... Pretty sure it's not a 5 and the 65 doesn't look close to it the 612 is smaller in diameter than the 64 and 65 (photo to come)