Type IV 2.4 L carb jetting. Options

[Keith914]

Here is my brief experience tuning my recently built 2.4 (104mm bore, 71mm stroke =2.365 L) with twin Weber carbs, as I attempted to set the engine to + 13.5 A/F. Carbs are 44's with 36 venture's. Based on various references and test running the car, finally achieved smooth running through the full rpm range plus "good" color on the plugs with: 60 idle jets, 160 main jets, and 220 air jets at sea level. Have a new Innovative A/F gauge which I relied on through early trials, but found it to be non sensitive to main jet and air jet changes when running above 3000 rpm, -- showing 12 to 12.5 A/F's with main jets from 135 up to 160 and air jets from 215 to 220(?). It did show higher A/F's at idle with larger main jets, and now shows idle A/F's ranging from 12.5 to 13.5 with the above adopted jet selection. So I reverted to old school spark plug color inspection method. I found a stretch of road that allowed a good 10 minute run in 5th gear at 3000 rpm, then switched the engine off coasting to a stop , then pulled a spark plug to inspect. The attached pics (poor focus!) show the spark plug colors on the last two tests, one with 140 main jets (lighter color), and one with the 160 main jets. I have thus settled on the 160 main jets. When cold the engine starts instantly, idles slowly (about 500 rpm) then slowly increases to 1100 rpm after about 2 minutes. While the engine ran well with the 140's there was occasionally a flat spot around 2000 rpm.