 Type IV 2.4 L carb jetting. |
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| Keith914 |
 Dec 17 2016, 04:38 PM
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Member  Group: Members Posts: 248 Joined: 29-April 16 From: Laguna Beach, California 92651 Member No.: 19,948 Region Association: Southern California  |
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.
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| jmill |
Dec 17 2016, 05:53 PM
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#2
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Green Hornet Group: Members Posts: 2,449 Joined: 9-May 08 From: Racine, Wisconsin Member No.: 9,038 Region Association: Upper MidWest |
Glad you got it running well! (IMG:style_emoticons/default/piratenanner.gif)
Strange that your AFR gauge is acting up. Did you place the sensor at the collector? |
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| Keith914 |
Dec 17 2016, 06:28 PM
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#3
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Member Group: Members Posts: 248 Joined: 29-April 16 From: Laguna Beach, California 92651 Member No.: 19,948 Region Association: Southern California |
QUOTE(jmill @ Dec 17 2016, 03:53 PM)  Glad you got it running well! (IMG:style_emoticons/default/piratenanner.gif) Strange that your AFR gauge is acting up. Did you place the sensor at the collector? I have headers that collect together and the sensor is about 12 inches down stream before the muffler |
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| jmill |
Dec 17 2016, 08:01 PM
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#4
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Green Hornet Group: Members Posts: 2,449 Joined: 9-May 08 From: Racine, Wisconsin Member No.: 9,038 Region Association: Upper MidWest |
Most place the sensor at the collector. It has the least amount of back pressure and the most turbulence. Your readings will be artificially high/low if the sensor is placed in an area of high back pressure. I'm no scientist but more pressure equals more atoms per unit volume which equates to screwy readings.
If you have a high overlap cam you may also see lean idle readings when you're actually good. AFR gauges are sweet but reading plugs may be more accurate if the sensor isn't placed just right. I think that's why a lot of folks run with EGT. |
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