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-   -   AFR and Jet size (http://www.clubcobra.com/forums/holley-tuning/134493-afr-jet-size.html)

genolan 08-28-2015 06:11 AM
AFR and Jet size
 
I had a friend hook his AFR gauge up to my 408CU stroker mild CAM with a 770 street avenger (Vacuum secondary) Primary jets 77 Secondary Jets 83 and got the following


AFR
Idle 14.0
WOT 12.5
Cruise 3k load 10.9

I assume I need to down size my primary jets but by how many Jet Sizes. How many Jet sizes will raise AFR by 1 full point. Will that affect my WOT by a percentage and if so approx how much, I will be able to use the gauge again but wanted to use some logic in jet selection.

MOTORHEAD 08-28-2015 07:01 AM
General opinion is 2 sizes at a time. I would start at 72 based on your eng specs.

zrayr 08-28-2015 10:53 AM
A carb with replaceable air bleeds does wonders for improving cruising A/F numbers.

That's the only area I'd want to change in your situation.

Z

genolan 08-28-2015 01:13 PM
Quote:
Originally Posted by zrayr (Post 1360792)
A carb with replaceable air bleeds does wonders for improving cruising A/F numbers.

That's the only area I'd want to change in your situation.

Z
The Holly doesn't have replaceable air bleeds, The car runs great with no bog or hesitation I just wanted to fine tune it in and thought 10.8 AFR was high and may cause some cylinder wash out.

zrayr 08-28-2015 01:23 PM
Quote:
Originally Posted by genolan (Post 1360808)
The Holly doesn't have replaceable air bleeds, The car runs great with no bog or hesitation I just wanted to fine tune it in and thought 10.8 AFR was high and may cause some cylinder wash out.
Yes, I know that one doesn't. I just wanted to advise as an "FYI" that carbs that do have replaceable air bleeds, such as the Holley Street HP series (great carbs), are very responsive to changes in the air bleeds which yield the changes the OP is seeking; leaner A/F ratio in cruise circuit without changing the rest of the a/f curve.

PS. I don't think you have anything to really worry about with a 10.8/1 a/f at cruise, re cylinder washing out, but it's not optimal of course. I ran a set Weber's that had a rich cruise a/f ratio, but after thousands of miles there was no change in cylinder wall wear.



Z.

Gaz64 09-03-2015 04:56 AM
Can you hook up a vacuum gauge to monitor inlet manifold vacuum, while monitoring A/F ratio?

Then you need a really long slight upgrade where you can maintain a slowly decreasing vacuum level, increasing load.

If the vacuum IS above your power valve opening point when it goes rich, say 10 inches, then decrease your main jet to say 72, and retest.

If it has an A/F of 10.9 at say 14 inches, increase idle air bleed size by .002, and retest.

fastd 09-03-2015 05:12 AM
Never a simple answer but the way you set up the question can provide a simple answer:

You want to increase more than 1 point to more like 12.5-13.5. I'd try decreasing main jet to 72 from 77 as previously suggested. This change should effect your other 2 reading at idle and WOT.


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