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Thread: Vacuum Leaks

  1. #11

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    It becomes tricky because circuits overlap one another. Changing the main jets without touching anything else, will change everything all together as well. I went as far as drilling out my idle feed restrictors, tapping, and drilling out blanks in .001" increments chasing the best fuel curve.

    I would say 65-70% of street driving is done on the front two barrels without every opening the secondaries. Because you have four corner idle, your rear two idle adjustment screws will affect your idle & cruise AFR. When you tune with an AFR gauge, don't get so focused on getting an EXACT AFR. Your AFR can change based on AIR TEMP, FUEL PRESSURE, etc., etc. Instead watch AFR as more of a fuel curve. Also, always make sure the car is up temp before you make changes.

    It will take some time & getting used to. Make one change at time. Making multiple changes will do nothing but cause headaches. There are some real good articles that I think were title "HOW TO TUNE YOUR CARB WITH EFI RESULTS". Also look up "idle feed restrictor tuning", you'll find lots of good articles on how to fine-tune carbs. Sean

  2. #12

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    To be totally honest my real effort has been to just get the car into some kind of position that would approximate driveable. The off idle stumble is excruciating. Let me recap where I am. I adjusted the four corner idle screws to 15 one half turns from close. Throttle response improved incredibly. The car is almost normal. AFR at idle went up to the 10.9-11.4 range from 10.4. I say that with your advice in mind. It literally swings between those numbers. Same with any sort of partial throttle, it will lean out then curve downward. Tonight I worked on the accelerator pump. I moved back to the orange cam from the blue one. I'm at orange position 1. I still have the white secondary spring in it. Car responds well to WOT, but the transition is still a pain. I did take your advice, and looked up some of the articles on idle restrictor tuning. One thing I have not done is drill the 3/32 holes in the blades. I thought I would try that next. If I do so, I should be able to close down the idle transition slots shouldn't I? Since I have been through the cams and springs up & down, I'm thinking the blade holes might be where I want to go.
    Last edited by wrp; 02-07-2017 at 10:54 PM.

  3. #13
    Join Date
    Nov 2014
    Location
    Ont. Canada
    Posts
    222

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    Before you drill the plates, have you tried opening the rear plates, and closing the front plates?

  4. #14

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    Not in that sequence. I have tried opening & closing them both the same distance.

    Back on the Dyno today. Put down 275/325. That is about a 12-15 rear wheel horsepower & 30 rear wheel torque pickup. Got this going back to the orange cam, adjusting the accelerator pump, and working the four corner idle screws out to 15 one half turns. I'm now certain my problem is the idle system. It idles at an AFR of about 10. It stumbles transitioning, but once going it pulls hard. You can see the curve below the line. From about 4,000 RPM & up, it is in pretty good shape. So I went and tried to adjust the secondary butterflies with the adjustment screw. Opened them quite a bit in order to try to lean the secondary by exposing the idle transition slots. Picked up nearly a full point from 10-11, but I think that's all I can adjust it. Is it time to get out the drill?


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    Last edited by wrp; 02-09-2017 at 09:30 PM.

  5. #15

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    So the Holley Tech guys nearly fainted when I told them the four corner adjustment screws were 15 one half turns from seated. I moved them to 3/4 of a turn from seated. AFR went way up. Backed them out to about two turns from seat. Throttle hesitation from idle is gone. Made an amazing difference. So now I have a stumble at mid-high range. It now idles around 12 and has a WOT AFR in the 13-14 range. Had a problem adjusting the idle down from 1050 to around 870. The fast idle adjustment is barely engaged. I intend to work on through the idle cams and secondary springs. Currently, I have the brown cam & yellow spring in. I'm wondering if I should just go back to the black spring & black cam, then work backwards. Any other ideas on the mid-range-WOT stumble?
    Last edited by wrp; 02-11-2017 at 07:55 PM.

  6. #16

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    Solved the problem. I went back in and changed the power valve to a 7.5, since I'm on 15 inHg of vacuum. I double checked the transition slots, and I set the float level in the bowl off the carburetor. I also moved the Jets up from .68 primary to .72 primary, and the secondaries from .73 to 78. That brought the idle down, so now I have adjustment in the four corner idle adjustment screws, and I can set the engine to around 14 AFR on the idle. When I go WOT she smooths out to 13.1. The car is an absolute beast. Cannot wait to get her back to the dyno.
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  7. #17

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    So I replaced my 1 1/4" headers with 1 1/2" headers. Now it's running rich. I have twisted the four corner adjustment screws both directions, but no change. Could the accelerator pump cam be causing a problem?

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