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Thread: Accelerator Pump Cam/Nozzle Tuning

  1. Default

    I have a similar issue with my car, a mild 318 with a 600 vac carburetor.

    I'm working on it being a PV issue, as it seems to me that my enrichment comes in a bit late from the PV-I.
    Dump the pedal, and it's fine until I stop mashing it, then it bogs until the PV opens up.

    Holley specifies to read your vacuum at idle and halve that, then the PV you choose is the next lowest after that number. From what I've read, this is not an ideal solution. Since you & I have WBO2 tuning, we can be a lot more accurate than Holley would have expected us to be in the pre-WBO2 sensor days.

    I'm changing from a 65 to an 85 later in the week. Will be interesting to see if it solves my problem, and yours too.

    Cheers, Robert

  2. #12


    Hey Robert. Did the power valve change help you? I changed my idle AFR to 12.9. That's where the motor seems to like it best. When I initially give it some gas from a stop, I dip into the 11. But when I'm cruising at 45 MPH and open the throttle a little quicker, I see it going lean into 17 for a second, and then it goes. No clue how to tackle this.

  3. #13


    Knebel, try picking up your main jets 3/4 sizes, readjust your idle mixture screws, and then see if that changes your transition cruise situation.

    This is really an emulsion issue. But emulsion is not adjustable on your carb, so you have to try & "massage" some of the other circuits to help out.

  4. #14


    I tried upping the jets to 70, that made the cruise way too rich. And the tip-in isn't 17:1 anymore, but 15 or 16:1. I can mess with emulsion, have some drill bits. I just need to know where to start. Another thought, could it be my timing? Maybe the curve too slow?

  5. #15


    Quote Originally Posted by Knebel View Post
    I tried upping the jets to 70, that made the cruise way too rich. And the tip-in isn't 17:1 anymore, but 15 or 16:1. I can mess with emulsion, have some drill bits. I just need to know where to start. Another thought, could it be my timing? Maybe the curve too slow?
    Knebel, I'm not good enough to advise anyone where to start with emulsion issues. Not to mention they should only be changed .001" at a time. Read around on There are some good carb tuner guys, and some really good posts over there. From there, you'll pickup a couple of website of some serious carb tuners.

  6. #16


    Knebel, any luck on your lean spot? I'm hoping not to hi-jack your thread, but this thread and your other one are very relevant to me. I'm running a 670 SA with WBO2, and getting up into the 18:1-20:1 when I barely crack & maintain throttle between 1250-1700 RPM (very slow maintained acceleration). I know you mentioned blipping the throttle, but in your other post, you mentioned cracking the throttle too. Does yours do this too? Cruise AFR is around 14:1 when up above 2k RPM. My predicament is that my two idle screws are turned out only 3/4, and trying to uncover the secondaries to get more turns seems to make the AFR bounce around more. It also causes my RPM drop to be excessive, from Park (1250 RPM) to Drive (750 RPM). Timing set at 16° initial, and vacuum in Park is 15 in-Hg. I was thinking it may not be worth worrying about with the low load, but I've found loading it up in the range (going up a steep hill) causes it to die out. Let me know what you've found, or perhaps one of the other guys can give me some pointers.

    Thanks! Delvin

  7. #17


    I just typed a long reply and it got lost now...darn. You say it stays lean when you try to maintain the throttle in a certain spot? Does the lean then go away when you give it some more throttle? I had this issue too, and enlarged the idle feed restrictor with a .031" drill bit. That solved that. I now still have that tip-in lean spot where my AFR jumps from 15 to 18 for 2 seconds, when accelerating with just a quick small throttle movement. My carb is off the car right now cause I just installed some annular boosters, but I noticed I have the primary throttles open a lot, had the same issues you describe with opening the secondaries more. Try that in very small increments of 1/8th of a turn and find the compromise.

    Oh & try maybe und 18° initial. The RPM drop is when it's too lean or not enough timing. I currently run 18° initial and a 12.9 to 13.0 AFR at idle, and barely have a 50 RPM drop. Still only have 9" vacuum in drive.

  8. #18


    Delvin, I just got finished putting my carb back onto the car. I had way too much of the Transition slot exposed (about just under half of it). I fixed that so that the slot is square and opened up the stop screw for the secondaries 1/2 turn more. That half turn gave me a 1100 idle in park and about 950 in gear. I slowly closed the secondaries a tiny bit (maybe 1/8th of a turn) til I got just under 1000 rpm in park. With the engine warm I have 750 in Drive, I also looked for the smallest change in AFR from Park to Drive. I did not touch the Idle speed screw anymore on the primaries. I adjusted the AFR with the Idle screws to about 14.5 in Drive. (It jumps a lot, sometimes in the 15s, sometimes in the 13s.) I gotta balance it a bit more and maybe enrich it up a tad. However, my AFR in park is about 13:1 and it goes a bunch leaner in Drive, that is also because I lose some vacuum when I put it in Drive, but it Idles pretty okay so far. I have not noticed the lean spot yet, but only took it around the block.

    When I crack the throttle from a stop, I still have it dipping in the 11:1 AFR. I might have to go back down to my 28 squirter, and see how that goes. I'll keep you posted!

  9. #19


    Knebel, try raising your initial timing up about 4-5 degrees. Get it up to 19 degrees or so, and see how your AFR acts at idle. It should stabilize it out to where it only swings about a half a point. If you decide to keep your initial timing there, you need to check your total timing as well. In park, run it up to 2500-3000 rpm and check the timing. Make sure its not in the 40s. If you have a vacuum advance, make sure that is disconnected & blocked off. If you don't have a degreed balancer wheel, then you will need a dial back timing light.
    Last edited by 69cam; 01-21-2016 at 02:41 AM.

  10. #20


    Thank you for the advice! I'm running 18° initial right now. I used to have it at 22° initial and it seemed to idle better. Maybe I have to go back to that. I know that when I got the car, it had 40° initial, which skyrocketed the total through the roof. That said, it idles super smooth when I adjust the timing to most vacuum, but it ends me up with 30 something degrees. I might have to consider plugging the vacuum advance into manifold vacuum. That would give me about 26° at idle.

    When I adjust the mixture to 12.9 AFR at idle, it just jumps .4 or so, and is way more stable. Right now with the lean mixture the engine shakes like crazy.

    Any idea on how to get that initial rich spot out of it? I'm thinking I get the idle fuel, transition, accelerator pump & booster all overlapping, because the vacuum increases a bit with the rpm raising 2 or 300 rpm, and it pulls fuel from everywhere.

    Oh, and I did a booster swap to annular boosters. Went from 28 air bleed to 32, and jetted from 66 down to 65. Puts me right in the 14s AFR and wow, it feels like the car picked up 100 HP and ton of torque. It was a little sluggish before, and now drives almost like EFI. I even noticed that the cold start with the choke is way more smooth. It's crazy how that changed the behavior of the engine.
    Last edited by Knebel; 01-21-2016 at 07:32 PM.

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