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Thread: Super Tuning A 670 Street Avenger

  1. #11

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    So you saying that when you "STEP ON IT HARDER BUT NOT WOT" that dropping down 3 jet sizes takes you from an 11:1 AFR to 15:1 + AFR? Or are you saying that after making the jet change that your lean spot of just cracking the throttle gets even worse ?

    Keep in mind what the intention of the accelerator pump circuit is used for. Since air can react way faster then liquid can, when you get on it air flow happens instantaneously. There is a delay for the pressure differential to happen which causes fuel to flow. That inherent delay is where you get a hesitation. That's why we have the accelerator pump circuit. Its a MOMENTARY shot of fuel to make up the difference in reaction time of air flow vs fuel flow. This is why its so important to make sure the linkage is adjusted properly. (I'm not saying yours isn't) You could have the biggest pump cam, huge squirters, and the bigger 50cc accel pump and still have a hesitation if your linkage isn't right.

    After making the above main jet change your going to have to adjust your IDLE MIXTURE SCREWS to a more rich condition. Check out this web page. TONS of good information on tuning. I borrowed this picture from him. As he states the bars are not scale, but he shows where "fuel" comes from during different driving conditions. During cruise and light acceleration your fuel comes from idle circuit, transition circuit, and maybe the mains. The mains don't start flowing until about 2000 rpm or so. The only user adjustable of those are the idle mixture and main jets. You can't adjust your the transfer slot.

    http://www.bob2000.com/carb.htm

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    Last edited by 69cam; 03-26-2015 at 08:47 AM.

  2. #12
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    It looks like he needs to put his PV back to what it was, enrich the idle a little to help with the lean spot, then drill & tap the PVCR so he can adjust the WOT.

  3. #13

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    Quote Originally Posted by 81 TransAm View Post
    It looks like he needs to put his PV back to what it was, enrich the idle a little to help with the lean spot, then drill & tap the PVCR so he can adjust the WOT.
    A higher PV would help. That's why I suggest to drive around with a vacuum gauge, and see where his vacuum is. The problem with drilling & tapping the PVCRs, is that they're at an angle and requires you to build a "JIG" if you will, to make sure the drill bit goes in at said angle. Second thing is, that after drilling & tapping the PVCRs, it can require you to clean up the threads of the PV. If you don't already have the taps, drill bit, and 6/32 hex blanks by the time you acquire them, you could probably purchase aftermarket ready to go metering blocks, already drilled & tapped for PVCR, IFR, and EMULSION for a very modest difference.

  4. #14

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    65/68 was standard jetting. Like this, with the 65 in the mains, that lean spot we are talking about goes all the way up to 22:1 AFR, instead of the 17:1. Also, the whole light cruise goes to about 16.5:1. Compared to the 14.7-15.5 I have now, that's why I said it runs best overall with the 68s. The rich condition when accelerating would still be there. I had the idle adjusted all the way to 12.5:1, and it barely made a difference, other than letting off the throttle went to 10:1. And the acceleration richness was about 1 point richer too, high 10s

    So do I understand that right that the accelerator pump is only for stepping on it harder? And for easing in its the transfer slot, mainly?
    Last edited by Knebel; 03-26-2015 at 01:15 PM.

  5. #15

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    OK, so with the 68 your getting your desired affect in cruise & light acceleration. But for the hard acceleration and not WOT, or activating the vacuum secondary acceleration your getting a rich condition. That being said, my advice is "determine where the fuel that is causing the rich condition" is coming from. Since we know we are only dealing with 4 circuits; idle circuit, transfer circuit, main circuit, and PVCRs. Only 3 of those are adjustable. So lets drop a circuit all together. Go get a PV plug. Lets see if your rich condition is caused by by the PVCRs.

    You lean spot is going to be strictly from your IDLE MIXTURE screw setting. Since we can adjust the transfer circuit thats your only other option. Well that and adjusting the secondaries as i mentioned before.
    Last edited by 69cam; 03-27-2015 at 02:28 PM.

  6. #16

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    So could I open up the IFR a little to solve this? It idles best at the 14:1. I measured .028" on the IFR, I could go to .029" or .030". I have the drill bits for that. This way, I can keep the clean idle and have the transition a little richer. Just don't want to throw the "economy" out the window.

  7. #17

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    Took the carb off today and tapped the PVCR to 6-32. Installed .038" bleeds in it. I readjusted the primary throttles to show about 0.028" transition slot. Then I adjusted the secondaries to show roughly the same amount of light around the edges. Cleaned out the metering block good, and put the carb on the car and adjust the idle mixture screws to the 1 1/4 (exactly how I had it before). Car starts up, and after it ran for a little bit, I tapped the throttle to get the idle down. It instantly stalled. A bunch of tinkering later, I have it running with the secondaries closed some more, and the primaries open some more. I have a good 13.5:1 in park and 14.5ish in gear, it can idle like this all day long. I rev it in the driveway, snap it open, slowly accelerate...great! Maintains a 14:1 all the way to 1500 or so. (Now mind you, with the secondaries more open and the 0.028" transition slot showing, I rev it and it goes to 22.8:1, and I also got a 15:1 idle with the screws IDK 3 or 4turns out. And I put it in gear, it goes even leaner, car was not driveable).

    Now I have them out about 2 1/2 turns or so and it idles good like I said. I tinkered with the secondary opening (the stop screw) to get it close to what I had it at (13:1 in park, 14:1 in drive), but I have to have the mixture screws out more!?? Anyway, I rev it in park, slowly, snappy, all good, goes back to idle fine and so on. I pull it out the driveway, it goes a little lean at slow acceleration (just like before, maybe not as much though). Then I get out our side street and get on it hard, no hesitation, AFR 11 something for a brief moment, then 12.2:1, great I think, lets try the smaller PVCR next. I let off the throttle and it...IDK what the hell it did, first I saw the idle really lean, then it goes super lean and I see the 22.8 on the meter (all with a normal 750 rpm idle speed), then it stalled. I coast to a stop, it instantly starts back up, idles way leaner than had it setup, and I could pull it in the driveway. I don't know why it won't hold my idle AFR. The only other thing I did yesterday was a new PCV valve...the car is not driveable like this.

    Oh, when I slowly bring up the rpms, my vacuum climbs from 14 to about 19 or 20 within the idle to 1500 where it was so lean, and this is when it stays at about 14:1. Before when it would go way lean, the vacuum would just stay where it was and then I would get over the lean spot and it jumps up to 19" Hg or so.

    I already sprayed the base of the carb to check for leaks but didn't find any! Is it correct, that the more I close the secondaries, the more vacuum the primaries pull?
    Last edited by Knebel; 03-28-2015 at 09:22 PM.

  8. #18

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    I know what's going on. Fuel gauge sits at 9 psi and doesn't move rhythmically like before. Tap the nut for the needle & seat and it twitches. I just managed to go around the block and it started the lean out again, opened the hood. 9 psi. I tap the fuel bowl and the pressure drops to almost 0, and the car stalled and showed 22.8:1 on the meter. I could also see it gradually getting leaner & leaner. I opened the plug for the fuel bowl and there was barely any fuel in there...sigh, so I need to fix that now.

    Before that I changed the PVCR to .033" (I actually read, that they should be half the diameter of the jets to increase fuel by 25%), so that should be close. I also opened the IFR to 0.031" And adjusted the idle mixture to 13.5:1. I think I can go a little leaner with this now that I have a little more fuel at the T-slot. I reset the secondary stop to 1/2 turn open. Then I set the idle with the primaries, so I have a good 750 rpm. After this, I took the carb off and measured the T-slot...0.0315". I think that's okay right?
    Last edited by Knebel; 03-29-2015 at 10:23 PM.

  9. #19

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    Don't try to adjust your T-slot width...you end up making it a boat anchor, and have to get a new carb body or new carb all together.

    Before you go making any changes, need to get your fuel bowl issue resolved first. That's throwing a monkey wrench into everything.

    It's an absolute must you have constant fuel pressure and fuel delivery at all times.
    Last edited by 69cam; 03-30-2015 at 11:52 AM.

  10. #20

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    Got a pressure regulator today, and cleaned the needle & seat. I'm gonna report back when I have an update!

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