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Thread: Lean spot just as throttle blades open & cruising.

  1. #1

    Default Lean spot just as throttle blades open & cruising.

    Holley 80457SA 600 CFM carburetor. The car is a 1964 Nova with 250 inline 6 cylinder. The issue is most noticeable when cruising with the throttle barely open. The air/fuel ratio goes over 17:1. This engine doesn’t like anything much over 15:1, so it surges & loses power until the accelerator pump is activated by accelerating. It idles at 1000/800 in gear and 13:1 AFR. The surge doesn’t seem to improve with changes to the idle screws or primary jets. Power valve is an 8.5". Are there fixed jets I can drill out to increase fuel just off idle?

  2. #2

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    Sounds like you're needing more fuel sooner since the squirters help the problem. Try using a numerically smaller power valve and see what happens. What's your idle vacuum reading?

  3. #3
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    Your idle gets its fuel from the idle discharge ports, which is controlled by your idle mixture screws. Your just off idle gets it fuel from your transfer slots. The fuel for your TS is controlled by your idle restrictor in the metering block and the idle air bleed at the top of the carb. To enrich your TS you would need to reduce the idle air bleed or increase the size of the idle restrictor. Since you do not have adjustable air bleeds, you could increase the idle restrictor. It may not take much, maybe .001". Just guessing, but your idle restrictors are probably in the .030" range. I found some small drill bits at a model shop that worked for me.

  4. #4

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    Is L the correct hole to drill out?
    Attachment 3089

  5. #5
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    K is the hole to drill out. The fuel comes from the main well through the hole marked L, then through the idle restrictor (K) to the idle well.

  6. #6

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    UPDATE: I haven’t had a lot of time to work on this car because I’ve been working on a different one lately. I did drill out the idle restrictor about .002" and it helped the stumble, but didn’t clear it up completely. Somewhere I was reading about how to set up the transfer slot and realized that mine was open 1/8” to get it to idle at 800 RPM. I forgot to put in my original post that I have a hotter cam than stock-212° duration @ .050" lift, .512" lift at valve, 110° lobe separation.

    1. What are the thoughts on putting a set screw that can be drilled out in hole B on the picture above and opening up K?
    2. When do I decide to drill a small hole in the butterfly plates to increase the amount of air at idle?
    3. Is there any point in opening up the secondaries with the small set screw on the base plate when there is no idle circuit on that side?

    I’m not real clear on where to start but from what I’ve read it seems that ideally the transfer slot should start as a square. For this carb that’s basically as low as the idle setting can go. I’m thinking put the drillable set screw in hole B and try them at .025-.030” plus a 1/16”(.062) hole in each of the primary throttle plates and see where that gets me. Is that way off base?

  7. #7
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    Don't drill the plates. Open the secondaries. And open the idle mixture screws a little.

  8. #8

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    I closed the primaries down so the transfer slot is about square, and opened the secondaries about the same amount. Now I'm having trouble getting the idle to come down below 1500 RPM. The idle speed screw is backed all the way out. Maybe I need a smaller idle restrictor? BTW, thanks for the help 81TransAM.

  9. #9
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    Don't worry about the idle restrictor size. Your idle is controlled by your idle mixture screws and throttle shaft position.

  10. #10

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    So I got it to run better than it was. This engine (inline 6) doesn't like the secondaries to be cracked open at idle, so I closed them and added some air to the idle circuit with my adjustable PCV valve (ME Wagner brand). It has adjustable idle and cruise circuits so I just opened the idle circuit a bit, increased the idle speed screw about 1/2 turn from all the way backed off, and closed the idle mixture screws to about 3/8 of a turn open. Seems to work well and idles at about 13.5:1 or so. I'm pretty happy with it now.

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