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Thread: BBC 540 WOT lean spot.

  1. #1

    Default BBC 540 WOT lean spot.

    Good day. I'm having a really annoying issue with a customer's 540 BBC. It's running a mild cam and Holley 950 mechanical double pumper. When doing a WOT pull, it goes dead lean till roughly 2750 RPM. From that point on, it's at a beautiful 12.5 AFR till redline. Being a manual transmission it's really annoying while driving. Cruise AFRs are OK, Idle too at 3/4 turn. Current setup:
    76 front and 80 rear jets
    6.5 power valves
    Non adjustable air bleeds, so still OEM.
    Click image for larger version. 

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  2. #2
    Join Date
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    Default

    Looks like a rather large flat spot there. What size pump nozzles do you have? I'd step them up, 1-2 thousands on the primary, and 2-3 on the secondary. Gary

  3. #3

    Default

    I'll check the pump nozzles next time I got the car here. I assume the nozzles will only fix the problem when hitting the gas in 1st/2nd gear, since it only enriches for a short period of time? it'd be nice to have a fix that also works in 5th gear. Air bleeds? Power valves?

  4. #4
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    Your Lambda is fine from 2500 RPM onwards, it's the 1500 - 2500 RPM where it goes lean. Assuming that's a wide open slam at 1500 RPM, the pump volume is fast enough. The pump cams determine the volume, the nozzles determine the rate. I would think you need 35s with green cams. Gary

  5. #5

    Default

    I'll try it out. I'm certain it'd help with in the lower gears, but it's more masking the problem than actually fixing it. Why would it even go lean there? The air is flowing in, so why won't it take fuel with it?

  6. #6
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    This carb has normal downleg boosters? There are other options. I build all of my carbs with 12 hole annular boosters. Much more responsive than any downleg.

  7. #7

    Default

    It seems like we're on the right track. I'll buy David Vizard's Holley book. Could you send a link to the booster you're using?

  8. #8
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    From one in the US: https://www.ebay.com.au/itm/17157721...frcectupt=true. He has a lot of stuff. You also need to special tool for booster installation, which costs about $300, or a GOOD carb shop. Gary

  9. #9

    Default

    Damn, those are expensive! I'd be happy to order them if it's sure to fix the problem. Since we're living in Holland, a Holley specialized carb shop is out of the question. What else would need to be modified?

  10. #10
    Join Date
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    I've collected my own tools over the last 10 years, so now rebush throttle bodies or fitting any booster is easy job for me. Have you tried any other carbs too see how the engine responds? Believe it or not, the early Holley 4010 all aluminium carbs are fairly good responsive carbs, and they have annular boosters. Here's another option:
    https://www.ebay.com.au/itm/19337367...MAAOSwIkNcK~0d. Gary

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