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Thread: 870 Avenger rich off-idle, but too lean at cruise shift points.

  1. #1

    Default 870 Avenger rich off-idle, but too lean at cruise shift points.

    I have a Holley 870 Avenger, and have had a "time of it" trying to tune this carb. It idles good with 13.0-13.5 AFR (have wideband) @ 950 rpm, with having idle mixture screws out 1.5 turns. As I raise idle from 950 (while in Park), the AFR goes progressively richer to 11.9 at around 1700 rpm, then it starts to lean out back to around 13.5 AFR by 2500 rpm. In gear, the lower rich off-idle isn't as bad but once I get to my 1-2 and 2-3 shift points (2500-3000 rpm at about 1/3 throttle, not WOT shift points), the gauge goes too lean to about 15.8 AFR with just a bit of surge. I have 15.5 in/Hg at idle in P and 12.5 in/Hg in gear. I have installed a 10.0-10.5" power valve to help with lean tip-in issues. Just trying how to figure out what changes are needed, to help the lower rpm rich then the lean shift point issue. Where should I focus my attention to cure these issues, IAB, IFR or Emulsion bleeds in metering block. The IFRs are in the lower position in the metering block. Please advise. Thank you in advance.

  2. #2
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    As a test, you could increase your idle AFR to 12 to 12.5. Then test it at the higher rpm. That will tell you if your idle circuit has any impact at that rpm. If it does, you may want to look at the IR. It may be too small and running out of fuel.

  3. #3

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    81 TransAm,
    Thank you for the reply. I have previously adjusted the idle AFR down to 12.5, and have roughly the same 15.5 in/Hg @ idle, but my just off-idle richness goes real rich, like 11.3 AFR. The shift point leanness was helped a bit, but not enough to suffer the MPG penalty at low cruise rpm. I'm thinking reducing the upper E-bleed orifice from .028" to .026 or even lower could be my next move, but emulsion changes are really my last resort. To do this would possibly require a IAB/IFR reduction to lean the just off-idle, low cruise rpm area and make up the reduction of fuel with the E-bleeds. Does this sound feasible, or am I chasing my tail. Once again, thank you for your reply.

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    The purpose of my test was to determine what affect the idle circuit has on the higher rpm areas. Of course, it's going to make the lower rpm areas too rich. If you open up the IR it may enrich the higher rpm areas. The transfer slot could be running out of fuel before the mains start to flow.

    The e-bleeds will not affect the idle circuit. Don't mess with them.

  5. #5

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    OK, so if I open the IFR up to supply additional fuel to the transfer slot, how would I lessen the amount of richness when rpms are between 1200 to 1700 rpm? Would an increase of the IFR along with an increase of IAB, cause a leaner low end, but a slightly richer transfer slot fueling curve? Or would a smaller MAB start the mains fueling sooner, cure my shift point lean issue? Really wanting to learn and REALLY appreciate your replies.

  6. #6
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    With so many different engines out there, most tuning is trial & error. You need to find what works for your engine. I would probably work with the IFR & IAB. How comfortable are you with modifying these? Have you ever drilled & tapped these before. You can buy all the bleeds, but I find it's a lot cheaper to use a brass set screw in the same thread size as the bleeds, then drill them out to the sizes you need. Just tap the hole deep enough that the set screw stops at the depth you want. It has been a long time since I have done this, but I think the thread sizes are 10-32, 8-32 & 6-32.

    The thing that usually causes the mains to open later is the kill bleed. If you look at the metering block, you will see the E-bleeds. Follow that channel up to the dogleg. It may have a hole there. If it does that slows the mains.

  7. #7

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    I have no problem with removing the factory brass air bleed & IFR inserts, and installing brass set screws. Have done this intricate work on a 770 Avenger I have. I realize it sounds like I'm asking for an accurate-to the number IAB/IFR sizes, and I also realize every engine combo is different, I really need a direction in which to concentrate my efforts to overcome this combination of rich/lean condition being so close in rpm. I do have kill bleeds in both of the metering blocks, about .028", and have found info where there are recommended for a down-leg booster. Would a smaller orifice, say .026" help me with my shift point leaning issue? Would a larger IAB (upsize .005-.008") and slightly larger IFR (upsize .001-.002"), help my low rpm/just off-idle richness, while helping with my shift point leanness? I have read where a smaller IAB starts the fueling curve sooner, but extends the idle circuit fueling duration; whereas a larger IAB starts a bit leaner, but has a more aggressive fuel curve, but will also reduce the overall idle circuit fueling duration. It has proven difficult for me to get all this carbs individual circuits to work together in harmony. Please advise. Thank you so much for your replies.

  8. #8
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    I would look at the IFR & IAB. A little bigger IFR may give the transfer slots a little more fuel and extend their range.

  9. #9

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    Thank you X2, maybe 100! I will try the IFR increase, .001-.002". Would reducing the IAB be a good move? I appreciate your replies, 81 TransAm, believe me I do.

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    Make one change at a time, then see what it does.

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