Expert Carberator Tuning Question - lean rpm hole

[aifilaw]

Yesterday I spent the entire day on a Dyno fighting with a Carberator, and I've run into a problem I can't figure out so I'm hoping someone will have an answer.
Before you say anything, yes its a @#$%@#% vacuum secondaries.... its just in there until I can get fuel injection, but that may be a couple years off, so I've got to make do.

Holley 870 Street Avenger (Vacuum Secondaries), standard 78/82 Jets, Metering Block on both sides, accelerator pump on the primaries, 6.5" Power Valves on both sides. There is a huge lean spot at WOT, and somewhat even under moderate acceleration between 2600 RPM and 3500 RPM. The spike is dramatic, going from slightly rich (12.5:1) to incredibly lean (16-17+:1) within a 1-150 RPM change at 2600 RPM. Then as you continue to accelerate at about 3300 RPM it starts to come back and by 3500-3600 it is back to a perfect mix, it stays at or near the perfect mix continuing all the way out to 5500 RPM. My best guess is the 4 metering ports seen closest to the venturi's need to be adjuster/drilled out/changed, but I am not that knowledgeable in how.

I have tried the black, brown, white, and yellow springs in the vacuum secondaries it makes no noticeable difference in the results ( I assumed it would shorten the end of the lean spot, lighter springs bring the secondaries in sooner, but it did not).I can't figure out why this is, but at the end of the day it felt like we were beating a dead horse to death.
I visually checked the secondaries opening, and they operate very well, exponentially opening starting right about 3500 RPM. Vacuum is quite good, brand new gaskets on the metering blocks and the bowls, new needle and seats just in case, new power valves just in case, re-adjusted the floats to about 3/4 of the sight glasses.

Application:
426ci Chrysler BB B Wedge Motor
Aluminum Headed ~280 CFM
Camshaft: comp cams XR280HR-10
Advertised 280/288
duration @ 0.50: 230/236
Lobe Center Angle: 110
Intake Centerline: 106

[68chargerAG]

[maxwellwedge]

Maybe a longer and/or bigger pump shot will help...which may lead to a 50cc pump. Is this bog on an engine dyno or a chassis dyno?

[firefighter3931]

With a Roller cam and good flowing heads that carb is not going to have the right fuel curve. The metering is going to be "off" for that type of application, inmo. A 500hp stroker is going to want a more agressive fuel curve. 

Ron

[aifilaw]

So far, this is the best direction I've gotten...although I wonder if it wouldn't be the main air bleeds on the secondary side...

Hi Bill

It appears that the main air bleeds are too large.  At high flows, the air bleeds let in more or less air to the mix.  A large air bleed lets in a lot of air at the top end, making the mixture lean at high rpm.  What may help is to decrease the size of the high speed air bleeds, which are the ones closest to the accelerator pump nozzle.  You may need to rejet leaner to compensate for the increased signal to the main jets.

If you have press in bleeds, this may be difficult to do.  You can use a punch to distort the hole a bit to make it smaller, but it's hard to control the size this way.

This article may help:

http://www.carcraft.com/techarticles/ccrp_0807_holley_carburetors_basics_guide/stable_air_fuel_ratio.html

If I find a kit to change these to replaceable style, I'll let you know!

Gerard

[71383bee]

With a Roller cam and good flowing heads that carb is not going to have the right fuel curve. The metering is going to be "off" for that type of application, inmo. A 500hp stroker is going to want a more agressive fuel curve. 

Ron

I agree the 870 street avenger is more of a one size fits all carb.  It can't be fine tuned very easily.  I think an 850 Mighty Demon is more up you alley.  FBO prepped my 650 for the 383 and literally it was a drop in deal.  A little bit to set the idle, and go.  No hesitations...period!  It has adjustable air bleed and the idle ease feature plus 4 corner mixture set which once you get the hang of it is a nice feature. 

[aifilaw]

Discovered from the links and pics that the high speed air bleeds were most probably my issue. Since tey are not threaded and changeable, and I didn't want to modify with a punch or shove something in there, I pulled the manifold off the carb and blocked them both with a piece of metal tape. Took it for a spin and I can't tell the lean spot is there anymore, or at least its several points better tremendous improvement. I will have to get a wideband back on there and see just how far I am off now, but it pulls like a freight train all the way, or as well as a vacuum secondary can anyways.

Will do for now until I do fuel injection.