7041540 Buick 455 goes lean at WOT

[Rhett]

Hey all - I've got a Buick 540 carb that goes lean at WOT, above 4900 rpm. I firdt saw this on the dyno where the carb was being fed direct, above 4900 the GPM went down while the RPM went up, and the HP went flat. We tried a Holley spread bore and the HP continued to climb. I didnt have an O2 sensor there, but on the car at WOT it reads 13.5 from a tailpipe bung. The car is a 4000 lb GS, 3.73 gears, 35 deg total timing stocker with with ported iron manifolds and a 228/239 114 cam. It idles great, has crisp throttle response through the range, pulls to 5700 and smells great. If I have detonation I cant hear it, the plugs look clean and show just slighly lean on the porcelain. Car ran 12.67@106.5 with no fuel delivery issues first time out. SInce then, I chaged the needle/seat from .135 to .145, and the primary jet/rod combo from 76/44 to 77/45. Cruise got a little fatter @ 14.5 but no significant change at WOT. This carb already delivers way too much fuel, but the car loves it and still gets 13.5 MPG. My main concern is hurting the engine at the high RPM. Carb spcs are as follows and the back to back Q-Jet/Holley sheets are attached. Interested in your thoughts, thanks in advance.

Fuel Inlet   0.145
Float Level   0.250
Idle Tube   0.036
Idle Channel Restriction   0.055
Accel Pump Discharge   0.033
Main Air Bleed (upper)   0.070
Main Air Bleed (lower)   0.070
Idle Air Bleed (upper)   0.067
Idle Air Bleed (lower)   0.070
Mixture Hole   0.090
Primary Jets   0.077
Primary Rods   0.045
Secondary Well restriction   0.039
Secondary POE Restriction   0.059
Secondary Tube Restriction   0.036
Secondary Rods   .00300 CC
Secondary Valve Opening   1.287
Secondary Hanger   F
Idle Air bypass   0.194

[Rhett]

Just to update, I have reviewed the similar post with the Chevy 468 and I did bring my needle/seat out to.152, no significant change noted..

[77cruiser]

Have you checked your fuel pressure? What pump do you have?

[Rhett]

No, but it showed up with the fuel hooked direct from a 6 lb electric pump on the dyno too. Honestly, engine feels great in the car. I'd never know if not for the 02 sensor..

[77cruiser]

I guess I wouldn't worry about it.

[Rhett]

That'd be fine except I'm giving up 30 hp to a workbench Holley, and we pride ourselves on that not being the case...

[lightning boy]

what was the vacuum reading at WOT?

[Rhett]

2-3 inches going by memory? Not quite zero is what I remember.

[77cruiser]

I should have had a vac. gauge when I had mine dynoed. I guess I could do it in car.

[77cruiser]

What size speadbore Holley?

[Rhett]

It was a 6211 800 CFM. And a leaky, uncalibrated POS that wouldnt idle too

[lightning boy]

looks like it's too fat to me. What makes you think it's lean?

[Rhett]

It should be fat, way fat but doesnt act like it. plugs, tailpipe, smell all normal if not slightly lean. The thing that concerns me is with the Q-Jet the fuel GPM went backward, and the HP levelled off where as with the Holley it kept climbing. Yeah the 02 sensor shows lean at WOT, but cant count on that cause its too far back in the system to be entirely accurate. I have another 540 that has a stock bypass, 76/44's and .067/.070 idle bleeds and .052 ICR and it wont idle at all on this engine. It likes the bypass, but I always suspected too much bypass is contributing to a lean condition. I keep thinking a .135 needle/seat was causing the fuel turbulance/stand off at high volumes, but a .145 and a .152 seat didnt change the condition.

[Kenth]

This is what i would have started with:

Fuel Inlet   0.135"
Float Level   13/32"
Idle Tube   0.039"-0.040"
Idle Channel Restriction   0.062"-0.064"
Accel Pump Discharge   0.033
Main Air Bleed (upper)   0.069
Main Air Bleed (lower)   0.039
Idle Air Bleed (upper)   0.054
Idle Air Bleed (lower)   0.080
Mixture Hole   0.095"-0.100"
Primary Jets   0.074
Primary Rods   0.045
Secondary Well restriction   0.039
Secondary POE Restriction   0.059
Secondary Tube Restriction   0.036
Secondary Rods   0.0527 CV
Secondary Valve Opening   1.287
Secondary Hanger   F
Idle Air bypass   0.110"

JMHO

[Rhett]

Well I can still set the other 540 up that way and see how it likes it..