Author Topic: Quadrajet leanning out at WOT  (Read 251 times)

Offline Chris Goodrich

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Quadrajet leanning out at WOT
« on: February 09, 2024, 11:31:00 AM »
I re-built an 800 CFM late model chevy q-jet for my 468 BBC (full specs below) to run in my 1976 C10. I toasted the bearings in the 468 and have built a 496 (specs below).

On the 468 it ran too lean and I do not want to run into that issue with the new 496. I have an air fuel ratio gauge hooked up to two oxygen sensors, one in each header collector. The AFR gauge shows it idling between 14.5:1 to 15:1, cruising around 17:1, mash on it and it dropped down to about 12.5:1 momentarily and then steadily starts climbing up and the engine started breaking up around 13.5:1. Fuel pressure gauge shows steady PSI around 7.5.

468 was:
Edelbrock RPM dual plane manifold
40 over 18.3cc dome pistons by ICON
factory forged crank and factory rods
781 Heads 119cc combustion chamber
.027 steel shim gasket
CR roughly 9.5:1
Comp Cams hydraulic flat tappet single pattern cam 253 @.050, lift was .550ish(?) 110 LSA
Crane gold 1.7 ratio roller rockers on intake and exhaust
Long tube headers, 1 and 7/8 inch primaries, 3.5 inch collector with 18inch long collector extension

The 468 combo was run with both the factory HEI distributor and Petronix points with CD box and rev limiter set at 6,800 shifted the TH400 at 6,500 RPM. Ran fine through first gear, did not go lean until top of second gear. Fuel pump was a Holley Black electric fuel pump running through 3/8 fuel line to pressure regulator and then -6 AN line to the carb, return line on factory tank pick up was only 1/4 so even though I tried running a return style regulator I had to run a deadhead regulator after it to bring pressure down from 14psi to 7.5 PSI although I could run up to 10 psi before it started flooding out of the top of the carb, PSI remained steady at whatever I set the deadhead at.

496 is:
Edelbrock RPM dual plane manifold
60 over 12.5cc dome pistons by RaceTec
Scat 4.25 inch forged crank, scat I-beam forged rods
AFR 265cc CNC ported heads 112cc combustion chamber
.030 gaskets
Zero deck
CR 10.5:1
Erson hydraulic roller, 246/252 duration @ .050, .620 lift on exhaust .656 lift on intake, 108 LSA
Crane gold 1.7 ratio roller rockers on exhaust, Harland Sharp 1.8 ratio on intake
Long tube headers, 1 and 7/8 inch primaries, 3.5 inch collector with 18inch long collector extension (might go to 2inch primaries)

Quadrajet Specs:
Model # 17080206
choke flaps removed
75 primary jets
50 custom M primary rods from Cliff's
Secondary Rods are DG rods with an M hanger (I also tried several other rod and hanger combos I would have to look at my box of rods to remember what else I tried besides DP rods and AX rods but the DG rods are the richest ones I have)
New primary shaft bushings
Air horn is nice and flat and always properly torqued with new gaskets between every rebuild
New cam gear for secondary hanger
Secondary flaps open to 90 degrees
HP accelerator pump from Cliff's
new needle and seat from Cliff's (I forgot the size and did not write it down but it is .130 or .135)

I have Cliff's book and used that for my rebuild. I started with recipe #2 then #3 and went up from there, finally got it to idle on the idle circuit (no nozzle drip) with:
.038 idle tubes
.0635 down channels
.1094 idle bypass air
.094 idle screw holes
light power piston spring

Please tell me what you think? Is 13.5 too lean for WOT as it seems to be, what can I do to make it richer?
« Last Edit: February 09, 2024, 11:35:19 AM by Chris Goodrich »

Offline Chris Goodrich

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Re: Quadrajet leanning out at WOT
« Reply #1 on: February 09, 2024, 11:32:48 AM »
The 496 is already built and going into the truck now, it will be run with a new fuel system. Aeromotive 340 in tank fuel pump installed at the back of a blazer tank retrofitted to fit the truck, -8 AN line to and from the Aeromotive return style regulator, -6 AN line from regulator to Carb (could easily switch that to -8AN but I already have the -6 AN line and fitting for carb inlet)

Th400 tranny with a 3,500 stall converter, 4.10 gears. 315/60 Mickey Thompson Et streets, 8.3 best ET in 1/8 mile with the 468. Have not run the 496 yet.

I was thinking about running a .145 needle and playing with the primary jets if it runs lean with the 496 but don't know what else to change as the DG secondary rods are the richest ones I have. I would like to keep the AFR ratio in the 12s at WOT (at least I would think that would be the smart thing to do and that 13.5 is way too lean for WOT).

Secondary wells and primary wells do not leak.

Offline Kenth

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Re: Quadrajet leanning out at WOT
« Reply #2 on: February 10, 2024, 02:39:13 AM »
Secondary flaps open to 90 degrees will restrict fuel flow from the nozzles.
I would not go further than 1.38" measured from back of open valve to back of chokeblade housing.
If more than that install a 6-32 set screw at the axle stop to adjust opening degree of the air valves.

Offline 77cruiser

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Re: Quadrajet leanning out at WOT
« Reply #3 on: February 11, 2024, 10:28:20 AM »
If your meter is reading right 13.5 is no problem. Would definitely go with the .145 N/S, might have to drop fuel pressure a bit.

Offline lightning boy

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Re: Quadrajet leanning out at WOT
« Reply #4 on: February 12, 2024, 12:39:34 PM »
Chris, I would install the fuel pressure regulator after (downstream) of the carb. Tee the fuel line coming from the pump into the carb, then put the FPR in, set it to 7 psi or whatever, then what the engine does not use goes back to the tank instead of trying to dead head it from the supply to the carb.

Offline 73ss

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Re: Quadrajet leanning out at WOT
« Reply #5 on: February 13, 2024, 03:33:22 PM »
I have a very similar set-up. 489 stroker with Brodix Race rite heads. Chet Herbert HR cam with almost identical specs except its on a 114 LSA. 200 4-R trans with 2500 ish lockup converter and 3.73's in the rear. I have one O2 sensor & AFR gauge.

Carb is 17080212. (Factory BB truck carb) Cliff set me up with a kit. 76 jets & the custom 50M rods.
.140 N&S
.040 idle tubes (originals were .039)
.054 idle downs.
.135 bypass.
.090 idle ports.
In tank holley pump with external regulator set to 6.5 PSI. It holds 5 PSI during a WOT run.

In 4th gear with the convertor locked, 2200 RPM, 72 MPH I'm seeing 15.0~15.3. I can lean/richen this with the APT. Car seems to run best at this range. No surging or audible detonation and actually some decent fuel economy.

At WOT I was getting 11.9 ~ 12.1  I leaned it out with a rod change, It's approx 12.4 now. It does not slowly climb as you described.
(If you want exact rod specs I'll get them, My notes are in garage and I'm in for the night.)

I have not had any nozzle drip with this setup. I opened up the bypass to .135 to get it off the transfer slot. It's still on the transfer slot more than I would like but am hesitant to go any bigger on the bypass.

I'm curious as to what your main air bleed sizes are and if the carb is the single MAB type? Also, .063 idle downs seem big to me and .038 idle tube a tad small? I'm not an expert though.

One thing I did notice on the AFR is it goes pig rich after letting off of WOT. It did the same thing with a holley that I was playing with. I assume this is because of the high vacuum created and it's really pulling lots of fuel from the idle circuit? Has anyone else seen this with a AFR?

Offline Cliff Ruggles

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Re: Quadrajet leanning out at WOT
« Reply #6 on: February 15, 2024, 01:58:51 AM »
"new needle and seat from Cliff's (I forgot the size and did not write it down but it is .130 or .135) "

It's probably .135" as I don't sell anything smaller than that.  Still not big enough for what you are doing.  I'd go to a .145" seat and make sure you have at least  7psi fuel pressure.....