Carb questions

[GRTfast]

I have a street big block with the following specs in a ~2500 lb car, 4 speed, 3.27 rear end, 29 inch tall tires.

.030 over 454 '72 cast iron block.
brodix race rite bb2 heads
comp cams thumpr hydraulic roller
rpm air gap
forged bottom end, H beam rods
9.5:1
Annular booster 850 DP holley, 2 corner idle
HEI distributor, 30 degrees initial timing, 39 all in @3000, 10 degrees vacuum advance (idles at 40 degrees advance).

In general, the car rips. Has some low speed surging/bucking issues that I have almost cured (with the 850 carb).

I recently acquired a quick fuels Q-750 DP with annular boosters and 4 corner idle. Car seems to like this carb better. Low end response is better, seems like it has more torque in the low and mid range, doesn't seem to make less power up top. Surging/bucking is all but gone. This new carb takes way less accel pump shot to run correctly as compared to the 850.

I have never measured a/f ratio on this car, only old school tuning techniques for any carb that's been on it (vacuum gauge for setting idle mixture, reading plugs for the mains).

Now for the questions.

1. I will be installing a wideband o2 kit for tuning purposes (already have the kit on the way). Do I need a sensor and gauge for each header (cylinder bank), or is tuning while only reading one bank generally acceptable?

2. I have to run the rear idle mixture screws all the way closed, and the fronts at about 7/8 on this new QF carb to get the car to idle correctly. If I open all 4 screws to 7/16 it is too rich (based on smell, and the way it runs). If I try going in further than that using all 4 screws, it seems like a tiny tiny change makes a huge difference, and it seems I can't find an acceptable idle mixture setting using all 4 corners. Does that sound normal? Should I be playing with idle air bleeds to get the mixture right with all the adjuster screws open to 1 turn (once I have my wideband installed)?

3. Any other advice that comes to mind when reading my post?

[427dart]

Are the idle air bleeds same size in Secondary as the Primary? Make sure no debris is in the secondary air bleed jets.
What happens if you have the mixture screws out 1/2 turn front and rear?

[GRTfast]

Are the idle air bleeds same size in Secondary as the Primary? Make sure no debris is in the secondary air bleed jets.
What happens if you have the mixture screws out 1/2 turn front and rear?

No debri, just can’t seem to get it lean enough at idle using the rear mixture screws. I’ll check the air bleed sizes tonight. I’m excited to get the wideband hooked up.

[427dart]

Might want to verify the sizes of Idle Feed Restrictor in both metering blocks and measure sizes of the Idle Air Bleeds front and rear.

When using your AFR gauge fire up the engine and let it run for 30 seconds before turning the AFR unit on.
This helps prevent moisture from blowing onto the sensor at cold start up.

[1972ho]

http://www.quickfueltechnology.com.au/c ... s-AUST.pdf Here is a old QF spec sheet see if the idle air bleeds match up to your carb.and you make changes to the air bleeds it will have a effect on the mix screw adjustments.

[GRTfast]

Got my wideband hooked up... this thing rocks. I've got the carb almost dialed at this point, I just have a lean spot at part throttle on the primaries before the power valve comes in (it's a 6.5). I ordered an assortment from 7.5-10.5 to tune out that lean spot. Here is how it's running:

idle: 14:1 (4 corner idle, all adjusters at 9/16 turn)
cruise: 14:1
part throttle accel on primaries before power valve opens: 15.5:1
part throttle once PV opens: 13:1
wide open: 12.8-13:1

Wide open was 11:1 but I reduced the secondary mains from 82-76 and bumped up the main air bleeds from .031 to .050, did the trick. I think it was too rich because this 750 is a tad small for the engine. I'm getting 2.5 inches of vacuum at wide open throttle above 4500 rpm or so. Apparently that scenario makes the carb act rich, which makes sense because the engine is "sucking" on it harder.

I find it interesting that Holley's method for choosing the correct PV (divide idle vacuum in half) seems to be flawed. It looks like this thing is going to want a 8.5 or so based on the vacuum readings through that throttle range. My idle vacuum is 12 inches. Strange that Holley would use idle vacuum as a metric to set a circuit that provides fuel at mid/upper level throttle settings.

Tuning with the a/f gauge is great, literally every adjustment I've made is in the right direction.

[prairiehotrodder]

Don't trust the AFR gage numbers. It would be better if they were just A, B, C etc.... Just use them to compare against themselves. For instance i would bet that your WOT 12.8 - 13 is way to lean for best power. Even though everything you read says thats where you want it to be. I'm using the AEM failsafe gage and my motor was way to rich according to the gage at WOT. I leaned it up to 12.5 ish and it went way slower at the track.

Also if you have to change the carb more than 4 or 5 jets from the factory calibration beware.

[GRTfast]

Don't trust the AFR gage numbers. It would be better if they were just A, B, C etc.... Just use them to compare against themselves. For instance i would bet that your WOT 12.8 - 13 is way to lean for best power. Even though everything you read says thats where you want it to be. I'm using the AEM failsafe gage and my motor was way to rich according to the gage at WOT. I leaned it up to 12.5 ish and it went way slower at the track.

Also if you have to change the carb more than 4 or 5 jets from the factory calibration beware.

Huh. I guess if I was drag racing, I would jet for ET. That said, this is a street car. With the settings I have now, it is the most responsive and best feeling setup I've had, and the plugs look good. I wonder if there is something about your engine that makes it like being super rich at WOT. I've heard that some old head designs were so bad that an overly rich mixture made the best power. I'm guessing you have good heads on your engine though.

When I worked in the motorcycle biz, we used a superflow eddy current dyno to build maps for sportbikes. Max torque/power was almost always in the 12.5:1 to 13:1 a/f ratio range. I have been reading that the same holds true for just about any IC engine on gasoline.

[jmarkaudio]

The idle calibration is rich on the QF carb, and the emulsion package makes it overly rich at part throttle. Also the original Holley you have has the idle feed restrictions pressed in at the bottom, the QF is threaded at the top and needs them threaded on the bottom to work best.

[BigBlockMopar]

Using an AFR-gauge is like being able to see for the first time what is actually happening and what/how the carb is 'doing'/reacting to the engine.
On the street is where the AFR gauge shines, as most of the time the engine runs well below 3000rpm.
Even better is when you're actually seeing the effects of your tuning and also feeling them during driving.

[GRTfast]

The idle calibration is rich on the QF carb, and the emulsion package makes it overly rich at part throttle. Also the original Holley you have has the idle feed restrictions pressed in at the bottom, the QF is threaded at the top and needs them threaded on the bottom to work best.

I saw a guide for making that mod on your site. I will probably tackle that one weekend coming up. I’ve been wanting to try a 750 for a while. When I stripped the primary squirted hole in my 850, it was the straw that broke the camel’s back .

I am going to fix the 850 using that BLP repair kit, and play with it some too. Having the afr gauge is cool!

[GRTfast]

Using an AFR-gauge is like being able to see for the first time what is actually happening and what/how the carb is 'doing'/reacting to the engine.
On the street is where the AFR gauge shines, as most of the time the engine runs well below 3000rpm.
Even better is when you're actually seeing the effects of your tuning and also feeling them during driving.

Agreed, it’s fun!