Tuning with a wideband

[jxu109]

I recently installed a wideband O2 setup in my truck.  I was happy to see the cruise mixture hanging right around the high 15's to low 16 range.  AT WOT, It would initially go down to low 10's, and creep to low 11's.  I switched out the secondary rods, and now it's in the low-mid 13's.

My idle is way high.  Standing still, it's around 16-17.  I opened the idle mixture screws to about 4 turns out, and it didn't get any better.  When moving at low speeds and barely being on the gas, it would go as high as 20 (guess that would explain the very low speed bucking problem I experience).  I'm going to try to increase the down channel a bit.

Under light-moderate acceleration, The numbers where in the 14-15 range, which seems a little high???

My main problem is that when idling, and flicking the throttle open from any RPM starting point, the AFR drops to 9 or 10, and it starts stumbling.  I moved the accelerator pump linkage back to the outermost hole, and I'm still seeing the same thing.  All of this seems a little strange, because when cruising and just giving the pedal a good shot, I don't notice any stumble or sub 13 AFR readings.

Is there a good procedure to follow when tuning with a wideband?  Is it beneficial to disconnect the accelerator pump and lock-out the secondaries to try to isolate the systems during tuning? 

Thanks

[jxu109]

Forgot to mention this is a late 70's 800 CFM Qjet.

68 C10 2wd
Super T-10 4 speed
350 ci bored .030,
 ~10:1 compression,
Comp Cams 270H
Performer RPM Heads,
Performer intake
MSD digital E-curve Distributor
Thorley Tri-Y's,
 Magnaflow x-pipe and mufflers.

[Cliff Ruggles]

My advice, tune every area without the wide ban in place.  Set everything, idle mixture, part throttle, full throttle, accellerator pump, etc, to the very best you can get them.  This may take several tanks of fuel.

It really helps to have a drag strip or chassis dyno for the full throttle runs.

Every single time I have spent the needed time to custom tune the carb in every area, when it performed the best, best fuel economy, throttle response, etc, the wideban showed that the carb was within an acceptable range in every area.

Some engines will simply require more or less fuel than others.  Don't forget to tune the distributor as well, as timing requirements go hand in hand with fuel curves.....Cliff

[jxu109]

What I like about using this is that I can see exactly how my changes are affecting the mixture across the board, and it's made me aware of my problem areas without much guessing involved.   I've never been successful doing things by feel, probably because I've never had it running 100% right, so I don't know what it's supposed to feel like.  For example, I lost 4 MPH in the 1/4 last time I was out, and I think some of that was because it was a little rich at WOT, but to me, it felt like it was running as good as ever. 

I realize that I can't tune this to the wideband, and shoot for magic numbers across the board, but I feel more comfortable tweaking things when I can see what effects my modifications are having on driving conditions.

I'm not very good at figuring out what needs to be changed where by feel (unless something has an obvious problem).  I've been tinkering with quadrajets on this truck for 3 years, and I'm really looking to find an approach to tuning that can take my poor sense of good tuning out of the equation.

On a side note, do you have new idle down channel restrictions, or some type of replacement for them?

[Cliff Ruggles]

Yes, we have new idle down channel restrictions, call the shop if you need them or anything else.

I wasn't talking about tuning strictly by "feel".

Idle mixture screws are set by finding the best idle quality at the leanest settings, either by listening to the engine or using a tachometer.  Lean them up one at a time until you hear a change in the engine rpm's, then back them out about 1/2 turn each, taking the time to balance them out.

Set the part throttle A/F ratio using our "tip-in" method in the book.  If you have apprx 50-100rpm tip in rpm increase, the carb is very, very close to perfect.

Then drive the car, and evaluate the performance, and measure the fuel economy over several tanks of fuel.

This is done BEFORE hooking up the wide-ban, so you know your settings are close to what the engine wants, prior to chasing around any particular setting(s) using the meter.

Each and every time I have done this, the carb was very, very close when the meter was installed.

What we see these days, is folks hooking up the meter, then chasing numbers all over the place, failing to evaluate engine performance, throttle response, fuel economy etc, simply because they are trying to stay within certain parameters on the LM1.

It really matters not what range or particular numbers you end up with, vehicle performance is more important......Cliff