All,
I would like to get your tips on engine (carb & timing) tuning using wide-band O2 sensors. I recently purchased a F.A.S.T system with dual O2 sensors inputs. Over the weekend I welded in the O2 sensor bungs. Tomorrow evening, I would like to get started on the tuning process. I am not looking for step by step beginner advice rather, I am looking for tips you've learned along the way using a similar 02 sensor tuning tool. With the F.A.S.T system I can watch each sensor individually or, average them together. I am tuning on the original 327 modifications include headers, a Holley, MSD distributor w/MSD 6AL box, mild cam, & 4speed trans. Looking forward to your posts.
Dan

 

target afr's to tune for.....around 14 at idle, 15-16 afr at cruise depending on throttle pressure (15 for when you are under load, 16 under more of a coast, 17 or higher if you're off the gas completely), and around 12.5-13 WOT is safe. You might tune a bit leaner at WOT and might get a bit more power out of it.

 

I hooked up the F.A.S.T system with the engine at operating temp in neutral t 500 rpm idle speed the A/F ratio is 14.2:1-15.2:1 Starting out in 1st gear leaving a stop I get a bad stumble the A/F goes way lean 20+:1. At part throttle and cruise the A/F ratio is 12.4-13.2:1 the same for WOT conditions. Needless to say I am running rich under part throttle and cruise conditions nearly the same ratio as I am under WOT conditions yet lean on of idle throttle tip in. I did install larger accelerator squirters originally they were 28 I went to 35 no change 38's no change 42's and a slight change improvement. I also increased the accelerator pump cam. I believe my next move will be to install a 50 cc pump however, I have to install a 1/4 carb spacer to clear the intake. I will also install a hollow squirter hold down bolt to flow more fuel.

As for the rich condition, I need to check the power valve & make sure it is not damaged. If it is not I will change step the primary jet sizes down a couple sizes and again test drive. This is a square bore double pumper I know that I have to be careful as simple changing the jet sizes may affect the power valve channel restriction circuits an modify those as need be.

Please continue to share your experiences.
DC

 

float level may be set wrong.

Also sounds like primary jets are too big, secondary might be right or smaller than needed when you swap in the proper primary.

It goes lean right at throttle tip in because you allow in more air but the fuel hasn't caught up to it.....the accelerator pump isn't adjust right still either.

 

Squid,
I was not able to get the test drive in before dark after making the changes to the carb. The primary jets were 71 and the rears were 80's I stepped the primary's down to #69's. Based on my cruise manifold vacuum I swapped in a 8.5 power valve. Regarding the off idle transition bog, the float level is adjusted correctly so & installed a 50cc accelerator pump & adjusted the arm to a .015in clearance.

I have learned when changing the primary jets on a square bore double pumper the power valve circuit restrictions may need to be altered by drilling. This is to maintain the original restriction size Holley engineered it for. I am going to hold off on any drilling until I get some more miles out of this.

Should have more to post tomorrow.
DC

 

I've had some time to make several test drives with the F.A.S.T system plumbed into the exhaust & let me say what a cool tool this is...forget the hassle of "reading the plugs" after every pass while tuning your carb/timing. The results of the tests drives showed I was running rich during part throttle & w.o.t also, I was running lean on the enrichment (transition) circuit about a ratio of nearly 20 to 1 between idle and take off. The car was stumbled bad & nearly died when I'd let off the clutch pedal & go to take off from a stop light. Adjusting the floats did improve the enrichment circuit however, the stumble was still present and the f.a.s.t system showed it lean. I installed a larger primary squirter nozzle...better, still a tad lean so I installed a 50 cc accelerator pump along with a larger primary banjo bolt to allow increased fuel to pass to the squirter nozzle and the car now launches hard. For the part throttle and w.o.t problem rich (15.5-16.3:1) first, I measured manifold vacuum at cruise then installed the appropriate power valve, second I changed the primary jets from 70's to 68's. Test drove the car still rich when getting into the secondary’s so I backed them down from 80's to 78's. I then bumped the timing up 3 degrees and this is running better than ever. Looking forward to assisting with your issues & thanks for the tips.

 

""For the part throttle and w.o.t problem rich (15.5-16.3:1) first""

at WOT, you should be between 12 and 13.5 on the AFR....if you're reading 15.5...you're running way lean on accel and will be running very hot and very dangerous.

At cruise throttle, 15.5-16.3 is perfectly fine.

Glad it's pulling better for you...just want to make sure that's clarified about the AFR at WOT....hopefully it's between 12 and 13.5....and hopefully around 13 the whole way through the RPM band.

 

Squid,
You are correct thanks for pointing out my mistake, I flip flopped some of my numbers in yesterdays post. During the enrichment circuit between idle and acceleration my numbers spiked dangerously lean nearly 20:1 it would either stall or, I'd have to pump the life out of the pedal to keep it running when I did, the engine would roar back to life and it would make for miserable take off from any stop. Now, I get solid squirt of fuel, taking off from a dead stop is smooth.

Originally, during part throttle and cruise, a/f ratio was rich looking more like w.o.t numbers 12's. Now I at cruise I am right around on 14.7-15:1 on average. At w.o.t a/f ratio is right around 12.4-13.2 overall solid numbers. I will continue to tune on the thing I could go a hair leaner on cruise to get it up around closer to 15.5-16:1 and the same for w.o.t to bump it up closer to 13.5:1. For now, it is running good & I feel safe running a hair rich vs. lean on the street and around town. Next post I will work on keeping my numbers straight hopefully I didn't mislead anyone.
Thanks Again.

 

those are good numbers for accel....and I bet it really rockets now.