Playing with a wideband

[steve91tt]

I'm a scientist by training so I can't resist the chance to gather data and over analyze things when I get the chance.

To that end, I picked up a wideband O2 meter...

Wide band

It's not my intention to leave the wideband installed permanently but rather just hook it up for tuning or trouble shooting. I installed the sensor in a bung in one of the down tubes in my header. The bung had always been there, just nothing but a bolt in it till now. I left the bulk of the wiring zip tied under the hood and only ran the necessary wires for the gauge which I zip tied to the steering column for easy viewing.

The installation went quickly and smoothly. After calibration I started the car up and and went for a test drive. I was surprised to see that the car was running very rich. So I stopped, leaned the carbs out a click or two, drove, adjusted, drove, etc. In the end, I adjusted both carbs leaner by 1 full turn. After doing so I think I'm still a little rich in spots.

Here are a few observations after leaning out the carbs by 1 turn...

1. Idle: AFR 10.5-12 depending on how hot the engine is. Generally close to 11ish.

2. 40 MPH steady speed, 3rd gear. AFR = 11-12.

3. 75 MPH steady speed, 4th gear. AFR = 14ish

4. WOT 2nd gear to redline AFR 12-13

5. moderate acceleration from 40 MPH in 3rd. AFR = 15-17

1-4 look rich to me but 5 looks very lean. Anyone have experience with these sort of tests?

[ozconnection]

I have an Innovate wide band sensor.

Your idle is rich. It gets richer the hotter the engine is right? Mine is 12.5ish HOT!

2 is still too rich, get 13.8-14.8

3 is ok

4 is ok too, mine is closer to 13 than 12 say 12.7-13

5 call in vacuum advance, extended timing tends to show a leaner mixture....so long as you have no surging, your mixtures will be fine.

Now the test is to get everything to where you need it. That's where the hours come into it. Ask me!

What's your induction setup? Webers, Su's?

They're a cool tool....

Tuning for power or economy?

Cheers.

[JonnyRock]

To those reading this who may not be familiar with Air-Fuel Ratios,

AFR < 14.7 is RICH

AFR = 14.7 is stoichiometric (meaning that theoretically all of the air and fuel should burn completely)

AFR > 14.7 is LEAN

These numbers apply to gasoline engines.

Edited March 27, 2011 by JonnyRock
specified gasoline

[steve91tt]

Sorry, I should have left more information about the car.

details: 3.0 liter Rebello bottom end, stock 1973 e88 top end, Ztherapy round tops, ATF fluid in dampers, SM needles, stock air box, K&N filter, mechanical and electric fuel pump, Mallory distributor, vacuum advance is disabled, MSD ignition, no emission equipment currently installed.

I am currently tuning for power.

Edited March 27, 2011 by steve91tt

[steve91tt]

Would changing to a higher viscosity oil in the SU's allow me to stay richer in case 5 above and therefore allow me to lean them out with the adjustment screws for idle and cruise?

I am currently using ATF in the SU's which is about 7wt oil as I understand it. This seems to be a popular oil but I have also read that folks use 20wt. If they can find it.

[steve91tt]

Update....

I leaned it out 0.5 turns on each carb. The idle went to 12.2 and the low speed cruise went to mid 14's. However 75MPH cruise went to 16ish. This seems a little high to me and I am getting popping from the exhaust on the down shift. So I backed it down a quarter turn. Now high speed cruise is about 15.2, idle 11.5.

It seems that I can't get the idle up without going lean on the highway.

Is there something I can do to the needles to help?

[Gary in NJ]

The needle profile will have more effect then the dampening oil. The dampening oil only effects the rate that the suction piston moves during throttle transitions (to compensate for throttle valve movement). While this will effect the A:F ratio during wide transitions, its effect is negligible.

Talk to Bruce about a a needle profile that will help you to maintain a 14.7 A:F ratio for the widest range of throttle valve setting (not necessarily rpm).

[conedodger]

Actually, I am surprised at just how good your original numbers are considering you're working with carbs and not EFI. I am not familiar with SU's as far as what adjustment they have. If they were Webers I would just go down a number on the idle jets.

[steve91tt]

Found something interesting and unexpected this evening. It seems that engaging my choke actually makes the car move slightly lean rather than rich. If I'm sitting and idling at 900 RPM and 12.1 AFR and engage the choke the RPM's jump to 1500 but the AFR goes up to 12.5. If I use throttle and no choke to get to 1500 RPM the AFR would be near 13 so the choke is still richening up the mixture but I thought it was an interesting observation and counter intuitive to me.

[tlorber]

Steve, you might be able to tweak your cable adjustment to change the nozzle movement with respect to the idle position of the butterfly.

As stated above, I think it is good that you are not leaning out at high rpms, which is a bit more typical with SU's.

On my roadster, I have to compromise a fat idle so that I don't get too lean-this could be adjusted with different needles. There is a huge number of needle profiles, and I remember reading here about someone that had a loaner program so that you could try the different profiles until you found the ones you like.

[steve91tt]

Steve, you might be able to tweak your cable adjustment to change the nozzle movement with respect to the idle position of the butterfly.

I agree, it looks like it is possible to adjust the amount the amount the nozzle moves with respect to butterfly as the choke is applied. However, I am running so rich at idle the car starts great in all weather as it is. I'm thinking that the ability to control the idle with the choke lever with minimal effect on AFR is going to be handy when I kick on the AC in the summer time.

[LeonV]

It seems that I can't get the idle up without going lean on the highway.

Bingo! You have just witnessed the inherent flaw in round top SUs.